300698留学生辅导、讲解Systems Programming、c/c++语言讲解、辅导c/c++编程 讲解留学生Processing|讲解
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Operating Systems Programming – 300698
1 Introduction
In this workshop you will investigate file I/O and file copy operations.
2 Specification
Following on from the demonstration program cp1.c presented in the lecture, we will make a series of modifi-
cations to the program.
Firstly, what happens when the cp1.c program is asked to copy a file onto itself, i.e. cp1 input input?
Is this what you expect? Modify the program to do something more sensible! As a hint, two files are the same if
they are on the same device and have the same i-node number (which stat() can give you), simply comparing
the names is not enough.
Secondly, a real copy program will assign the same file permissions to the destination as were on the source,
modify your answer to the last part to do this.
Thirdly, real copy programs allow the second argument to be a directory, modify the answer to the last part
to include this functionality. You should allocate the space for the new name dynamically.
13 Sample Code
3.1 cp1.c
#include
#include
#include
#define BUFFERSIZE 4096
#define COPYMODE 0644
void oops(char *, char *);
main(int ac, char *av[])
{
int in_fd, out_fd, n_chars;
char buf[BUFFERSIZE];
if ( ac != 3 ){
fprintf( stderr, "usage: %s source destination\n", *av);
exit(1);
}
if ( (in_fd=open(av[1], O_RDONLY)) == -1 )
oops("Cannot open ", av[1]);
if ( (out_fd=creat( av[2], COPYMODE)) == -1 )
oops( "Cannot creat", av[2]);
while ( (n_chars = read(in_fd , buf, BUFFERSIZE)) > 0 )
if ( write( out_fd, buf, n_chars ) != n_chars )
oops("Write error to ", av[2]);
if ( n_chars == -1 )
oops("Read error from ", av[1]);
if ( close(in_fd) == -1 || close(out_fd) == -1 )
oops("Error closing files","");
}
void oops(char *s1, char *s2)
{
fprintf(stderr,"Error: %s ", s1);
perror(s2);
exit(1);
}
24 Supplementary Materials
The material on the following pages is an extract of the linux system documentation and may prove useful in
implementing this Workshop. These manual pages are taken from the Linux man-pages Project available at:
http://www.kernel.org/doc/man-pages/.
3OPEN(2) Linux Programmer’s Manual OPEN(2)
NAME
open, creat ? open and possibly create a file or device
SYNOPSIS
#include
#include
#include
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
int creat(const char * pathname, mode_t mode);
DESCRIPTION
Given a pathname for a file, open() returns a file descriptor,asmall, non-negative integer for use in subsequent
system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful
call will be the lowest-numbered file descriptor not currently open for the process.
The new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor
flag described in fcntl(2) is initially disabled). The file offset is set to the beginning of the file (see
lseek(2)).
A call to open() creates a new open file description, an entry in the system-wide table of open files. This
entry records the file offset and the file status flags (modifiable via the fcntl() F_SETFL operation). A file
descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently
removed or modified to refer to a different file. The new open file description is initially not shared with
any other process, but sharing may arise via fork(2).
The parameter flags must include one of the following access modes: O_RDONLY, O_WRONLY, or
O_RDWR. These request opening the file read-only, write-only, or read/write, respectively.
In addition, zero or more file creation flags and file status flags can be bitwise-or’d in flags. The file creation
flags are O_CREAT, O_EXCL, O_NOCTTY, and O_TRUNC. The file status flags are all of the
remaining flags listed below. The distinction between these two groups of flags is that the file status flags
can be retrieved and (in some cases) modified using fcntl(2). The full list of file creation flags and file status
flags is as follows:
O_APPEND
The file is opened in append mode. Before each write(), the file offset is positioned at the end of
the file, as if with lseek(). O_APPEND may lead to corrupted files on NFS file systems if more
than one process appends data to a file at once. This is because NFS does not support appending
to a file, so the client kernel has to simulate it, which can’t be done without a race condition.
O_ASYNC
Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via
fcntl(2)) when input or output becomes possible on this file descriptor. This feature is only available
for terminals, pseudo-terminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2)
for further details.
O_CREAT
If the file does not exist it will be created. The owner (user ID) of the file is set to the effective
user ID of the process. The group ownership (group ID) is set either to the effective group ID of
the process or to the group ID of the parent directory (depending on filesystem type and mount
options, and the mode of the parent directory, see, e.g., the mount options bsdgroups and sysvgroups
of the ext2 filesystem, as described in mount(8)).
O_DIRECT
Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance,
but it is useful in special situations, such as when applications do their own caching. File
I/O is done directly to/from user space buffers. The I/O is synchronous, i.e., at the completion of a
read(2) or write(2), data is guaranteed to have been transferred. Under Linux 2.4 transfer sizes,
Linux 2.6.12 2005-06-22 1OPEN(2) Linux Programmer’s Manual OPEN(2)
and the alignment of user buffer and file offset must all be multiples of the logical block size of the
file system. Under Linux 2.6 alignment must fit the block size of the device.
A semantically similar (but deprecated) interface for block devices is described in raw(8).
O_DIRECTORY
If pathname is not a directory, cause the open to fail. This flag is Linux-specific, and was added in
kernel version 2.1.126, to avoid denial-of-service problems if opendir(3) is called on a FIFO or
tape device, but should not be used outside of the implementation of opendir.
O_EXCL
When used with O_CREAT, if the file already exists it is an error and the open() will fail. In this
context, a symbolic link exists, regardless of where it points to. O_EXCL is broken on NFS file
systems; programs which rely on it for performing locking tasks will contain a race condition.
The solution for performing atomic file locking using a lockfile is to create a unique file on the
same file system (e.g., incorporating hostname and pid), use link(2) to makealink to the lockfile.
If link() returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if its
link count has increased to 2, in which case the lock is also successful.
O_LARGEFILE
(LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an
off64_t) to be opened.
O_NOATIME
(Since Linux 2.6.8) Do not update the file last access time (st_atime in the inode) when the file is
read(2). This flag is intended for use by indexing or backup programs, where its use can signifi-
cantly reduce the amount of disk activity. This flag may not be effective on all filesystems. One
example is NFS, where the server maintains the access time.
O_NOCTTY
If pathname refers to a terminal device — see tty(4) — it will not become the process’s controlling
terminal even if the process does not have one.
O_NOFOLLOW
If pathname is a symbolic link, then the open fails. This is a FreeBSD extension, which was added
to Linux in version 2.1.126. Symbolic links in earlier components of the pathname will still be
followed.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in non-blocking mode. Neither the open() nor any subsequent
operations on the file descriptor which is returned will cause the calling process to wait. For the
handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK
in conjunction with mandatory file locks and with file leases, see fcntl(2).
O_SYNC
The file is opened for synchronous I/O. Any write()s on the resulting file descriptor will block the
calling process until the data has been physically written to the underlying hardware. But see
RESTRICTIONS below.
O_TRUNC
If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or
O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the
O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified.
Some of these optional flags can be altered using fcntl() after the file has been opened.
The argument mode specifies the permissions to use in case a new file is created. It is modified by the process’s
umask in the usual way: the permissions of the created file are (mode & ?umask). Note that this
mode only applies to future accesses of the newly created file; the open() call that creates a read-only file
Linux 2.6.12 2005-06-22 2OPEN(2) Linux Programmer’s Manual OPEN(2)
may well return a read/write file descriptor.
The following symbolic constants are provided for mode:
S_IRWXU
00700 user (file owner) has read, write and execute permission
S_IRUSR
00400 user has read permission
S_IWUSR
00200 user has write permission
S_IXUSR
00100 user has execute permission
S_IRWXG
00070 group has read, write and execute permission
S_IRGRP
00040 group has read permission
S_IWGRP
00020 group has write permission
S_IXGRP
00010 group has execute permission
S_IRWXO
00007 others have read, write and execute permission
S_IROTH
00004 others have read permission
S_IWOTH
00002 others have write permission
S_IXOTH
00001 others have execute permission
mode must be specified when O_CREAT is in the flags, and is ignored otherwise.
creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
RETURN VALUE
open() and creat() return the new file descriptor, or ?1 if an error occurred (in which case, errno is set
appropriately).
NOTES
Note that open() can open device special files, but creat() cannot create them; use mknod(2) instead.
On NFS file systems with UID mapping enabled, open() may return a file descriptor but e.g. read(2)
requests are denied with EACCES. This is because the client performs open() by checking the permissions,
but UID mapping is performed by the server upon read and write requests.
If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of
last status change, and time of last modification; see stat(2)) are set to the current time, and so are the
st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the
O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time.
ERRORS
EACCES
The requested access to the file is not allowed, or search permission is denied for one of the directories
in the path prefix of pathname, or the file did not exist yet and write access to the parent
directory is not allowed. (See also path_resolution(2).)
Linux 2.6.12 2005-06-22 3OPEN(2) Linux Programmer’s Manual OPEN(2)
EEXIST
pathname already exists and O_CREAT and O_EXCL were used.
EFAULT
pathname points outside your accessible address space.
EISDIR
pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or
O_RDWR is set).
ELOOP
Too many symbolic links were encountered in resolving pathname, or O_NOFOLLOW was
specified but pathname wasasymbolic link.
EMFILE
The process already has the maximum number of files open.
ENAMETOOLONG
pathname was too long.
ENFILE
The system limit on the total number of open files has been reached.
ENODEV
pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel
bug; in this situation ENXIO must be returned.)
ENOENT
O_CREAT is not set and the named file does not exist. Or,adirectory component in pathname
does not exist or is a dangling symbolic link.
ENOMEM
Insufficient kernel memory was available.
ENOSPC
pathname was to be created but the device containing pathname has no room for the new file.
ENOTDIR
A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was
specified and pathname was not a directory.
ENXIO
O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the file open
for reading. Or, the file is a device special file and no corresponding device exists.
EOVERFLOW
pathname refers to a regular file, too large to be opened; see O_LARGEFILE above.
EPERM
The O_NOATIME flag was specified, but the effective user ID of the caller did not match the
owner of the file and the caller was not privileged (CAP_FOWNER).
EROFS
pathname refers to a file on a read-only filesystem and write access was requested.
ETXTBSY
pathname refers to an executable image which is currently being executed and write access was
requested.
EWOULDBLOCK
The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see
fcntl(2)).
Linux 2.6.12 2005-06-22 4OPEN(2) Linux Programmer’s Manual OPEN(2)
NOTE
Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the
intention to read or write. This is typically used to open devices in order to get a file descriptor for use with
ioctl(2).
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. The O_NOATIME, O_NOFOLLOW, and O_DIRECTORY flags are
Linux-specific. One may have to define the _GNU_SOURCE macro to get their definitions.
The (undefined) effect of O_RDONLY|O_TRUNC varies among implementations. On many systems the
file is actually truncated.
The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of
Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced
a flag of same name, but without alignment restrictions. Support was added under Linux in kernel
version 2.4.10. Older Linux kernels simply ignore this flag. One may have to define the _GNU_SOURCE
macro to get its definition.
BUGS
"The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and
was probably designed by a deranged monkey on some serious mind-controlling substances." — Linus
Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling open(); use
fcntl(2) to enable this flag.
RESTRICTIONS
There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and
O_NDELAY.
POSIX provides for three different variants of synchronised I/O, corresponding to the flags O_SYNC,
O_DSYNC and O_RSYNC. Currently (2.1.130) these are all synonymous under Linux.
SEE ALSO
close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mount(2), mmap(2), openat(2), path_resolution(2),
read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), fifo(7), feature_test_macros(7)
Linux 2.6.12 2005-06-22 5CLOSE(2) Linux Programmer’s Manual CLOSE(2)
NAME
close ? close a file descriptor
SYNOPSIS
#include
int close(int fd);
DESCRIPTION
close() closes a file descriptor, so that it no longer refers to any file and may be reused. Any record locks
(see fcntl(2)) held on the file it was associated with, and owned by the process, are removed (regardless of
the file descriptor that was used to obtain the lock).
If fd is the last copy of a particular file descriptor the resources associated with it are freed; if the descriptor
was the last reference to a file which has been removed using unlink(2) the file is deleted.
RETURN VALUE
close() returns zero on success. On error, ?1 is returned, and errno is set appropriately.
ERRORS
EBADF
fd isn’tavalid open file descriptor.
EINTR
The close() call was interrupted by a signal.
EIO An I/O error occurred.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
Not checking the return value of close() is a common but nevertheless serious programming error. It is
quite possible that errors on a previous write(2) operation are first reported at the final close(). Not checking
the return value when closing the file may lead to silent loss of data. This can especially be observed
with NFS and with disk quota.
A successful close does not guarantee that the data has been successfully saved to disk, as the kernel defers
writes. It is not common for a filesystem to flush the buffers when the stream is closed. If you need to be
sure that the data is physically stored use fsync(2). (It will depend on the disk hardware at this point.)
It is probably unwise to close file descriptors while they may be in use by system calls in other threads in
the same process. Since a file descriptor may be re-used, there are some obscure race conditions that may
cause unintended side effects.
When dealing with sockets, you have to be sure that there is no recv(2) still blocking on it on another
thread, otherwise it might block forever, since no more messages will be sent via the socket. Be sure to use
shutdown(2) to shut down all parts the connection before closing the socket.
SEE ALSO
fcntl(2), fsync(2), open(2), shutdown(2), unlink(2), fclose(3)
Linux 2001-12-13 1READ(2) Linux Programmer’s Manual READ(2)
NAME
read ? read from a file descriptor
SYNOPSIS
#include
ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the
result is unspecified.
RETURN VALUE
On success, the number of bytes read is returned (zero indicates end of file), and the file position is
advanced by this number. It is not an error if this number is smaller than the number of bytes requested;
this may happen for example because fewer bytes are actually available right now (maybe because we were
close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was
interrupted by a signal. On error, ?1 is returned, and errno is set appropriately. In this case it is left unspecified
whether the file position (if any) changes.
ERRORS
EAGAIN
Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available
for reading.
EBADF
fd is not a valid file descriptor or is not open for reading.
EFAULT
buf is outside your accessible address space.
EINTR
The call was interrupted by a signal before any data was read.
EINVAL
fd is attached to an object which is unsuitable for reading; or the file was opened with the
O_DIRECT flag, and either the address specified in buf, the value specified in count, or the current
file offset is not suitably aligned.
EIO I/O error. This will happen for example when the process is in a background process group, tries to
read from its controlling tty, and either it is ignoring or blocking SIGTTIN or its process group is
orphaned. It may also occur when there is a low-level I/O error while reading from a disk or tape.
EISDIR
fd refers to a directory.
Other errors may occur, depending on the object connected to fd. POSIX allows a read() that is interrupted
after reading some data to return ?1 (with errno set to EINTR) or to return the number of bytes already
read.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
RESTRICTIONS
On NFS file systems, reading small amounts of data will only update the time stamp the first time, subsequent
calls may not do so. This is caused by client side attribute caching, because most if not all NFS
clients leave st_atime (last file access time) updates to the server and client side reads satisfied from the
client’s cache will not cause st_atime updates on the server as there are no server side reads. UNIX semantics
can be obtained by disabling client side attribute caching, but in most situations this will substantially
increase server load and decrease performance.
Linux 2.0.32 1997-07-12 1READ(2) Linux Programmer’s Manual READ(2)
Many filesystems and disks were considered to be fast enough that the implementation of O_NONBLOCK
was deemed unnecessary. So, O_NONBLOCK may not be available on files and/or disks.
SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2), readlink(2), readv(2), select(2),
write(2), fread(3)
Linux 2.0.32 1997-07-12 2WRITE(2) Linux Programmer’s Manual WRITE(2)
NAME
write write to a file descriptor
SYNOPSIS
#include
ssize_t write(int fd, const void *buf, size_t count);
DESCRIPTION
write() writes up to count bytes to the file referenced by the file descriptor fd from the buffer starting at
buf. POSIX requires that a read() which can be proved to occur after a write() has returned returns the
new data. Note that not all file systems are POSIX conforming.
RETURN VALUE
On success, the number of bytes written are returned (zero indicates nothing was written). On error, 1 is
returned, and errno is set appropriately. If count is zero and the file descriptor refers to a regular file, 0 may
be returned, or an error could be detected. Foraspecial file, the results are not portable.
ERRORS
EAGAIN
Non-blocking I/O has been selected using O_NONBLOCK and the write would block.
EBADF
fd is not a valid file descriptor or is not open for writing.
EFAULT
buf is outside your accessible address space.
EFBIG
An attempt was made to write a file that exceeds the implementation-defined maximum file size or
the process’ file size limit, or to write at a position past the maximum allowed offset.
EINTR
The call was interrupted by a signal before any data was written.
EINVAL
fd is attached to an object which is unsuitable for writing; or the file was opened with the
O_DIRECT flag, and either the address specified in buf, the value specified in count, or the current
file offset is not suitably aligned.
EIO A low-level I/O error occurred while modifying the inode.
ENOSPC
The device containing the file referred to by fd has no room for the data.
EPIPE fd is connected to a pipe or socket whose reading end is closed. When this happens the writing
process will also receive a SIGPIPE signal. (Thus, the write return value is seen only if the program
catches, blocks or ignores this signal.)
Other errors may occur, depending on the object connected to fd.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.
NOTES
A successful return from write() does not make any guarantee that data has been committed to disk. In
fact, on some buggy implementations, it does not even guarantee that space has successfully been reserved
for the data. The only way to be sure is to call fsync(2) after you are done writing all your data.
SEE ALSO
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(3), fwrite(3)
Linux 2.0.32 2001-12-13 1STAT(2) Linux Programmer’s Manual STAT(2)
NAME
stat, fstat, lstat ? get file status
SYNOPSIS
#include
#include
#include
int stat(const char * path, struct stat *buf );
int fstat(int filedes, struct stat *buf );
int lstat(const char *path, struct stat *buf );
DESCRIPTION
These functions return information about a file. No permissions are required on the file itself, but — in the
case of stat() and lstat() — execute (search) permission is required on all of the directories in path that
lead to the file.
stat() stats the file pointed to by path and fills in buf.
lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file
that it refers to.
fstat() is identical to stat(), except that the file to be stat-ed is specified by the file descriptor filedes.
All of these system calls return a stat structure, which contains the following fields:
struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for filesystem I/O */
blkcnt_t st_blocks; /* number of blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */
};
The st_dev field describes the device on which this file resides.
The st_rdev field describes the device that this file (inode) represents.
The st_size field gives the size of the file (if it is a regular file or a symbolic link) in bytes. The size of a
symlink is the length of the pathname it contains, without a trailing null byte.
The st_blocks field indicates the number of blocks allocated to the file, 512-byte units. (This may be
smaller than st_size/512, for example, when the file has holes.)
The st_blksize field gives the "preferred" blocksize for efficient file system I/O. (Writing to a file in smaller
chunks may cause an inefficient read-modify-rewrite.)
Not all of the Linux filesystems implement all of the time fields. Some file system types allow mounting in
such a way that file accesses do not cause an update of the st_atime field. (See ‘noatime’ in mount(8).)
The field st_atime is changed by file accesses, e.g. by execve(2), mknod(2), pipe(2), utime(2) and read(2)
Linux 2.6.7 2004-06-23 1STAT(2) Linux Programmer’s Manual STAT(2)
(of more than zero bytes). Other routines, like mmap(2), may or may not update st_atime.
The field st_mtime is changed by file modifications, e.g. by mknod(2), truncate(2), utime(2) and write(2)
(of more than zero bytes). Moreover, st_mtime of a directory is changed by the creation or deletion of files
in that directory. The st_mtime field is not changed for changes in owner, group, hard link count, or mode.
The field st_ctime is changed by writing or by setting inode information (i.e., owner, group, link count,
mode, etc.).
The following POSIX macros are defined to check the file type using the st_mode field:
S_ISREG(m) is it a regular file?
S_ISDIR(m) directory?
S_ISCHR(m) character device?
S_ISBLK(m) block device?
S_ISFIFO(m) FIFO (named pipe)?
S_ISLNK(m) symbolic link? (Not in POSIX.1-1996.)
S_ISSOCK(m) socket? (Not in POSIX.1-1996.)
The following flags are defined for the st_mode field:
S_IFMT 0170000 bitmask for the file type bitfields
S_IFSOCK 0140000 socket
S_IFLNK 0120000 symbolic link
S_IFREG 0100000 regular file
S_IFBLK 0060000 block device
S_IFDIR 0040000 directory
S_IFCHR 0020000 character device
S_IFIFO 0010000 FIFO
S_ISUID 0004000 set UID bit
S_ISGID 0002000 set-group-ID bit (see below)
S_ISVTX 0001000 sticky bit (see below)
S_IRWXU 00700 mask for file owner permissions
S_IRUSR 00400 owner has read permission
S_IWUSR 00200 owner has write permission
S_IXUSR 00100 owner has execute permission
S_IRWXG 00070 mask for group permissions
S_IRGRP 00040 group has read permission
S_IWGRP 00020 group has write permission
S_IXGRP 00010 group has execute permission
S_IRWXO 00007 mask for permissions for others (not in group)
S_IROTH 00004 others have read permission
S_IWOTH 00002 others have write permission
S_IXOTH 00001 others have execute permission
The set-group-ID bit (S_ISGID) has several special uses. Foradirectory it indicates that BSD semantics is
to be used for that directory: files created there inherit their group ID from the directory, not from the effective
group ID of the creating process, and directories created there will also get the S_ISGID bit set. For a
file that does not have the group execution bit (S_IXGRP) set, the set-group-ID bit indicates mandatory
file/record locking.
The ‘sticky’ bit (S_ISVTX) on a directory means that a file in that directory can be renamed or deleted only
by the owner of the file, by the owner of the directory, and by a privileged process.
Linux 2.6.7 2004-06-23 2STAT(2) Linux Programmer’s Manual STAT(2)
LINUX NOTES
Since kernel 2.5.48, the stat structure supports nanosecond resolution for the three file timestamp fields.
Glibc exposes the nanosecond component of each field using names either of the form st_atim.tv_nsec, if
the _BSD_SOURCE or _SVID_SOURCE feature test macro is defined, or of the form st_atimensec, if neither
of these macros is defined. On file systems that do not support sub-second timestamps, these nanosecond
fields are returned with the value 0.
For most files under the /proc directory, stat() does not return the file size in the st_size field; instead the
field is returned with the value 0.
RETURN VALUE
On success, zero is returned. On error, ?1 is returned, and errno is set appropriately.
ERRORS
EACCES
Search permission is denied for one of the directories in the path prefix of path. (See also
path_resolution(2).)
EBADF
filedes is bad.
EFAULT
Bad address.
ELOOP
Too many symbolic links encountered while traversing the path.
ENAMETOOLONG
File name too long.
ENOENT
A component of the path path does not exist, or the path is an empty string.
ENOMEM
Out of memory (i.e. kernel memory).
ENOTDIR
A component of the path is not a directory.
CONFORMING TO
These system calls conform to SVr4, 4.3BSD, POSIX.1-2001.
Use of the st_blocks and st_blksize fields may be less portable. (They were introduced in BSD. The interpretation
differs between systems, and possibly on a single system when NFS mounts are involved.)
POSIX does not describe the S_IFMT, S_IFSOCK, S_IFLNK, S_IFREG, S_IFBLK, S_IFDIR, S_IFCHR,
S_IFIFO, S_ISVTX bits, but instead demands the use of the macros S_ISDIR(), etc. The S_ISLNK and
S_ISSOCK macros are not in POSIX.1-1996, but both are present in POSIX.1-2001; the former is from
SVID 4, the latter from SUSv2.
Unix V7 (and later systems) had S_IREAD, S_IWRITE, S_IEXEC, where POSIX prescribes the synonyms
S_IRUSR, S_IWUSR, S_IXUSR.
OTHER SYSTEMS
Values that have been (or are) in use on various systems:
hex name ls octal description
f000 S_IFMT 170000 mask for file type
0000 000000 SCO out-of-service inode, BSD unknown type
SVID-v2 and XPG2 have both 0 and 0100000 for ordinary file
1000 S_IFIFO p| 010000 FIFO (named pipe)
Linux 2.6.7 2004-06-23 3STAT(2) Linux Programmer’s Manual STAT(2)
2000 S_IFCHR c 020000 character special (V7)
3000 S_IFMPC 030000 multiplexed character special (V7)
4000 S_IFDIR d/ 040000 directory (V7)
5000 S_IFNAM 050000 XENIX named special file
with two subtypes, distinguished by st_rdev values 1, 2:
0001 S_INSEM s 000001 XENIX semaphore subtype of IFNAM
0002 S_INSHD m 000002 XENIX shared data subtype of IFNAM
6000 S_IFBLK b 060000 block special (V7)
7000 S_IFMPB 070000 multiplexed block special (V7)
8000 S_IFREG - 100000 regular (V7)
9000 S_IFCMP 110000 VxFS compressed
9000 S_IFNWK n 110000 network special (HP-UX)
a000 S_IFLNK l@ 120000 symbolic link (BSD)
b000 S_IFSHAD 130000 Solaris shadow inode for ACL (not
Operating Systems Programming – 300698
1 Introduction
In this workshop you will investigate file I/O and file copy operations.
2 Specification
Following on from the demonstration program cp1.c presented in the lecture, we will make a series of modifi-
cations to the program.
Firstly, what happens when the cp1.c program is asked to copy a file onto itself, i.e. cp1 input input?
Is this what you expect? Modify the program to do something more sensible! As a hint, two files are the same if
they are on the same device and have the same i-node number (which stat() can give you), simply comparing
the names is not enough.
Secondly, a real copy program will assign the same file permissions to the destination as were on the source,
modify your answer to the last part to do this.
Thirdly, real copy programs allow the second argument to be a directory, modify the answer to the last part
to include this functionality. You should allocate the space for the new name dynamically.
13 Sample Code
3.1 cp1.c
#include
#include
#include
#define BUFFERSIZE 4096
#define COPYMODE 0644
void oops(char *, char *);
main(int ac, char *av[])
{
int in_fd, out_fd, n_chars;
char buf[BUFFERSIZE];
if ( ac != 3 ){
fprintf( stderr, "usage: %s source destination\n", *av);
exit(1);
}
if ( (in_fd=open(av[1], O_RDONLY)) == -1 )
oops("Cannot open ", av[1]);
if ( (out_fd=creat( av[2], COPYMODE)) == -1 )
oops( "Cannot creat", av[2]);
while ( (n_chars = read(in_fd , buf, BUFFERSIZE)) > 0 )
if ( write( out_fd, buf, n_chars ) != n_chars )
oops("Write error to ", av[2]);
if ( n_chars == -1 )
oops("Read error from ", av[1]);
if ( close(in_fd) == -1 || close(out_fd) == -1 )
oops("Error closing files","");
}
void oops(char *s1, char *s2)
{
fprintf(stderr,"Error: %s ", s1);
perror(s2);
exit(1);
}
24 Supplementary Materials
The material on the following pages is an extract of the linux system documentation and may prove useful in
implementing this Workshop. These manual pages are taken from the Linux man-pages Project available at:
http://www.kernel.org/doc/man-pages/.
3OPEN(2) Linux Programmer’s Manual OPEN(2)
NAME
open, creat ? open and possibly create a file or device
SYNOPSIS
#include
#include
#include
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
int creat(const char * pathname, mode_t mode);
DESCRIPTION
Given a pathname for a file, open() returns a file descriptor,asmall, non-negative integer for use in subsequent
system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful
call will be the lowest-numbered file descriptor not currently open for the process.
The new file descriptor is set to remain open across an execve(2) (i.e., the FD_CLOEXEC file descriptor
flag described in fcntl(2) is initially disabled). The file offset is set to the beginning of the file (see
lseek(2)).
A call to open() creates a new open file description, an entry in the system-wide table of open files. This
entry records the file offset and the file status flags (modifiable via the fcntl() F_SETFL operation). A file
descriptor is a reference to one of these entries; this reference is unaffected if pathname is subsequently
removed or modified to refer to a different file. The new open file description is initially not shared with
any other process, but sharing may arise via fork(2).
The parameter flags must include one of the following access modes: O_RDONLY, O_WRONLY, or
O_RDWR. These request opening the file read-only, write-only, or read/write, respectively.
In addition, zero or more file creation flags and file status flags can be bitwise-or’d in flags. The file creation
flags are O_CREAT, O_EXCL, O_NOCTTY, and O_TRUNC. The file status flags are all of the
remaining flags listed below. The distinction between these two groups of flags is that the file status flags
can be retrieved and (in some cases) modified using fcntl(2). The full list of file creation flags and file status
flags is as follows:
O_APPEND
The file is opened in append mode. Before each write(), the file offset is positioned at the end of
the file, as if with lseek(). O_APPEND may lead to corrupted files on NFS file systems if more
than one process appends data to a file at once. This is because NFS does not support appending
to a file, so the client kernel has to simulate it, which can’t be done without a race condition.
O_ASYNC
Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via
fcntl(2)) when input or output becomes possible on this file descriptor. This feature is only available
for terminals, pseudo-terminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2)
for further details.
O_CREAT
If the file does not exist it will be created. The owner (user ID) of the file is set to the effective
user ID of the process. The group ownership (group ID) is set either to the effective group ID of
the process or to the group ID of the parent directory (depending on filesystem type and mount
options, and the mode of the parent directory, see, e.g., the mount options bsdgroups and sysvgroups
of the ext2 filesystem, as described in mount(8)).
O_DIRECT
Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance,
but it is useful in special situations, such as when applications do their own caching. File
I/O is done directly to/from user space buffers. The I/O is synchronous, i.e., at the completion of a
read(2) or write(2), data is guaranteed to have been transferred. Under Linux 2.4 transfer sizes,
Linux 2.6.12 2005-06-22 1OPEN(2) Linux Programmer’s Manual OPEN(2)
and the alignment of user buffer and file offset must all be multiples of the logical block size of the
file system. Under Linux 2.6 alignment must fit the block size of the device.
A semantically similar (but deprecated) interface for block devices is described in raw(8).
O_DIRECTORY
If pathname is not a directory, cause the open to fail. This flag is Linux-specific, and was added in
kernel version 2.1.126, to avoid denial-of-service problems if opendir(3) is called on a FIFO or
tape device, but should not be used outside of the implementation of opendir.
O_EXCL
When used with O_CREAT, if the file already exists it is an error and the open() will fail. In this
context, a symbolic link exists, regardless of where it points to. O_EXCL is broken on NFS file
systems; programs which rely on it for performing locking tasks will contain a race condition.
The solution for performing atomic file locking using a lockfile is to create a unique file on the
same file system (e.g., incorporating hostname and pid), use link(2) to makealink to the lockfile.
If link() returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if its
link count has increased to 2, in which case the lock is also successful.
O_LARGEFILE
(LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an
off64_t) to be opened.
O_NOATIME
(Since Linux 2.6.8) Do not update the file last access time (st_atime in the inode) when the file is
read(2). This flag is intended for use by indexing or backup programs, where its use can signifi-
cantly reduce the amount of disk activity. This flag may not be effective on all filesystems. One
example is NFS, where the server maintains the access time.
O_NOCTTY
If pathname refers to a terminal device — see tty(4) — it will not become the process’s controlling
terminal even if the process does not have one.
O_NOFOLLOW
If pathname is a symbolic link, then the open fails. This is a FreeBSD extension, which was added
to Linux in version 2.1.126. Symbolic links in earlier components of the pathname will still be
followed.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in non-blocking mode. Neither the open() nor any subsequent
operations on the file descriptor which is returned will cause the calling process to wait. For the
handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK
in conjunction with mandatory file locks and with file leases, see fcntl(2).
O_SYNC
The file is opened for synchronous I/O. Any write()s on the resulting file descriptor will block the
calling process until the data has been physically written to the underlying hardware. But see
RESTRICTIONS below.
O_TRUNC
If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or
O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the
O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified.
Some of these optional flags can be altered using fcntl() after the file has been opened.
The argument mode specifies the permissions to use in case a new file is created. It is modified by the process’s
umask in the usual way: the permissions of the created file are (mode & ?umask). Note that this
mode only applies to future accesses of the newly created file; the open() call that creates a read-only file
Linux 2.6.12 2005-06-22 2OPEN(2) Linux Programmer’s Manual OPEN(2)
may well return a read/write file descriptor.
The following symbolic constants are provided for mode:
S_IRWXU
00700 user (file owner) has read, write and execute permission
S_IRUSR
00400 user has read permission
S_IWUSR
00200 user has write permission
S_IXUSR
00100 user has execute permission
S_IRWXG
00070 group has read, write and execute permission
S_IRGRP
00040 group has read permission
S_IWGRP
00020 group has write permission
S_IXGRP
00010 group has execute permission
S_IRWXO
00007 others have read, write and execute permission
S_IROTH
00004 others have read permission
S_IWOTH
00002 others have write permission
S_IXOTH
00001 others have execute permission
mode must be specified when O_CREAT is in the flags, and is ignored otherwise.
creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
RETURN VALUE
open() and creat() return the new file descriptor, or ?1 if an error occurred (in which case, errno is set
appropriately).
NOTES
Note that open() can open device special files, but creat() cannot create them; use mknod(2) instead.
On NFS file systems with UID mapping enabled, open() may return a file descriptor but e.g. read(2)
requests are denied with EACCES. This is because the client performs open() by checking the permissions,
but UID mapping is performed by the server upon read and write requests.
If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of
last status change, and time of last modification; see stat(2)) are set to the current time, and so are the
st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the
O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time.
ERRORS
EACCES
The requested access to the file is not allowed, or search permission is denied for one of the directories
in the path prefix of pathname, or the file did not exist yet and write access to the parent
directory is not allowed. (See also path_resolution(2).)
Linux 2.6.12 2005-06-22 3OPEN(2) Linux Programmer’s Manual OPEN(2)
EEXIST
pathname already exists and O_CREAT and O_EXCL were used.
EFAULT
pathname points outside your accessible address space.
EISDIR
pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or
O_RDWR is set).
ELOOP
Too many symbolic links were encountered in resolving pathname, or O_NOFOLLOW was
specified but pathname wasasymbolic link.
EMFILE
The process already has the maximum number of files open.
ENAMETOOLONG
pathname was too long.
ENFILE
The system limit on the total number of open files has been reached.
ENODEV
pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel
bug; in this situation ENXIO must be returned.)
ENOENT
O_CREAT is not set and the named file does not exist. Or,adirectory component in pathname
does not exist or is a dangling symbolic link.
ENOMEM
Insufficient kernel memory was available.
ENOSPC
pathname was to be created but the device containing pathname has no room for the new file.
ENOTDIR
A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was
specified and pathname was not a directory.
ENXIO
O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the file open
for reading. Or, the file is a device special file and no corresponding device exists.
EOVERFLOW
pathname refers to a regular file, too large to be opened; see O_LARGEFILE above.
EPERM
The O_NOATIME flag was specified, but the effective user ID of the caller did not match the
owner of the file and the caller was not privileged (CAP_FOWNER).
EROFS
pathname refers to a file on a read-only filesystem and write access was requested.
ETXTBSY
pathname refers to an executable image which is currently being executed and write access was
requested.
EWOULDBLOCK
The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see
fcntl(2)).
Linux 2.6.12 2005-06-22 4OPEN(2) Linux Programmer’s Manual OPEN(2)
NOTE
Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily have the
intention to read or write. This is typically used to open devices in order to get a file descriptor for use with
ioctl(2).
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. The O_NOATIME, O_NOFOLLOW, and O_DIRECTORY flags are
Linux-specific. One may have to define the _GNU_SOURCE macro to get their definitions.
The (undefined) effect of O_RDONLY|O_TRUNC varies among implementations. On many systems the
file is actually truncated.
The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of
Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced
a flag of same name, but without alignment restrictions. Support was added under Linux in kernel
version 2.4.10. Older Linux kernels simply ignore this flag. One may have to define the _GNU_SOURCE
macro to get its definition.
BUGS
"The thing that has always disturbed me about O_DIRECT is that the whole interface is just stupid, and
was probably designed by a deranged monkey on some serious mind-controlling substances." — Linus
Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling open(); use
fcntl(2) to enable this flag.
RESTRICTIONS
There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and
O_NDELAY.
POSIX provides for three different variants of synchronised I/O, corresponding to the flags O_SYNC,
O_DSYNC and O_RSYNC. Currently (2.1.130) these are all synonymous under Linux.
SEE ALSO
close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mount(2), mmap(2), openat(2), path_resolution(2),
read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), fifo(7), feature_test_macros(7)
Linux 2.6.12 2005-06-22 5CLOSE(2) Linux Programmer’s Manual CLOSE(2)
NAME
close ? close a file descriptor
SYNOPSIS
#include
int close(int fd);
DESCRIPTION
close() closes a file descriptor, so that it no longer refers to any file and may be reused. Any record locks
(see fcntl(2)) held on the file it was associated with, and owned by the process, are removed (regardless of
the file descriptor that was used to obtain the lock).
If fd is the last copy of a particular file descriptor the resources associated with it are freed; if the descriptor
was the last reference to a file which has been removed using unlink(2) the file is deleted.
RETURN VALUE
close() returns zero on success. On error, ?1 is returned, and errno is set appropriately.
ERRORS
EBADF
fd isn’tavalid open file descriptor.
EINTR
The close() call was interrupted by a signal.
EIO An I/O error occurred.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
Not checking the return value of close() is a common but nevertheless serious programming error. It is
quite possible that errors on a previous write(2) operation are first reported at the final close(). Not checking
the return value when closing the file may lead to silent loss of data. This can especially be observed
with NFS and with disk quota.
A successful close does not guarantee that the data has been successfully saved to disk, as the kernel defers
writes. It is not common for a filesystem to flush the buffers when the stream is closed. If you need to be
sure that the data is physically stored use fsync(2). (It will depend on the disk hardware at this point.)
It is probably unwise to close file descriptors while they may be in use by system calls in other threads in
the same process. Since a file descriptor may be re-used, there are some obscure race conditions that may
cause unintended side effects.
When dealing with sockets, you have to be sure that there is no recv(2) still blocking on it on another
thread, otherwise it might block forever, since no more messages will be sent via the socket. Be sure to use
shutdown(2) to shut down all parts the connection before closing the socket.
SEE ALSO
fcntl(2), fsync(2), open(2), shutdown(2), unlink(2), fclose(3)
Linux 2001-12-13 1READ(2) Linux Programmer’s Manual READ(2)
NAME
read ? read from a file descriptor
SYNOPSIS
#include
ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the
result is unspecified.
RETURN VALUE
On success, the number of bytes read is returned (zero indicates end of file), and the file position is
advanced by this number. It is not an error if this number is smaller than the number of bytes requested;
this may happen for example because fewer bytes are actually available right now (maybe because we were
close to end-of-file, or because we are reading from a pipe, or from a terminal), or because read() was
interrupted by a signal. On error, ?1 is returned, and errno is set appropriately. In this case it is left unspecified
whether the file position (if any) changes.
ERRORS
EAGAIN
Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available
for reading.
EBADF
fd is not a valid file descriptor or is not open for reading.
EFAULT
buf is outside your accessible address space.
EINTR
The call was interrupted by a signal before any data was read.
EINVAL
fd is attached to an object which is unsuitable for reading; or the file was opened with the
O_DIRECT flag, and either the address specified in buf, the value specified in count, or the current
file offset is not suitably aligned.
EIO I/O error. This will happen for example when the process is in a background process group, tries to
read from its controlling tty, and either it is ignoring or blocking SIGTTIN or its process group is
orphaned. It may also occur when there is a low-level I/O error while reading from a disk or tape.
EISDIR
fd refers to a directory.
Other errors may occur, depending on the object connected to fd. POSIX allows a read() that is interrupted
after reading some data to return ?1 (with errno set to EINTR) or to return the number of bytes already
read.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
RESTRICTIONS
On NFS file systems, reading small amounts of data will only update the time stamp the first time, subsequent
calls may not do so. This is caused by client side attribute caching, because most if not all NFS
clients leave st_atime (last file access time) updates to the server and client side reads satisfied from the
client’s cache will not cause st_atime updates on the server as there are no server side reads. UNIX semantics
can be obtained by disabling client side attribute caching, but in most situations this will substantially
increase server load and decrease performance.
Linux 2.0.32 1997-07-12 1READ(2) Linux Programmer’s Manual READ(2)
Many filesystems and disks were considered to be fast enough that the implementation of O_NONBLOCK
was deemed unnecessary. So, O_NONBLOCK may not be available on files and/or disks.
SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2), readlink(2), readv(2), select(2),
write(2), fread(3)
Linux 2.0.32 1997-07-12 2WRITE(2) Linux Programmer’s Manual WRITE(2)
NAME
write write to a file descriptor
SYNOPSIS
#include
ssize_t write(int fd, const void *buf, size_t count);
DESCRIPTION
write() writes up to count bytes to the file referenced by the file descriptor fd from the buffer starting at
buf. POSIX requires that a read() which can be proved to occur after a write() has returned returns the
new data. Note that not all file systems are POSIX conforming.
RETURN VALUE
On success, the number of bytes written are returned (zero indicates nothing was written). On error, 1 is
returned, and errno is set appropriately. If count is zero and the file descriptor refers to a regular file, 0 may
be returned, or an error could be detected. Foraspecial file, the results are not portable.
ERRORS
EAGAIN
Non-blocking I/O has been selected using O_NONBLOCK and the write would block.
EBADF
fd is not a valid file descriptor or is not open for writing.
EFAULT
buf is outside your accessible address space.
EFBIG
An attempt was made to write a file that exceeds the implementation-defined maximum file size or
the process’ file size limit, or to write at a position past the maximum allowed offset.
EINTR
The call was interrupted by a signal before any data was written.
EINVAL
fd is attached to an object which is unsuitable for writing; or the file was opened with the
O_DIRECT flag, and either the address specified in buf, the value specified in count, or the current
file offset is not suitably aligned.
EIO A low-level I/O error occurred while modifying the inode.
ENOSPC
The device containing the file referred to by fd has no room for the data.
EPIPE fd is connected to a pipe or socket whose reading end is closed. When this happens the writing
process will also receive a SIGPIPE signal. (Thus, the write return value is seen only if the program
catches, blocks or ignores this signal.)
Other errors may occur, depending on the object connected to fd.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.
NOTES
A successful return from write() does not make any guarantee that data has been committed to disk. In
fact, on some buggy implementations, it does not even guarantee that space has successfully been reserved
for the data. The only way to be sure is to call fsync(2) after you are done writing all your data.
SEE ALSO
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(3), fwrite(3)
Linux 2.0.32 2001-12-13 1STAT(2) Linux Programmer’s Manual STAT(2)
NAME
stat, fstat, lstat ? get file status
SYNOPSIS
#include
#include
#include
int stat(const char * path, struct stat *buf );
int fstat(int filedes, struct stat *buf );
int lstat(const char *path, struct stat *buf );
DESCRIPTION
These functions return information about a file. No permissions are required on the file itself, but — in the
case of stat() and lstat() — execute (search) permission is required on all of the directories in path that
lead to the file.
stat() stats the file pointed to by path and fills in buf.
lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file
that it refers to.
fstat() is identical to stat(), except that the file to be stat-ed is specified by the file descriptor filedes.
All of these system calls return a stat structure, which contains the following fields:
struct stat {
dev_t st_dev; /* ID of device containing file */
ino_t st_ino; /* inode number */
mode_t st_mode; /* protection */
nlink_t st_nlink; /* number of hard links */
uid_t st_uid; /* user ID of owner */
gid_t st_gid; /* group ID of owner */
dev_t st_rdev; /* device ID (if special file) */
off_t st_size; /* total size, in bytes */
blksize_t st_blksize; /* blocksize for filesystem I/O */
blkcnt_t st_blocks; /* number of blocks allocated */
time_t st_atime; /* time of last access */
time_t st_mtime; /* time of last modification */
time_t st_ctime; /* time of last status change */
};
The st_dev field describes the device on which this file resides.
The st_rdev field describes the device that this file (inode) represents.
The st_size field gives the size of the file (if it is a regular file or a symbolic link) in bytes. The size of a
symlink is the length of the pathname it contains, without a trailing null byte.
The st_blocks field indicates the number of blocks allocated to the file, 512-byte units. (This may be
smaller than st_size/512, for example, when the file has holes.)
The st_blksize field gives the "preferred" blocksize for efficient file system I/O. (Writing to a file in smaller
chunks may cause an inefficient read-modify-rewrite.)
Not all of the Linux filesystems implement all of the time fields. Some file system types allow mounting in
such a way that file accesses do not cause an update of the st_atime field. (See ‘noatime’ in mount(8).)
The field st_atime is changed by file accesses, e.g. by execve(2), mknod(2), pipe(2), utime(2) and read(2)
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(of more than zero bytes). Other routines, like mmap(2), may or may not update st_atime.
The field st_mtime is changed by file modifications, e.g. by mknod(2), truncate(2), utime(2) and write(2)
(of more than zero bytes). Moreover, st_mtime of a directory is changed by the creation or deletion of files
in that directory. The st_mtime field is not changed for changes in owner, group, hard link count, or mode.
The field st_ctime is changed by writing or by setting inode information (i.e., owner, group, link count,
mode, etc.).
The following POSIX macros are defined to check the file type using the st_mode field:
S_ISREG(m) is it a regular file?
S_ISDIR(m) directory?
S_ISCHR(m) character device?
S_ISBLK(m) block device?
S_ISFIFO(m) FIFO (named pipe)?
S_ISLNK(m) symbolic link? (Not in POSIX.1-1996.)
S_ISSOCK(m) socket? (Not in POSIX.1-1996.)
The following flags are defined for the st_mode field:
S_IFMT 0170000 bitmask for the file type bitfields
S_IFSOCK 0140000 socket
S_IFLNK 0120000 symbolic link
S_IFREG 0100000 regular file
S_IFBLK 0060000 block device
S_IFDIR 0040000 directory
S_IFCHR 0020000 character device
S_IFIFO 0010000 FIFO
S_ISUID 0004000 set UID bit
S_ISGID 0002000 set-group-ID bit (see below)
S_ISVTX 0001000 sticky bit (see below)
S_IRWXU 00700 mask for file owner permissions
S_IRUSR 00400 owner has read permission
S_IWUSR 00200 owner has write permission
S_IXUSR 00100 owner has execute permission
S_IRWXG 00070 mask for group permissions
S_IRGRP 00040 group has read permission
S_IWGRP 00020 group has write permission
S_IXGRP 00010 group has execute permission
S_IRWXO 00007 mask for permissions for others (not in group)
S_IROTH 00004 others have read permission
S_IWOTH 00002 others have write permission
S_IXOTH 00001 others have execute permission
The set-group-ID bit (S_ISGID) has several special uses. Foradirectory it indicates that BSD semantics is
to be used for that directory: files created there inherit their group ID from the directory, not from the effective
group ID of the creating process, and directories created there will also get the S_ISGID bit set. For a
file that does not have the group execution bit (S_IXGRP) set, the set-group-ID bit indicates mandatory
file/record locking.
The ‘sticky’ bit (S_ISVTX) on a directory means that a file in that directory can be renamed or deleted only
by the owner of the file, by the owner of the directory, and by a privileged process.
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LINUX NOTES
Since kernel 2.5.48, the stat structure supports nanosecond resolution for the three file timestamp fields.
Glibc exposes the nanosecond component of each field using names either of the form st_atim.tv_nsec, if
the _BSD_SOURCE or _SVID_SOURCE feature test macro is defined, or of the form st_atimensec, if neither
of these macros is defined. On file systems that do not support sub-second timestamps, these nanosecond
fields are returned with the value 0.
For most files under the /proc directory, stat() does not return the file size in the st_size field; instead the
field is returned with the value 0.
RETURN VALUE
On success, zero is returned. On error, ?1 is returned, and errno is set appropriately.
ERRORS
EACCES
Search permission is denied for one of the directories in the path prefix of path. (See also
path_resolution(2).)
EBADF
filedes is bad.
EFAULT
Bad address.
ELOOP
Too many symbolic links encountered while traversing the path.
ENAMETOOLONG
File name too long.
ENOENT
A component of the path path does not exist, or the path is an empty string.
ENOMEM
Out of memory (i.e. kernel memory).
ENOTDIR
A component of the path is not a directory.
CONFORMING TO
These system calls conform to SVr4, 4.3BSD, POSIX.1-2001.
Use of the st_blocks and st_blksize fields may be less portable. (They were introduced in BSD. The interpretation
differs between systems, and possibly on a single system when NFS mounts are involved.)
POSIX does not describe the S_IFMT, S_IFSOCK, S_IFLNK, S_IFREG, S_IFBLK, S_IFDIR, S_IFCHR,
S_IFIFO, S_ISVTX bits, but instead demands the use of the macros S_ISDIR(), etc. The S_ISLNK and
S_ISSOCK macros are not in POSIX.1-1996, but both are present in POSIX.1-2001; the former is from
SVID 4, the latter from SUSv2.
Unix V7 (and later systems) had S_IREAD, S_IWRITE, S_IEXEC, where POSIX prescribes the synonyms
S_IRUSR, S_IWUSR, S_IXUSR.
OTHER SYSTEMS
Values that have been (or are) in use on various systems:
hex name ls octal description
f000 S_IFMT 170000 mask for file type
0000 000000 SCO out-of-service inode, BSD unknown type
SVID-v2 and XPG2 have both 0 and 0100000 for ordinary file
1000 S_IFIFO p| 010000 FIFO (named pipe)
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2000 S_IFCHR c 020000 character special (V7)
3000 S_IFMPC 030000 multiplexed character special (V7)
4000 S_IFDIR d/ 040000 directory (V7)
5000 S_IFNAM 050000 XENIX named special file
with two subtypes, distinguished by st_rdev values 1, 2:
0001 S_INSEM s 000001 XENIX semaphore subtype of IFNAM
0002 S_INSHD m 000002 XENIX shared data subtype of IFNAM
6000 S_IFBLK b 060000 block special (V7)
7000 S_IFMPB 070000 multiplexed block special (V7)
8000 S_IFREG - 100000 regular (V7)
9000 S_IFCMP 110000 VxFS compressed
9000 S_IFNWK n 110000 network special (HP-UX)
a000 S_IFLNK l@ 120000 symbolic link (BSD)
b000 S_IFSHAD 130000 Solaris shadow inode for ACL (not