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This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions described in section Input/Output on Streams and section Low-Level Input/Output, these functions are concerned with operating on the files themselves, rather than on their contents.
Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times.
Each process has associated with it a directory, called its current working directory or simply working directory, that is used in the resolution of relative file names (see section File Name Resolution).
When you log in and begin a new session, your working directory is
initially set to the home directory associated with your login account
in the system user database. You can find any user's home directory
using the getpwuid or getpwnam functions; see section User Database.
Users can change the working directory using shell commands like
cd. The functions described in this section are the primitives
used by those commands and by other programs for examining and changing
the working directory.
Prototypes for these functions are declared in the header file `unistd.h'.
Function: char * getcwd (char *buffer, size_t size)
The getcwd function returns an absolute file name representing
the current working directory, storing it in the character array
buffer that you provide. The size argument is how you tell
the system the allocation size of buffer.
The GNU library version of this function also permits you to specify a
null pointer for the buffer argument. Then getcwd
allocates a buffer automatically, as with malloc
(see section Unconstrained Allocation). If the size is greater than
zero, then the buffer is that large; otherwise, the buffer is as large
as necessary to hold the result.
The return value is buffer on success and a null pointer on failure.
The following errno error conditions are defined for this function:
EINVAL
ERANGE
EACCES
Here is an example showing how you could implement the behavior of GNU's
getcwd (NULL, 0) using only the standard behavior of
getcwd:
char *
gnu_getcwd ()
{
int size = 100;
char *buffer = (char *) xmalloc (size);
while (1)
{
char *value = getcwd (buffer, size);
if (value != 0)
return buffer;
size *= 2;
free (buffer);
buffer = (char *) xmalloc (size);
}
}
See section Examples of malloc, for information about xmalloc, which is
not a library function but is a customary name used in most GNU
software.
Function: char * getwd (char *buffer)
This is similar to getcwd. The GNU library provides getwd
for backwards compatibility with BSD. The buffer should be a
pointer to an array at least PATH_MAX bytes long.
Function: int chdir (const char *filename)
This function is used to set the process's working directory to filename.
The normal, successful return value from chdir is 0. A
value of -1 is returned to indicate an error. The errno
error conditions defined for this function are the usual file name
syntax errors (see section File Name Errors), plus ENOTDIR if the
file filename is not a directory.
The facilities described in this section let you read the contents of a directory file. This is useful if you want your program to list all the files in a directory, perhaps as part of a menu.
The opendir function opens a directory stream whose
elements are directory entries. You use the readdir function on
the directory stream to retrieve these entries, represented as
struct dirent objects. The name of the file for each entry is
stored in the d_name member of this structure. There are obvious
parallels here to the stream facilities for ordinary files, described in
section Input/Output on Streams.
This section describes what you find in a single directory entry, as you might obtain it from a directory stream. All the symbols are declared in the header file `dirent.h'.
This is a structure type used to return information about directory entries. It contains the following fields:
char *d_name
ino_t d_fileno
d_ino.
size_t d_namlen
This structure may contain additional members in the future.
When a file has multiple names, each name has its own directory entry.
The only way you can tell that the directory entries belong to a
single file is that they have the same value for the d_fileno
field.
File attributes such as size, modification times, and the like are part of the file itself, not any particular directory entry. See section File Attributes.
This section describes how to open a directory stream. All the symbols are declared in the header file `dirent.h'.
The DIR data type represents a directory stream.
You shouldn't ever allocate objects of the struct dirent or
DIR data types, since the directory access functions do that for
you. Instead, you refer to these objects using the pointers returned by
the following functions.
Function: DIR * opendir (const char *dirname)
The opendir function opens and returns a directory stream for
reading the directory whose file name is dirname. The stream has
type DIR *.
If unsuccessful, opendir returns a null pointer. In addition to
the usual file name syntax errors (see section File Name Errors), the
following errno error conditions are defined for this function:
EACCES
dirname.
EMFILE
ENFILE
The DIR type is typically implemented using a file descriptor,
and the opendir function in terms of the open function.
See section Low-Level Input/Output. Directory streams and the underlying
file descriptors are closed on exec (see section Executing a File).
This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it. All the symbols are declared in the header file `dirent.h'.
Function: struct dirent * readdir (DIR *dirstream)
This function reads the next entry from the directory. It normally returns a pointer to a structure containing information about the file. This structure is statically allocated and can be rewritten by a subsequent call.
Portability Note: On some systems, readdir may not
return entries for `.' and `..'. See section File Name Resolution.
If there are no more entries in the directory or an error is detected,
readdir returns a null pointer. The following errno error
conditions are defined for this function:
EBADF
Function: int closedir (DIR *dirstream)
This function closes the directory stream dirstream. It returns
0 on success and -1 on failure.
The following errno error conditions are defined for this
function:
EBADF
Here's a simple program that prints the names of the files in the current working directory:
#include <stddef.h>
#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
int
main (void)
{
DIR *dp;
struct dirent *ep;
dp = opendir ("./");
if (dp != NULL)
{
while (ep = readdir (dp))
puts (ep->d_name);
(void) closedir (dp);
}
else
puts ("Couldn't open the directory.");
return 0;
}
The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see section Array Sort Function
This section describes how to reread parts of a directory that you have already read from an open directory stream. All the symbols are declared in the header file `dirent.h'.
Function: void rewinddir (DIR *dirstream)
The rewinddir function is used to reinitialize the directory
stream dirstream, so that if you call readdir it
returns information about the first entry in the directory again. This
function also notices if files have been added or removed to the
directory since it was opened with opendir. (Entries for these
files might or might not be returned by readdir if they were
added or removed since you last called opendir or
rewinddir.)
Function: off_t telldir (DIR *dirstream)
The telldir function returns the file position of the directory
stream dirstream. You can use this value with seekdir to
restore the directory stream to that position.
Function: void seekdir (DIR *dirstream, off_t pos)
The seekdir function sets the file position of the directory
stream dirstream to pos. The value pos must be the
result of a previous call to telldir on this particular stream;
closing and reopening the directory can invalidate values returned by
telldir.
In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others.
To add a name to a file, use the link function. (The new name is
also called a hard link to the file.) Creating a new link to a
file does not copy the contents of the file; it simply makes a new name
by which the file can be known, in addition to the file's existing name
or names.
One file can have names in several directories, so the the organization of the file system is not a strict hierarchy or tree.
Since a particular file exists within a single file system, all its
names must be in directories in that file system. link reports
an error if you try to make a hard link to the file from another file
system.
The prototype for the link function is declared in the header
file `unistd.h'.
Function: int link (const char *oldname, const char *newname)
The link function makes a new link to the existing file named by
oldname, under the new name newname.
This function returns a value of 0 if it is successful and
-1 on failure. In addition to the usual file name syntax errors
(see section File Name Errors) for both oldname and newname, the
following errno error conditions are defined for this function:
EACCES
EEXIST
EMLINK
LINK_MAX; see
section Limits on File System Capacity.)
Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.
ENOENT
ENOSPC
EPERM
EROFS
EXDEV
The GNU system supports soft links or symbolic links. This is a kind of "file" that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file.
The reason symbolic links work the way they do is that special things
happen when you try to open the link. The open function realizes
you have specified the name of a link, reads the file name contained in
the link, and opens that file name instead. The stat function
likewise operates on the file that the symbolic link points to, instead
of on the link itself. So does link, the function that makes a
hard link.
By contrast, other operations such as deleting or renaming the file
operate on the link itself. The functions readlink and
lstat also refrain from following symbolic links, because
their purpose is to obtain information about the link.
Prototypes for the functions listed in this section are in `unistd.h'.
Function: int symlink (const char *oldname, const char *newname)
The symlink function makes a symbolic link to oldname named
newname.
The normal return value from symlink is 0. A return value
of -1 indicates an error. In addition to the usual file name
syntax errors (see section File Name Errors), the following errno
error conditions are defined for this function:
EEXIST
EROFS
ENOSPC
EIO
Function: int readlink (const char *filename, char *buffer, size_t size)
The readlink function gets the value of the symbolic link
filename. The file name that the link points to is copied into
buffer. This file name string is not null-terminated;
readlink normally returns the number of characters copied. The
size argument specifies the maximum number of characters to copy,
usually the allocation size of buffer.
If the return value equals size, you cannot tell whether or not
there was room to return the entire name. So make a bigger buffer and
call readlink again. Here is an example:
char *
readlink_malloc (char *filename)
{
int size = 100;
while (1)
{
char *buffer = (char *) xmalloc (size);
int nchars = readlink (filename, buffer, size);
if (nchars < size)
return buffer;
free (buffer);
size *= 2;
}
}
A value of -1 is returned in case of error. In addition to the
usual file name syntax errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EINVAL
EIO
You can delete a file with the functions unlink or remove.
(These names are synonymous.)
Deletion actually deletes a file name. If this is the file's only name, then the file is deleted as well. If the file has other names as well (see section Hard Links), it remains accessible under its other names.
Function: int unlink (const char *filename)
The unlink function deletes the file name filename. If
this is a file's sole name, the file itself is also deleted. (Actually,
if any process has the file open when this happens, deletion is
postponed until all processes have closed the file.)
The function unlink is declared in the header file `unistd.h'.
This function returns 0 on successful completion, and -1
on error. In addition to the usual file name syntax errors
(see section File Name Errors), the following errno error conditions are
defined for this function:
EACCESS
EBUSY
ENOENT
EPERM
unlink cannot be used to delete the name of a
directory, or can only be used this way by a privileged user.
To avoid such problems, use rmdir to delete directories.
EROFS
Function: int remove (const char *filename)
The remove function is another name for unlink.
remove is the ANSI C name, whereas unlink is the POSIX.1
name. The name remove is declared in `stdio.h'.
Function: int rmdir (const char *filename)
The rmdir function deletes a directory. The directory must be
empty before it can be removed; in other words, it can only contain
entries for `.' and `..'.
In most other respects, rmdir behaves like unlink. There
are two additional errno error conditions defined for
rmdir:
EEXIST
ENOTEMPTY
These two error codes are synonymous; some systems use one, and some use the other.
The prototype for this function is declared in the header file `unistd.h'.
The rename function is used to change a file's name.
Function: int rename (const char *oldname, const char *newname)
The rename function renames the file name oldname with
newname. The file formerly accessible under the name
oldname is afterward accessible as newname instead. (If the
file had any other names aside from oldname, it continues to have
those names.)
The directory containing the name newname must be on the same file system as the file (as indicated by the name oldname).
One special case for rename is when oldname and
newname are two names for the same file. The consistent way to
handle this case is to delete oldname. However, POSIX says that
in this case rename does nothing and reports success--which is
inconsistent. We don't know what your operating system will do. The
GNU system, when completed, will probably do the right thing (delete
oldname) unless you explicitly request strict POSIX compatibility
"even when it hurts".
If the oldname is not a directory, then any existing file named
newname is removed during the renaming operation. However, if
newname is the name of a directory, rename fails in this
case.
If the oldname is a directory, then either newname must not
exist or it must name a directory that is empty. In the latter case,
the existing directory named newname is deleted first. The name
newname must not specify a subdirectory of the directory
oldname which is being renamed.
One useful feature of rename is that the meaning of the name
newname changes "atomically" from any previously existing file
by that name to its new meaning (the file that was called
oldname). There is no instant at which newname is
nonexistent "in between" the old meaning and the new meaning.
If rename fails, it returns -1. In addition to the usual
file name syntax errors (see section File Name Errors), the following
errno error conditions are defined for this function:
EACCES
EBUSY
EEXIST
ENOTEMPTY
EINVAL
EISDIR
EMLINK
Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.
ENOENT
ENOSPC
EROFS
EXDEV
Directories are created with the mkdir function. (There is also
a shell command mkdir which does the same thing.)
Function: int mkdir (const char *filename, mode_t mode)
The mkdir function creates a new, empty directory whose name is
filename.
The argument mode specifies the file permissions for the new directory file. See section The Mode Bits for Access Permission, for more information about this.
A return value of 0 indicates successful completion, and
-1 indicates failure. In addition to the usual file name syntax
errors (see section File Name Errors), the following errno error
conditions are defined for this function:
EACCES
EEXIST
EMLINK
Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.
ENOSPC
EROFS
To use this function, your program should include the header file `sys/stat.h'.
When you issue an `ls -l' shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, and the like. This kind of information is called the file attributes; it is associated with the file itself and not a particular one of its names.
This section contains information about how you can inquire about and modify these attributes of files.
When you read the attributes of a file, they come back in a structure
called struct stat. This section describes the names of the
attributes, their data types, and what they mean. For the functions
to read the attributes of a file, see section Reading the Attributes of a File.
The header file `sys/stat.h' declares all the symbols defined in this section.
The stat structure type is used to return information about the
attributes of a file. It contains at least the following members:
mode_t st_mode
ino_t st_ino
dev_t st_dev
st_ino and
st_dev, taken together, uniquely identify the file.
nlink_t st_nlink
uid_t st_uid
gid_t st_gid
off_t st_size
time_t st_atime
unsigned long int st_atime_usec
time_t st_mtime
unsigned long int st_mtime_usec
time_t st_ctime
unsigned long int st_ctime_usec
unsigned int st_nblocks
The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse--it may have "holes" which contain zeros but do not actually take up space on the disk.
You can tell (approximately) whether a file is sparse by comparing this
value with st_size, like this:
(st.st_blocks * 512 < st.st_size)
This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem.
unsigned int st_blksize
Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file `sys/types.h' as well as in `sys/stat.h'. Here is a list of them.
This is an integer data type used to represent file modes. In the
GNU system, this is equivalent to unsigned int.
This is an arithmetic data type used to represent file serial numbers.
(In Unix jargon, these are sometimes called inode numbers.)
In the GNU system, this type is equivalent to unsigned long int.
This is an arithmetic data type used to represent file device numbers.
In the GNU system, this is equivalent to int.
This is an arithmetic data type used to represent file link counts.
In the GNU system, this is equivalent to unsigned short int.
To examine the attributes of files, use the functions stat,
fstat and lstat. They return the attribute information in
a struct stat object. All three functions are declared in the
header file `sys/stat.h'.
Function: int stat (const char *filename, struct stat *buf)
The stat function returns information about the attributes of the
file named by filename in the structure pointed at by buf.
If filename is the name of a symbolic link, the attributes you get
describe the file that the link points to. If the link points to a
nonexistent file name, then stat fails, reporting a nonexistent
file.
The return value is 0 if the operation is successful, and -1
on failure. In addition to the usual file name syntax errors
(see section File Name Errors, the following errno error conditions
are defined for this function:
ENOENT
Function: int fstat (int filedes, struct stat *buf)
The fstat function is like stat, except that it takes an
open file descriptor as an argument instead of a file name.
See section Low-Level Input/Output.
Like stat, fstat returns 0 on success and -1
on failure. The following errno error conditions are defined for
fstat:
EBADF
Function: int lstat (const char *filename, struct stat *buf)
The lstat function is like stat, except that it does not
follow symbolic links. If filename is the name of a symbolic
link, lstat returns information about the link itself; otherwise,
lstat works like stat. See section Symbolic Links.
The file mode, stored in the st_mode field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits. This section discusses only the type code,
which you can use to tell whether the file is a directory, whether it is
a socket, and so on. For information about the access permission,
section The Mode Bits for Access Permission.
There are two predefined ways you can access the file type portion of the file mode. First of all, for each type of file, there is a predicate macro which examines a file mode value and returns true or false--is the file of that type, or not. Secondly, you can mask out the rest of the file mode to get just a file type code. You can compare this against various constants for the supported file types.
All of the symbols listed in this section are defined in the header file `sys/stat.h'.
The following predicate macros test the type of a file, given the value
m which is the st_mode field returned by stat on
that file:
This macro returns nonzero if the file is a directory.
This macro returns nonzero if the file is a character special file (a device like a terminal).
This macro returns nonzero if the file is a block special file (a device like a disk).
This macro returns nonzero if the file is a regular file.
Macro: int S_ISFIFO (mode_t m)
This macro returns nonzero if the file is a FIFO special file, or a pipe. See section Pipes and FIFOs.
This macro returns nonzero if the file is a symbolic link. See section Symbolic Links.
Macro: int S_ISSOCK (mode_t m)
This macro returns nonzero if the file is a socket. See section Sockets.
An alterate non-POSIX method of testing the file type is supported for
compatibility with BSD. The mode can be bitwise ANDed with
S_IFMT to extract the file type code, and compared to the
appropriate type code constant. For example,
S_ISCHR (mode)
is equivalent to:
((mode & S_IFMT) == S_IFCHR)
This is a bit mask used to extract the file type code portion of a mode value.
These are the symbolic names for the different file type codes:
S_IFDIR
S_IFCHR
S_IFBLK
S_IFREG
S_IFLNK
S_IFSOCK
S_IFIFO
Every file has an owner which is one of the registered user names defined on the system. Each file also has a group, which is one of the defined groups. The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control.
The file owner and group play a role in determining access because the file has one set of access permission bits for the user that is the owner, another set that apply to users who belong to the file's group, and a third set of bits that apply to everyone else. See section How Your Access to a File is Decided, for the details of how access is decided based on this data.
When a file is created, its owner is set from the effective user ID of the process that creates it (see section The Persona of a Process). The file's group ID may be set from either effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored. When you access a remote file system, it behaves according to its own rule, not according to the system your program is running on. Thus, your program must be prepared to encounter either kind of behavior, no matter what kind of system you run it on.
You can change the owner and/or group owner of an existing file using
the chown function. This is the primitive for the chown
and chgrp shell commands.
The prototype for this function is declared in `unistd.h'.
Function: int chown (const char *filename, uid_t owner, gid_t group)
The chown function changes the owner of the file filename to
owner, and its group owner to group.
Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID bits of the file's permissions. (This is because those bits may not be appropriate for the new owner.) The other file permission bits are not changed.
The return value is 0 on success and -1 on failure.
In addition to the usual file name syntax errors (see section File Name Errors),
the following errno error conditions are defined for this function:
EPERM
Only privileged users or the file's owner can change the file's group. On most file systems, only privileged users can change the file owner; some file systems allow you to change the owner if you are currently the owner. When you access a remote file system, the behavior you encounter is determined by the system that actually holds the file, not by the system your program is running on.
See section Optional Features in File Support, for information about the
_POSIX_CHOWN_RESTRICTED macro.
EROFS
Function: int fchown (int filedes, int owner, int group)
This is like chown, except that it changes the owner of the file
with open file descriptor filedes.
The return value from fchown is 0 on success and -1
on failure. The following errno error codes are defined for this
function:
EBADF
EINVAL
EPERM
chmod, above.
EROFS
The file mode, stored in the st_mode field of the file
attributes, contains two kinds of information: the file type code, and
the access permission bits. This section discusses only the access
permission bits, which control who can read or write the file.
See section Testing the Type of a File, for information about the file type code.
All of the symbols listed in this section are defined in the header file `sys/stat.h'.
These symbolic constants are defined for the file mode bits that control access permission for the file:
S_IRUSR
S_IREAD
S_IREAD is an obsolete synonym provided for BSD
compatibility.
S_IWUSR
S_IWRITE
S_IWRITE is an obsolete synonym provided for BSD compatibility.
S_IXUSR
S_IEXEC
S_IEXEC is an obsolete
synonym provided for BSD compatibility.
S_IRWXU
S_IRGRP
S_IWGRP
S_IXGRP
S_IRWXG
S_IROTH
S_IWOTH
S_IXOTH
S_IRWXO
S_ISUID
S_ISGID
S_ISVTX
On an executable file, it modifies the swapping policies of the system. Normally, when a program terminates, its pages in core are immediately freed and reused. If the sticky bit is set on the executable file, the system keeps the pages in core for a while as if the program were still running. This is advantageous for a program that is likely to be run many times in succession.
On a directory, the sticky bit gives permission to delete a file in the directory if you can write the contents of that file. Ordinarily, a user either can delete all the files in the directory or cannot delete any of them (based on whether the user has write permission for the directory). The sticky bit makes it possible to control deletion for individual files.
The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed.
Warning: Writing explicit numbers for file permissions is bad practice. It is not only nonportable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean, use the symbolic names.
Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process, and its supplementary group IDs, together with the file's owner, group and permission bits. These concepts are discussed in detail in section The Persona of a Process.
If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding "user" (or "owner") bits. Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the "group" bits. Otherwise, permissions are controlled by the "other" bits.
Privileged users, like `root', can access any file, regardless of its file permission bits. As a special case, for a file to be executable even for a privileged user, at least one of its execute bits must be set.
The primitive functions for creating files (for example, open or
mkdir) take a mode argument, which specifies the file
permissions for the newly created file. But the specified mode is
modified by the process's file creation mask, or umask,
before it is used.
The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files. For example, if you set all the "other" access bits in the mask, then newly created files are not accessible at all to processes in the "other" category, even if the mode argument specified to the creation function would permit such access. In other words, the file creation mask is the complement of the ordinary access permissions you want to grant.
Programs that create files typically specify a mode argument that includes all the permissions that make sense for the particular file. For an ordinary file, this is typically read and write permission for all classes of users. These permissions are then restricted as specified by the individual user's own file creation mask.
To change the permission of an existing file given its name, call
chmod. This function ignores the file creation mask; it uses
exactly the specified permission bits.
In normal use, the file creation mask is initialized in the user's login
shell (using the umask shell command), and inherited by all
subprocesses. Application programs normally don't need to worry about
the file creation mask. It will do automatically what it is supposed to
do.
When your program should create a file and bypass the umask for its
access permissions, the easiest way to do this is to use fchmod
after opening the file, rather than changing the umask.
In fact, changing the umask is usually done only by shells. They use
the umask function.
The functions in this section are declared in `sys/stat.h'.
Function: mode_t umask (mode_t mask)
The umask function sets the file creation mask of the current
process to mask, and returns the previous value of the file
creation mask.
Here is an example showing how to read the mask with umask
without changing it permanently:
mode_t
read_umask (void)
{
mask = umask (0);
umask (mask);
}
However, it is better to use getumask if you just want to read
the mask value, because that is reentrant (at least if you use the GNU
operating system).
Function: mode_t getumask (void)
Return the current value of the file creation mask for the current process. This function is a GNU extension.
Function: int chmod (const char *filename, mode_t mode)
The chmod function sets the access permission bits for the file
named by filename to mode.
If the filename names a symbolic link, chmod changes the
permission of the file pointed to by the link, not those of the link
itself. There is actually no way to set the mode of a link, which is
always -1.
This function returns 0 if successful and -1 if not. In
addition to the usual file name syntax errors (see section File Name Errors), the following errno error conditions are defined for
this function:
ENOENT
EPERM
EROFS
Function: int fchmod (int filedes, int mode)
This is like chmod, except that it changes the permissions of
the file currently open via descriptor filedes.
The return value from fchmod is 0 on success and -1
on failure. The following errno error codes are defined for this
function:
EBADF
EINVAL
EPERM
EROFS
When a program runs as a privileged user, this permits it to access
files off-limits to ordinary users--for example, to modify
`/etc/passwd'. Programs designed to be run by ordinary users but
access such files use the setuid bit feature so that they always run
with root as the effective user ID.
Such a program may also access files specified by the user, files which
conceptually are being accessed explicitly by the user. Since the
program runs as root, it has permission to access whatever file
the user specifies--but usually the desired behavior is to permit only
those files which the user could ordinarily access.
The program therefore must explicitly check whether the user would have the necessary access to a file, before it reads or writes the file.
To do this, use the function access, which checks for access
permission based on the process's real user ID rather than the
effective user ID. (The setuid feature does not alter the real user ID,
so it reflects the user who actually ran the program.)
There is another way you could check this access, which is easy to
describe, but very hard to use. This is to examine the file mode bits
and mimic the system's own access computation. This method is
undesirable because many systems have additional access control
features; your program cannot portably mimic them, and you would not
want to try to keep track of the diverse features that different systems
have. Using access is simple and automatically does whatever is
appropriate for the system you are using.
The symbols in this section are declared in `unistd.h'.
Function: int access (const char *filename, int how)
The access function checks to see whether the file named by
filename can be accessed in the way specified by the how
argument. The how argument either can be the bitwise OR of the
flags R_OK, W_OK, X_OK, or the existence test
F_OK.
This function uses the real user and group ID's of the calling
process, rather than the effective ID's, to check for access
permission. As a result, if you use the function from a setuid
or setgid program (see section How an Application Can Change Persona), it gives
information relative to the user who actually ran the program.
The return value is 0 if the access is permitted, and -1
otherwise. (In other words, treated as a predicate function,
access returns true if the requested access is denied.)
In addition to the usual file name syntax errors (see section File Name Errors), the following errno error conditions are defined for
this function:
EACCES
ENOENT
EROFS
These macros are defined in the header file `unistd.h' for use
as the how argument to the access function. The values
are integer constants.
Argument that means, test for read permission.
Argument that means, test for write permission.
Argument that means, test for execute/search permission.
Argument that means, test for existence of the file.
Each file has three timestamps associated with it: its access time,
its modification time, and its attribute modification time. These
correspond to the st_atime, st_mtime, and st_ctime
members of the stat structure; see section File Attributes.
All of these times are represented in calendar time format, as
time_t objects. This data type is defined in `time.h'.
For more information about representation and manipulation of time
values, see section Calendar Time.
When an existing file is opened, its attribute change time and modification time fields are updated. Reading from a file updates its access time attribute, and writing updates its modification time.
When a file is created, all three timestamps for that file are set to the current time. In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated.
Adding a new name for a file with the link function updates the
attribute change time field of the file being linked, and both the
attribute change time and modification time fields of the directory
containing the new name. These same fields are affected if a file name
is deleted with unlink, remove, or rmdir. Renaming
a file with rename affects only the attribute change time and
modification time fields of the two parent directories involved, and not
the times for the file being renamed.
Changing attributes of a file (for example, with chmod) updates
its attribute change time field.
You can also change some of the timestamps of a file explicitly using
the utime function--all except the attribute change time. You
need to include the header file `utime.h' to use this facility.
The utimbuf structure is used with the utime function to
specify new access and modification times for a file. It contains the
following members:
time_t actime
time_t modtime
Function: int utime (const char *filename, const struct utimbuf *times)
This function is used to modify the file times associated with the file named filename.
If times is a null pointer, then the access and modification times
of the file are set to the current time. Otherwise, they are set to the
values from the actime and modtime members (respectively)
of the utimbuf structure pointed at by times.
The attribute modification time for the file is set to the current time in either case (since changing the timestamps is itself a modification of the file attributes).
The utime function returns 0 if successful and -1
on failure. In addition to the usual file name syntax errors
(see section File Name Errors), the following errno error conditions
are defined for this function:
EACCES
ENOENT
EPERM
EROFS
Each of the three time stamps has a corresponding microsecond part,
which extends its resolution. These fields are called
st_atime_usec, st_mtime_usec, and st_ctime_usec;
each has a value between 0 and 999,999, which indicates the time in
microseconds. They correspond to the tv_usec field of a
timeval structure; see section High-Resolution Calendar.
The utimes function is like utime, but also lets you specify
the fractional part of the file times. The prototype for this function is
in the header file `sys/time.h'.
Function: int utimes (const char *filename, struct timeval tvp[2])
This function sets the file access and modification times for the file
named by filename. The new file access time is specified by
tvp[0], and the new modification time by
tvp[1]. This function comes from BSD.
The return values and error conditions are the same as for the utime
function.
The mknod function is the primitive for making special files,
such as files that correspond to devices. The GNU library includes
this function for compatibility with BSD.
The prototype for mknod is declared in `sys/stat.h'.
Function: int mknod (const char *filename, int mode, int dev)
The mknod function makes a special file with name filename.
The mode specifies the mode of the file, and may include the various
special file bits, such as S_IFCHR (for a character special file)
or S_IFBLK (for a block special file). See section Testing the Type of a File.
The dev argument specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created.
The return value is 0 on success and -1 on error. In addition
to the usual file name syntax errors (see section File Name Errors), the
following errno error conditions are defined for this function:
EPERM
ENOSPC
EROFS
EEXIST
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