This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions described in section 7 Input/Output on Streams and section 8 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 6.2.2 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 26.12 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'.
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 3.3 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 3.3.2 Examples of malloc, for information about xmalloc, which is
not a library function but is a customary name used in most GNU
software.
getcwd, but has no way to specify the size of
the buffer. The GNU library provides getwd only
for backwards compatibility with BSD.
The buffer argument should be a pointer to an array at least
PATH_MAX bytes long (see section 28.6 Limits on File System Capacity). In the GNU
system there is no limit to the size of a file name, so this is not
necessarily enough space to contain the directory name. That is why
this function is deprecated.
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 6.2.3 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 7 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'.
char d_name[]
ino_t d_fileno
d_ino. In the GNU system and most POSIX
systems, for most files this the same as the st_ino member that
stat will return for the file. See section 9.8 File Attributes.
unsigned char d_namlen
unsigned char because that is the integer
type of the appropriate size
unsigned char d_type
DT_UNKNOWN
DT_REG
DT_DIR
DT_FIFO
DT_SOCK
DT_CHR
DT_BLK
st_mode member of
struct statbuf. These two macros convert between d_type
values and st_mode values:
d_type value corresponding to mode.
st_mode value corresponding to dirtype.
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 9.8 File Attributes.
This section describes how to open a directory stream. All the symbols are declared in the header file `dirent.h'.
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.
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 errors (see section 6.2.3 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 8 Low-Level Input/Output. Directory streams and the underlying
file descriptors are closed on exec (see section 23.5 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'.
Portability Note: On some systems, readdir may not
return entries for `.' and `..', even though these are always
valid file names in any directory. See section 6.2.2 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
readdir is not thread safe. Multiple threads using
readdir on the same dirstream may overwrite the return
value. Use readdir_r when this is critical.
readdir. Like
readdir it returns the next entry from the directory. But to
prevent conflicts for simultaneously running threads the result is not
stored in some internal memory. Instead the argument entry has to
point to a place where the result is stored.
The return value is 0 in case the next entry was read
successfully. In this case a pointer to the result is returned in
*result. It is not required that *result is the same as
entry. If something goes wrong while executing readdir_r
the function returns -1. The errno variable is set like
described for readdir.
Portability Note: On some systems, readdir_r may not
return a terminated string as the file name even if no d_reclen
element is available in struct dirent and the file name as the
maximal allowed size. Modern systems all have the d_reclen field
and on old systems multi threading is not critical. In any case, there
is no such problem with the readdir function so that even on
systems without d_reclen field one could use multiple threads by
using external locking.
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 9.2.6 Scanning the Content of a Directory and section 15.3 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'.
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.)
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.
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.
A higher-level interface to the directory handling functions is the
scandir function. With its help one can select a subset of the
entries in a directory, possibly sort them and get as the result a list
of names.
The scandir function scans the contents of the directory selected
by dir. The result in namelist is an array of pointers to
structure of type struct dirent which describe all selected
directory entries and which is allocated using malloc. Instead
of always getting all directory entries returned, the user supplied
function selector can be used to decide which entries are in the
result. Only the entries for which selector returns a nonzero
value are selected.
Finally the entries in the namelist are sorted using the user
supplied function cmp. The arguments of the cmp function
are of type struct dirent **. I.e., one cannot directly use the
strcmp or strcoll function; see the function
alphasort below.
The return value of the function gives the number of entries placed in
namelist. If it is -1 an error occurred and the global
variable errno contains more information on the error.
As said above the fourth argument to the scandir function must be
a pointer to a sorting function. For the convenience of the programmer
the GNU C library contains an implementation of a function which is very
helpful for this purpose.
alphasort function behaves like the strcmp function
(see section 5.5 String/Array Comparison). The difference is that the arguments
are not string pointers but instead they are of type
struct dirent **.
Return value of is less than, equal to, or greater than zero depending on the order of the two entries a and b.
Here is a revised version of the directory lister found above
(see section 9.2.4 Simple Program to List a Directory). Using the scandir function we
can avoid using the functions which directly work with the directory
contents. After the call the found entries are available for direct
used.
#include <stdio.h>
#include <dirent.h>
static int
one (struct dirent *unused)
{
return 1;
}
int
main (void)
{
struct dirent **eps;
int n;
n = scandir ("./", &eps, one, alphasort);
if (n >= 0)
{
int cnt;
for (cnt = 0; cnt < n; ++cnt)
puts (eps[cnt]->d_name);
}
else
perror ("Couldn't open the directory");
return 0;
}
Please note the simple selector function for this example. Since
we want to see all directory entries we always return 1.
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.
In most implementations, it is not possible to have hard links to the
same file in multiple file systems. link reports an error if you
try to make a hard link to the file from another file system when this
cannot be done.
The prototype for the link function is declared in the header
file `unistd.h'.
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 errors
(see section 6.2.3 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 28.6 Limits on File System Capacity.)
ENOENT
ENOSPC
EPERM
EROFS
EXDEV
EIO
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.
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. So does link,
the function that makes a hard link--it makes a hard link to the
symbolic link, which one rarely wants.
Prototypes for the functions listed in this section are in `unistd.h'.
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 6.2.3 File Name Errors), the following errno
error conditions are defined for this function:
EEXIST
EROFS
ENOSPC
EIO
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 errors (see section 6.2.3 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.
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 9.3 Hard Links), it remains accessible under its other names.
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 errors
(see section 6.2.3 File Name Errors), the following errno error conditions are
defined for this function:
EACCES
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.
(In the GNU system unlink can never delete the name of a directory.)
EROFS
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:
ENOTEMPTY
EEXIST
These two error codes are synonymous; some systems use one, and some use
the other. The GNU system always uses ENOTEMPTY.
The prototype for this function is declared in the header file `unistd.h'.
unlink for files and like rmdir for directories.
remove is declared in `stdio.h'.
The rename function is used to change a file's name.
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 requires
that in this case rename do nothing and report success--which is
inconsistent. We don't know what your operating system will do.
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
there is a system crash during the operation, it is possible for both
names to still exist; but newname will always be intact if it
exists at all.
If rename fails, it returns -1. In addition to the usual
file name errors (see section 6.2.3 File Name Errors), the following
errno error conditions are defined for this function:
EACCES
EBUSY
ENOTEMPTY
EEXIST
ENOTEMPTY for this, but some other systems return EEXIST.
EINVAL
EISDIR
EMLINK
ENOENT
ENOSPC
EROFS
EXDEV
Directories are created with the mkdir function. (There is also
a shell command mkdir which does the same thing.)
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 9.8.5 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 6.2.3 File Name Errors), the following errno error
conditions are defined for this function:
EACCES
EEXIST
EMLINK
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 9.8.2 Reading the Attributes of a File.
The header file `sys/stat.h' declares all the symbols defined in this section.
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. The
st_dev value is not necessarily consistent across reboots or
system crashes, however.
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_blocks
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
st_blocks.)
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.
unsigned int.
unsigned long int.
int.
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'.
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 errors
(see section 6.2.3 File Name Errors, the following errno error conditions
are defined for this function:
ENOENT
fstat function is like stat, except that it takes an
open file descriptor as an argument instead of a file name.
See section 8 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
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 9.4 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 9.8.5 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:
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)
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 9.8.6 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 26.2 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'.
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 errors (see section 6.2.3 File Name Errors),
the following errno error conditions are defined for this function:
EPERM
_POSIX_CHOWN_RESTRICTED macro.
EROFS
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 9.8.3 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
chmod fails with EFTYPE;
see section 9.8.7 Assigning File Permissions.
Some systems (particularly SunOS) have yet another use for the sticky
bit. If the sticky bit is set on a file that is not executable,
it means the opposite: never cache the pages of this file at all. The
main use of this is for the files on an NFS server machine which are
used as the swap area of diskless client machines. The idea is that the
pages of the file will be cached in the client's memory, so it is a
waste of the server's memory to cache them a second time. In this use
the sticky bit also says that the filesystem may fail to record the
file's modification time onto disk reliably (the idea being that noone
cares for a swap file).
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 26.2 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'.
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).
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.
This function returns 0 if successful and -1 if not. In
addition to the usual file name errors (see section 6.2.3 File Name Errors), the following errno error conditions are defined for
this function:
ENOENT
EPERM
EROFS
EFTYPE
S_ISVTX bit (the "sticky bit") set,
and the named file is not a directory. Some systems do not allow setting the
sticky bit on non-directory files, and some do (and only some of those
assign a useful meaning to the bit for non-directory files).
You only get EFTYPE on systems where the sticky bit has no useful
meaning for non-directory files, so it is always safe to just clear the
bit in mode and call chmod again. See section 9.8.5 The Mode Bits for Access Permission,
for full details on the sticky bit.
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.
access is only only appropriate to use in setuid programs.
A non-setuid program will always use the effective ID rather than the
real ID.
The symbols in this section are declared in `unistd.h'.
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 26.4 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 errors (see section 6.2.3 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.
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 9.8 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 17.2 Calendar Time.
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.
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
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 errors
(see section 6.2.3 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 17.2.2 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'.
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'.
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 9.8.3 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 errors (see section 6.2.3 File Name Errors), the
following errno error conditions are defined for this function:
EPERM
ENOSPC
EROFS
EEXIST
If you need to use a temporary file in your program, you can use the
tmpfile function to open it. Or you can use the tmpnam
(better: tmpnam_r) function make a name for a temporary file and
then open it in the usual way with fopen.
The tempnam function is like tmpnam but lets you choose
what directory temporary files will go in, and something about what
their file names will look like. Important for multi threaded programs
is that tempnam is reentrant while tmpnam is not since it
returns a pointer to a static buffer.
These facilities are declared in the header file `stdio.h'.
fopen with mode "wb+". The file is deleted
automatically when it is closed or when the program terminates. (On
some other ISO C systems the file may fail to be deleted if the program
terminates abnormally).
This function is reentrant.
L_tmpnam characters,
and the result is written into that array.
It is possible for tmpnam to fail if you call it too many times
without removing previously created files. This is because the fixed
length of a temporary file name gives room for only a finite number of
different names. If tmpnam fails, it returns a null pointer.
tmpnam function. But it
does not allow result to be a null pointer. In the later case a
null pointer is returned.
This function is reentrant because the non-reentrant situation of
tmpnam cannot happen here.
tmpnam function.
TMP_MAX is a lower bound for how many temporary names
you can create with tmpnam. You can rely on being able to call
tmpnam at least this many times before it might fail saying you
have made too many temporary file names.
With the GNU library, you can create a very large number of temporary
file names--if you actually create the files, you will probably run out
of disk space before you run out of names. Some other systems have a
fixed, small limit on the number of temporary files. The limit is never
less than 25.
malloc; you should release its storage with free when
it is no longer needed.
Because the string is dynamically allocated this function is reentrant.
The directory prefix for the temporary file name is determined by testing each of the following, in sequence. The directory must exist and be writable.
TMPDIR, if it is defined. For security
reasons this only happens if the program is not SUID or SGID enabled.
P_tmpdir macro.
This function is defined for SVID compatibility.
Older Unix systems did not have the functions just described. Instead
they used mktemp and mkstemp. Both of these functions
work by modifying a file name template string you pass. The last six
characters of this string must be `XXXXXX'. These six `X's
are replaced with six characters which make the whole string a unique
file name. Usually the template string is something like
`/tmp/prefixXXXXXX', and each program uses a unique prefix.
Note: Because mktemp and mkstemp modify the
template string, you must not pass string constants to them.
String constants are normally in read-only storage, so your program
would crash when mktemp or mkstemp tried to modify the
string.
mktemp function generates a unique file name by modifying
template as described above. If successful, it returns
template as modified. If mktemp cannot find a unique file
name, it makes template an empty string and returns that. If
template does not end with `XXXXXX', mktemp returns a
null pointer.
mkstemp function generates a unique file name just as
mktemp does, but it also opens the file for you with open
(see section 8.1 Opening and Closing Files). If successful, it modifies
template in place and returns a file descriptor open on that file
for reading and writing. If mkstemp cannot create a
uniquely-named file, it makes template an empty string and returns
-1. If template does not end with `XXXXXX',
mkstemp returns -1 and does not modify template.
Unlike mktemp, mkstemp is actually guaranteed to create a
unique file that cannot possibly clash with any other program trying to
create a temporary file. This is because it works by calling
open with the O_EXCL flag bit, which says you want to
always create a new file, and get an error if the file already exists.
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