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GNU systems support 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. link, the
function that makes a hard link, does too. It makes a hard link to the
symbolic link, which one rarely wants.
Some systems have, for some functions operating on files, a limit on how many symbolic links are followed when resolving a path name. The limit if it exists is published in the sys/param.h header file.
The macro MAXSYMLINKS specifies how many symlinks some function
will follow before returning ELOOP. Not all functions behave the
same and this value is not the same as that returned for
_SC_SYMLOOP by sysconf. In fact, the sysconf
result can indicate that there is no fixed limit although
MAXSYMLINKS exists and has a finite value.
Prototypes for most of the functions listed in this section are in unistd.h.
Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
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 File Name Errors), the following errno
error conditions are defined for this function:
EEXISTThere is already an existing file named newname.
EROFSThe file newname would exist on a read-only file system.
ENOSPCThe directory or file system cannot be extended to make the new link.
EIOA hardware error occurred while reading or writing data on the disk.
Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.
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 (const char *filename)
{
size_t size = 50;
char *buffer = NULL;
while (1)
{
buffer = xreallocarray (buffer, size, 2);
size *= 2;
ssize_t nchars = readlink (filename, buffer, size);
if (nchars < 0)
{
free (buffer);
return NULL;
}
if (nchars < size)
return buffer;
}
}
A value of -1 is returned in case of error. In addition to the
usual file name errors (see File Name Errors), the following
errno error conditions are defined for this function:
EINVALThe named file is not a symbolic link.
EIOA hardware error occurred while reading or writing data on the disk.
In some situations it is desirable to resolve all the
symbolic links to get the real
name of a file where no prefix names a symbolic link which is followed
and no filename in the path is . or ... This is for
instance desirable if files have to be compared in which case different
names can refer to the same inode.
Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | See POSIX Safety Concepts.
The canonicalize_file_name function returns the absolute name of
the file named by name which contains no ., ..
components nor any repeated path separators (/) or symlinks. The
result is passed back as the return value of the function in a block of
memory allocated with malloc. If the result is not used anymore
the memory should be freed with a call to free.
If any of the path components are missing the function returns a NULL
pointer. This is also what is returned if the length of the path
reaches or exceeds PATH_MAX characters. In any case
errno is set accordingly.
ENAMETOOLONGThe resulting path is too long. This error only occurs on systems which have a limit on the file name length.
EACCESAt least one of the path components is not readable.
ENOENTThe input file name is empty.
ENOENTAt least one of the path components does not exist.
ELOOPMore than MAXSYMLINKS many symlinks have been followed.
This function is a GNU extension and is declared in stdlib.h.
The Unix standard includes a similar function which differs from
canonicalize_file_name in that the user has to provide the buffer
where the result is placed in.
Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | See POSIX Safety Concepts.
A call to realpath where the resolved parameter is
NULL behaves exactly like canonicalize_file_name. The
function allocates a buffer for the file name and returns a pointer to
it. If resolved is not NULL it points to a buffer into
which the result is copied. It is the callers responsibility to
allocate a buffer which is large enough. On systems which define
PATH_MAX this means the buffer must be large enough for a
pathname of this size. For systems without limitations on the pathname
length the requirement cannot be met and programs should not call
realpath with anything but NULL for the second parameter.
One other difference is that the buffer resolved (if nonzero) will
contain the part of the path component which does not exist or is not
readable if the function returns NULL and errno is set to
EACCES or ENOENT.
This function is declared in stdlib.h.
The advantage of using this function is that it is more widely available. The drawback is that it reports failures for long paths on systems which have no limits on the file name length.
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