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. 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 have 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 a 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'.
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
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 File Name Errors), the following
errno
error conditions are defined for this function:
EINVAL
EIO
In some situations it is desirable to resolve all the 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 compare in which case different
names can refer to the same inode.
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
.
In any of the path components except the last one is 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.
ENAMETOOLONG
EACCES
ENOENT
ENOENT
ELOOP
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.
The realpath
function behaves just like
canonicalize_file_name
but instead of allocating a buffer for the
result it is placed in the buffer pointed to by resolved.
One other difference is that the buffer resolved 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 path on systems which have no limits on the file name length.
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