READLINK(3P) POSIX Programmer's Manual READLINK(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
readlink, readlinkat — read the contents of a symbolic link
SYNOPSIS
#include <unistd.h>
ssize_t readlink(const char *restrict path, char *restrict buf,
size_t bufsize);
ssize_t readlinkat(int fd, const char *restrict path,
char *restrict buf, size_t bufsize);
DESCRIPTION
The readlink() function shall place the contents of the symbolic link
referred to by path in the buffer buf which has size bufsize. If the
number of bytes in the symbolic link is less than bufsize, the contents
of the remainder of buf are unspecified. If the buf argument is not
large enough to contain the link content, the first bufsize bytes shall
be placed in buf.
If the value of bufsize is greater than {SSIZE_MAX}, the result is
implementation-defined.
Upon successful completion, readlink() shall mark for update the last
data access timestamp of the symbolic link.
The readlinkat() function shall be equivalent to the readlink() func‐
tion except in the case where path specifies a relative path. In this
case the symbolic link whose content is read is relative to the direc‐
tory associated with the file descriptor fd instead of the current
working directory. If the file descriptor was opened without O_SEARCH,
the function shall check whether directory searches are permitted using
the current permissions of the directory underlying the file descrip‐
tor. If the file descriptor was opened with O_SEARCH, the function
shall not perform the check.
If readlinkat() is passed the special value AT_FDCWD in the fd parame‐
ter, the current working directory shall be used and the behavior shall
be identical to a call to readlink().
RETURN VALUE
Upon successful completion, these functions shall return the count of
bytes placed in the buffer. Otherwise, these functions shall return a
value of −1, leave the buffer unchanged, and set errno to indicate the
error.
ERRORS
These functions shall fail if:
EACCES Search permission is denied for a component of the path prefix
of path.
EINVAL The path argument names a file that is not a symbolic link.
EIO An I/O error occurred while reading from the file system.
ELOOP A loop exists in symbolic links encountered during resolution of
the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_MAX}.
ENOENT A component of path does not name an existing file or path is an
empty string.
ENOTDIR
A component of the path prefix names an existing file that is
neither a directory nor a symbolic link to a directory, or the
path argument contains at least one non-<slash> character and
ends with one or more trailing <slash> characters and the last
pathname component names an existing file that is neither a
directory nor a symbolic link to a directory.
The readlinkat() function shall fail if:
EACCES fd was not opened with O_SEARCH and the permissions of the
directory underlying fd do not permit directory searches.
EBADF The path argument does not specify an absolute path and the fd
argument is neither AT_FDCWD nor a valid file descriptor open
for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file
descriptor associated with a non-directory file.
These functions may fail if:
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
tion of a symbolic link produced an intermediate result with a
length that exceeds {PATH_MAX}.
The following sections are informative.
EXAMPLES
Reading the Name of a Symbolic Link
The following example shows how to read the name of a symbolic link
named /modules/pass1.
#include <unistd.h>
char buf[1024];
ssize_t len;
...
if ((len = readlink("/modules/pass1", buf, sizeof(buf)-1)) != -1)
buf[len] = '\0';
APPLICATION USAGE
Conforming applications should not assume that the returned contents of
the symbolic link are null-terminated.
RATIONALE
The type associated with bufsiz is a size_t in order to be consistent
with both the ISO C standard and the definition of read(). The behav‐
ior specified for readlink() when bufsiz is zero represents historical
practice. For this case, the standard developers considered a change
whereby readlink() would return the number of non-null bytes contained
in the symbolic link with the buffer buf remaining unchanged; however,
since the stat structure member st_size value can be used to determine
the size of buffer necessary to contain the contents of the symbolic
link as returned by readlink(), this proposal was rejected, and the
historical practice retained.
The purpose of the readlinkat() function is to read the content of sym‐
bolic links in directories other than the current working directory
without exposure to race conditions. Any part of the path of a file
could be changed in parallel to a call to readlink(), resulting in
unspecified behavior. By opening a file descriptor for the target
directory and using the readlinkat() function it can be guaranteed that
the symbolic link read is located relative to the desired directory.
FUTURE DIRECTIONS
None.
SEE ALSOfstatat(), symlink()
The Base Definitions volume of POSIX.1‐2008, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 READLINK(3P)
