READLINK(3POSIX) POSIX Programmer's Manual READLINK(3POSIX)
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 argu‐
ment 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 time‐
stamp of the symbolic link.
The readlinkat() function shall be equivalent to the readlink() function except in the
case where path specifies a relative path. In this case the symbolic link whose content is
read is relative to the directory associated with the file descriptor fd instead of the
current working directory. If the file descriptor was opened without O_SEARCH, the func‐
tion shall check whether directory searches are permitted using the current permissions of
the directory underlying the file descriptor. 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 parameter, the current
working directory shall be used and the behavior shall be identical to a call to read‐
link().
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 resolution 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 behavior 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 symbolic 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.
SEE ALSO
fstatat(), 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 Electrical 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 discrep‐
ancy 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 READLINK(3POSIX)