readlink

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 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() 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
       function 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 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
              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.

FUTURE DIRECTIONS
       None.

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
       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.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                   2013                      READLINK(3POSIX)