readlink(3p) — Linux manual page


READLINK(3P)            POSIX Programmer's Manual           READLINK(3P)

PROLOG         top

       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         top

       readlink, readlinkat — read the contents of a symbolic link

SYNOPSIS         top

       #include <unistd.h>

       ssize_t readlink(const char *restrict path, char *restrict buf,
           size_t bufsize);

       #include <fcntl.h>

       ssize_t readlinkat(int fd, const char *restrict path,
           char *restrict buf, size_t bufsize);

DESCRIPTION         top

       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 access mode of the open
       file description associated with the file descriptor is not
       O_SEARCH, the function shall check whether directory searches are
       permitted using the current permissions of the directory
       underlying the file descriptor. If the access mode is 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         top

       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         top

       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

       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.

              The length of a component of a pathname is longer than

       ENOENT A component of path does not name an existing file or path
              is an empty string.

              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 The access mode of the open file description associated
              with fd is not 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.

              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.

              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         top

   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';


       Conforming applications should not assume that the returned
       contents of the symbolic link are null-terminated.

RATIONALE         top

       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         top

       fstatat(3p), symlink(3p)

       The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  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 .

       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 .

IEEE/The Open Group               2017                      READLINK(3P)

Pages that refer to this page: unistd.h(0p)fstatat(3p)symlink(3p)