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);
       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

       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         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 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.

              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 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.

              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.1‐2008, unistd.h(0p)

COPYRIGHT         top

       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 .

       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                 2013                        READLINK(3P)

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