Ubuntu Manpage: This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

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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);

       #include <fcntl.h>

       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 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

       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 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.

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.

COPYRIGHT

       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 http://www.opengroup.org/unix/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                                   2017                                      READLINK(3POSIX)