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READLINK(3P) POSIX Programmer's Manual READLINK(3P)
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.
readlink, readlinkat — read the contents of a symbolic link
#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);
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().
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.
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.
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.
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.
None.
fstatat(3p), symlink(3p)
The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)
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(3P)
Pages that refer to this page: unistd.h(0p), fstatat(3p), symlink(3p)
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