.\" Copyright (c) 1983, 1991 The Regents of the University of California. .\" And Copyright (C) 2011 Guillem Jover .\" And Copyright (C) 2006, 2014 Michael Kerrisk .\" All rights reserved. .\" .\" SPDX-License-Identifier: BSD-4-Clause-UC .\" .\" @(#)readlink.2 6.8 (Berkeley) 3/10/91 .\" .\" Modified Sat Jul 24 00:10:21 1993 by Rik Faith (faith@cs.unc.edu) .\" Modified Tue Jul 9 23:55:17 1996 by aeb .\" Modified Fri Jan 24 00:26:00 1997 by aeb .\" 2011-09-20, Guillem Jover : .\" Added text on dynamically allocating buffer + example program .\" .TH readlink 2 2023-03-30 "Linux man-pages 6.04" .SH NAME readlink, readlinkat \- read value of a symbolic link .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .PP .BI "ssize_t readlink(const char *restrict " pathname ", char *restrict " buf , .BI " size_t " bufsiz ); .PP .BR "#include " "/* Definition of " AT_* " constants */" .B #include .PP .BI "ssize_t readlinkat(int " dirfd ", const char *restrict " pathname , .BI " char *restrict " buf ", size_t " bufsiz ); .PP .fi .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .PP .BR readlink (): .nf _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L .\" || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || /* glibc <= 2.19: */ _BSD_SOURCE .fi .PP .BR readlinkat (): .nf Since glibc 2.10: _POSIX_C_SOURCE >= 200809L Before glibc 2.10: _ATFILE_SOURCE .fi .SH DESCRIPTION .BR readlink () places the contents of the symbolic link .I pathname in the buffer .IR buf , which has size .IR bufsiz . .BR readlink () does not append a terminating null byte to .IR buf . It will (silently) truncate the contents (to a length of .I bufsiz characters), in case the buffer is too small to hold all of the contents. .SS readlinkat() The .BR readlinkat () system call operates in exactly the same way as .BR readlink (), except for the differences described here. .PP If the pathname given in .I pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor .I dirfd (rather than relative to the current working directory of the calling process, as is done by .BR readlink () for a relative pathname). .PP If .I pathname is relative and .I dirfd is the special value .BR AT_FDCWD , then .I pathname is interpreted relative to the current working directory of the calling process (like .BR readlink ()). .PP If .I pathname is absolute, then .I dirfd is ignored. .PP Since Linux 2.6.39, .\" commit 65cfc6722361570bfe255698d9cd4dccaf47570d .I pathname can be an empty string, in which case the call operates on the symbolic link referred to by .I dirfd (which should have been obtained using .BR open (2) with the .B O_PATH and .B O_NOFOLLOW flags). .PP See .BR openat (2) for an explanation of the need for .BR readlinkat (). .SH RETURN VALUE On success, these calls return the number of bytes placed in .IR buf . (If the returned value equals .IR bufsiz , then truncation may have occurred.) On error, \-1 is returned and .I errno is set to indicate the error. .SH ERRORS .TP .B EACCES Search permission is denied for a component of the path prefix. (See also .BR path_resolution (7).) .TP .B EBADF .RB ( readlinkat ()) .I pathname is relative but .I dirfd is neither .B AT_FDCWD nor a valid file descriptor. .TP .B EFAULT .I buf extends outside the process's allocated address space. .TP .B EINVAL .I bufsiz is not positive. .\" At the glibc level, bufsiz is unsigned, so this error can only occur .\" if bufsiz==0. However, the in the kernel syscall, bufsiz is signed, .\" and this error can also occur if bufsiz < 0. .\" See: http://thread.gmane.org/gmane.linux.man/380 .\" Subject: [patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall? .TP .B EINVAL The named file (i.e., the final filename component of .IR pathname ) is not a symbolic link. .TP .B EIO An I/O error occurred while reading from the filesystem. .TP .B ELOOP Too many symbolic links were encountered in translating the pathname. .TP .B ENAMETOOLONG A pathname, or a component of a pathname, was too long. .TP .B ENOENT The named file does not exist. .TP .B ENOMEM Insufficient kernel memory was available. .TP .B ENOTDIR A component of the path prefix is not a directory. .TP .B ENOTDIR .RB ( readlinkat ()) .I pathname is relative and .I dirfd is a file descriptor referring to a file other than a directory. .SH STANDARDS POSIX.1-2008. .SH HISTORY .TP .BR readlink () 4.4BSD (first appeared in 4.2BSD), POSIX.1-2001, POSIX.1-2008. .TP .BR readlinkat () POSIX.1-2008. Linux 2.6.16, glibc 2.4. .PP Up to and including glibc 2.4, the return type of .BR readlink () was declared as .IR int . Nowadays, the return type is declared as .IR ssize_t , as (newly) required in POSIX.1-2001. .SS glibc On older kernels where .BR readlinkat () is unavailable, the glibc wrapper function falls back to the use of .BR readlink (). When .I pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in .I /proc/self/fd that corresponds to the .I dirfd argument. .SH NOTES Using a statically sized buffer might not provide enough room for the symbolic link contents. The required size for the buffer can be obtained from the .I stat.st_size value returned by a call to .BR lstat (2) on the link. However, the number of bytes written by .BR readlink () and .BR readlinkat () should be checked to make sure that the size of the symbolic link did not increase between the calls. Dynamically allocating the buffer for .BR readlink () and .BR readlinkat () also addresses a common portability problem when using .B PATH_MAX for the buffer size, as this constant is not guaranteed to be defined per POSIX if the system does not have such limit. .SH EXAMPLES The following program allocates the buffer needed by .BR readlink () dynamically from the information provided by .BR lstat (2), falling back to a buffer of size .B PATH_MAX in cases where .BR lstat (2) reports a size of zero. .PP .\" SRC BEGIN (readlink.c) .EX #include #include #include #include #include int main(int argc, char *argv[]) { char *buf; ssize_t nbytes, bufsiz; struct stat sb; if (argc != 2) { fprintf(stderr, "Usage: %s \en", argv[0]); exit(EXIT_FAILURE); } if (lstat(argv[1], &sb) == \-1) { perror("lstat"); exit(EXIT_FAILURE); } /* Add one to the link size, so that we can determine whether the buffer returned by readlink() was truncated. */ bufsiz = sb.st_size + 1; /* Some magic symlinks under (for example) /proc and /sys report \[aq]st_size\[aq] as zero. In that case, take PATH_MAX as a "good enough" estimate. */ if (sb.st_size == 0) bufsiz = PATH_MAX; buf = malloc(bufsiz); if (buf == NULL) { perror("malloc"); exit(EXIT_FAILURE); } nbytes = readlink(argv[1], buf, bufsiz); if (nbytes == \-1) { perror("readlink"); exit(EXIT_FAILURE); } /* Print only \[aq]nbytes\[aq] of \[aq]buf\[aq], as it doesn't contain a terminating null byte (\[aq]\e0\[aq]). */ printf("\[aq]%s\[aq] points to \[aq]%.*s\[aq]\en", argv[1], (int) nbytes, buf); /* If the return value was equal to the buffer size, then the the link target was larger than expected (perhaps because the target was changed between the call to lstat() and the call to readlink()). Warn the user that the returned target may have been truncated. */ if (nbytes == bufsiz) printf("(Returned buffer may have been truncated)\en"); free(buf); exit(EXIT_SUCCESS); } .EE .\" SRC END .SH SEE ALSO .BR readlink (1), .BR lstat (2), .BR stat (2), .BR symlink (2), .BR realpath (3), .BR path_resolution (7), .BR symlink (7)