READLINK(3P)

📦 version 2017a, in alpine 3.22, alpine 3.23, alpine 3.24, arch rolling, nixpkgs unstable, opensuse-leap 16.0

okay bestie, we are in the POSIX Programmer's Manual today, and this one is called readlink (and its chaotic sibling readlinkat). buckle up 🎀

✨ PROLOG ✨

so, tea time ☕ — this manual page is part of the POSIX Programmer's Manual. that means the Linux implementation of this interface may differ (and if you wanna know the Linux deets you gotta go read the corresponding Linux manual page, no cap), or honestly? the interface may not even be implemented on Linux at all. vibe check your platform first, fr fr.

🌸 NAME 🌸

readlink, readlinkat — read the contents of a symbolic link. literally just peeking inside a symlink to see where it's pointing 🦋

💅 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 👑

okay so here's the lore 📖

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 (translation: don't go reading that leftover garbage and thinking it's data, bestie 🙈). If the buf argument is not large enough to contain the link content, the first bufsize bytes shall be placed in buf. so you get a lil sneaky truncated peek 👀

If the value of bufsize is greater than {SSIZE_MAX}, the result is implementation-defined. iykyk, that's the "we don't make promises here" zone 💀

Upon successful completion, readlink() shall mark for update the last data access timestamp of the symbolic link. yes it's touching the atime, she's got receipts ✨

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. main character energy: it does its own thing relative to fd, not to wherever your shell happens to be standing 🧚

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. periodt. no check, no drama 💅

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(). it's giving "same girl, different font" 🫶

🔥 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. so like, if it flops, your buffer is untouched — she didn't even touch your stuff, respectful queen 😭

💜 ERRORS 💜

these functions shall fail if:

  • EACCES — Search permission is denied for a component of the path prefix of path. (you don't have the pass, bestie 🚫)
  • EINVAL — The path argument names a file that is not a symbolic link. (you asked for a symlink, this is not that, wrong girl sorry 💀)
  • 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. (symlinks pointing at symlinks pointing at symlinks… it's a whole cycle 🔁)
  • 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. (bad fd energy 🙈)
  • 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}.
heads up: the following sections are informative. they're the commentary track, not the rules 📝

🌈 EXAMPLES 🌈

The following example shows how to read the name of a symbolic link named /modules/pass1. iconic lil snippet, watch the -1 on that sizeof(buf) by the way — she's leaving room for the null 🎀

#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. this is so important, bestie — readlink does NOT null-terminate for you. you gotta do it yourself, like the independent queen you are 💅

🎀 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. so yeah, the committee said "we're keeping it, it's fine" ✨

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. this is the whole reason readlinkat exists — race conditions are NOT the vibe, and fd-relative is the safe girlie way 🧚✨

⭐ FUTURE DIRECTIONS ⭐

None. nothing. no plans. she's perfect as she is 👑

🌸 SEE ALSO 🌸

fstatat(), symlink()

The Base Definitions volume of POSIX.1‐2017, <fcntl.h>, <unistd.h>

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.

and that's the whole page, bestie 🫶 brought to you by IEEE/The Open Group, circa 2017, filed under READLINK(3P). go forth and resolve your symlinks responsibly, mwah 💅✨

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