READLINK(3POSIX)

💖 READLINK(3POSIX) 💖

okay bestie, welcome to the POSIX Programmer's Manual, the READLINK(3POSIX) page 🎀

⚠️ real talk before we start: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so go check the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface may not be implemented on Linux at all. 🙈

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✨ PROLOG ✨

Yeah so — this page lives in the POSIX Programmer's Manual. Linux might do its own thing with this interface, so consult the matching Linux manual page if you need the Linux specifics, or plot twist: the interface might not even exist on Linux. No cap.

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🌸 NAME 🌸

readlink, readlinkat — read the contents of a symbolic link 🦋

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👑 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);

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💅 DESCRIPTION 💅

So readlink() is gonna take the contents of the symbolic link that path points at and drop them into the buffer buf, which has size bufsize. Iconic. If the number of bytes in the symbolic link is less than bufsize, then the rest of buf is just… unspecified, bestie. We don't know what's in there. 🙈 And if buf is not big enough to hold the link content? Then the first bufsize bytes get placed in buf. That's it, that's the deal.

If the value of bufsize is greater than {SSIZE_MAX}, the result is implementation-defined — so like, don't. 💀

Upon successful completion, readlink() shall mark for update the last data access timestamp of the symbolic link. ✨

Now readlinkat() is basically the same vibe as readlink() except when path specifies a relative path. In that 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, but make it directory-relative. 🫶

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. But if the access mode is O_SEARCH, the function shall not perform that check. Periodt.

And if you pass readlinkat() the special value AT_FDCWD in the fd parameter? Then the current working directory shall be used, and the behavior shall be identical to a call to readlink(). Slay, same function basically. 💖

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🔥 RETURN VALUE 🔥

Upon successful completion, these functions shall return the count of bytes placed in the buffer. Otherwise? They shall return a value of -1, leave the buffer unchanged, and set errno to indicate the error. So yeah, on failure your buffer is untouched — no half-written chaos. 🙅

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🌈 ERRORS 🌈

These functions shall fail if:

• EACCES — Search permission is denied for a component of the path prefix of path. rude tbh.
• EINVAL — The path argument names a file that is not a symbolic link. bestie that's not a symlink, what are you doing 😭
• 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. nothing there, ghosted 👻
• 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. (Yeah, that one's a mouthful, read it twice. 💀)

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}.

📌 the following sections are informative — so like, they're the commentary track, not the binding rules. 💅

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🎀 EXAMPLES 🎀

Reading the Name of a Symbolic Link

okay so here's how you 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';

look at her go 🫶

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🧚 APPLICATION USAGE 🧚

Conforming applications should not assume that the returned contents of the symbolic link are null-terminated. like, do not assume. fr fr. that's why the example above slaps a '\0' on there itself. 💅

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💜 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 committee said nope 💀

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. that's the whole glow-up, race-condition-free 🏆

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⭐ FUTURE DIRECTIONS ⭐

None. (a rare calm moment, enjoy it 🦋)

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🌸 SEE ALSO 🌸

• fstatat()
• symlink()

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

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📜 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 .

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and that's the whole page, bestie!! 🎀 written by IEEE/The Open Group, dated 2017, and it's the READLINK(3POSIX) page from the POSIX Programmer's Manual — go forth and read those symlinks responsibly, mwah 💖✨🦋