MKFIFO(3P)

🌸 POSIX Programmer's Manual 🌸 — the mkfifo(3P) era

okay bestie, before we even start: this page lives inside the POSIX Programmer's Manual, and the Linux version of this interface might not behave the same way (so like, go read the Linux man page for the actual Linux tea), or honestly it might not even be implemented on Linux at all. just so you know 💅

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✨ NAME ✨

mkfifo, mkfifoat — make a FIFO special file

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🎀 SYNOPSIS 🎀

#include <sys/stat.h>

int mkfifo(const char *path, mode_t mode);

#include <fcntl.h>

int mkfifoat(int fd, const char *path, mode_t mode);

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🦋 DESCRIPTION 🦋

so mkfifo() is gonna create a brand new FIFO special file, and it's named by the pathname that path is pointing at. the file permission bits of this new FIFO get initialized from mode, and then — plot twist — those permission bits from the mode argument get modified by the process' file creation mask. yeah, the umask said not today 💀

when bits in mode other than the file permission bits are set, the effect is implementation-defined. (iykyk, don't count on it being cute.)

if path names a symbolic link, mkfifo() shall fail and set errno to [EEXIST]. no symlink shenanigans allowed, periodt.

the FIFO's user ID gets set to the process' effective user ID. the FIFO's group ID gets set to the group ID of the parent directory or to the effective group ID of the process. implementations shall provide a way to initialize the FIFO's group ID to the group ID of the parent directory. implementations may, but do not have to, provide an implementation-defined way to initialize the FIFO's group ID to the effective group ID of the calling process. (yes that's a lot of group ID talk, it matters, stay with me 🫶)

upon successful completion, mkfifo() shall mark for update the last data access, last data modification, and last file status change timestamps of the file. also, the last data modification and last file status change timestamps of the directory that contains the new entry get marked for update. timestamps are literally the main characters here.

now, mkfifoat() is equivalent to mkfifo() except in the case where path specifies a relative path. in that case the newly created FIFO is created 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.

and if you pass mkfifoat() 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 mkfifo(). iconic, honestly.

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💖 RETURN VALUE 💖

upon successful completion, these functions shall return 0. otherwise, these functions shall return -1 and set errno to indicate the error. if -1 is returned, no FIFO shall be created. (so like, no half-created mess, we love a clean failure.)

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🔥 ERRORS 🔥

these functions shall fail if:

• EACCES — a component of the path prefix denies search permission, or write permission is denied on the parent directory of the FIFO to be created.
• EEXIST — the named file already exists.
• 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 the path prefix of path does not name an existing file or path is an empty string.
• ENOENT or ENOTDIR — the path argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters. if path without the trailing <slash> characters would name an existing file, an [ENOENT] error shall not occur.
• ENOSPC — the directory that would contain the new file cannot be extended or the file system is out of file-allocation resources.
• ENOTDIR — a component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory.
• EROFS — the named file resides on a read-only file system.

the mkfifoat() 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}.

also heads up: the following sections are informative. they're the vibes, not the law. 📖

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🌈 EXAMPLES 🌈

Creating a FIFO File

the following example shows how to create a FIFO file named /home/cnd/mod_done, with read/write permissions for owner, and with read permissions for group and others.

#include <sys/types.h>
#include <sys/stat.h>

int status;
...
status = mkfifo("/home/cnd/mod_done", S_IWUSR | S_IRUSR |
    S_IRGRP | S_IROTH);

look at that bitwise OR glow-up ✨

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💅 APPLICATION USAGE 💅

None.

(not a single note. she said nothing and left. respect.)

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🧚 RATIONALE 🧚

the syntax of this function is intended to maintain compatibility with historical implementations of mknod(). that function was included in the 1984 /usr/group standard, but only for use in creating FIFO special files. mknod() was originally excluded from the POSIX.1‐1988 standard as implementation-defined and replaced by mkdir() and mkfifo(). mknod() is now included for alignment with the Single UNIX Specification. (character development fr fr.)

the POSIX.1‐1990 standard required that the group ID of a newly created FIFO be set to the group ID of its parent directory or to the effective group ID of the creating process. FIPS 151‐2 required that implementations provide a way to have the group ID be set to the group ID of the containing directory, but did not prohibit implementations also supporting a way to set the group ID to the effective group ID of the creating process. conforming applications should not assume which group ID will be used. if it matters, an application can use chown() to set the group ID after the FIFO is created, or determine under what conditions the implementation will set the desired group ID. (tl;dr: don't guess, verify 💀)

the whole purpose of the mkfifoat() function is to create a FIFO special file 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 mkfifo(), resulting in unspecified behavior. by opening a file descriptor for the target directory and using the mkfifoat() function, it can be guaranteed that the newly created FIFO is located relative to the desired directory. no cap, that's the whole point.

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⭐ FUTURE DIRECTIONS ⭐

None.

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👑 SEE ALSO 👑

chmod(), mknod(), umask()

the Base Definitions volume of POSIX.1‐2017: <fcntl.h>, <sys_stat.h>, <sys_types.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 🫶 this one came from IEEE / The Open Group, dated 2017, and the manual page you just read is MKFIFO(3P) — go make some FIFOs, you absolute icon 🎀💖✨