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PROLOG
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.
NAME
mkfifo, mkfifoat — make a FIFO special file relative to directory file descriptor
SYNOPSIS
#include <sys/stat.h>
int mkfifo(const char *path, mode_t mode);
int mkfifoat(int fd, const char *path, mode_t mode);
DESCRIPTION
The mkfifo() function shall create a new FIFO special file named by the pathname pointed to by path. The
file permission bits of the new FIFO shall be initialized from mode. The file permission bits of the
mode argument shall be modified by the process' file creation mask.
When bits in mode other than the file permission bits are set, the effect is implementation-defined.
If path names a symbolic link, mkfifo() shall fail and set errno to [EEXIST].
The FIFO's user ID shall be set to the process' effective user ID. The FIFO's group ID shall be 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 need not, provide an implementation-defined way to initialize the FIFO's group ID to the
effective group ID of the calling process.
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 shall be marked for update.
The mkfifoat() function shall be equivalent to the mkfifo() function except in the case where path
specifies a relative path. In this 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 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 mkfifoat() 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 mkfifo().
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.
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 names 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 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.
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);
APPLICATION USAGE
None.
RATIONALE
The syntax of this function is intended to maintain compatibility with historical implementations of
mknod(). The latter function was included in the 1984 /usr/group standard but only for use in creating
FIFO special files. The mknod() function was originally excluded from the POSIX.1‐1988 standard as
implementation-defined and replaced by mkdir() and mkfifo(). The mknod() function is now included for
alignment with the Single UNIX Specification.
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.
The 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.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod(), mknod(), umask()
The Base Definitions volume of POSIX.1‐2008, <sys_stat.h>, <sys_types.h>
COPYRIGHT
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 .