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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
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);
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 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.
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 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}.
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.1‐2017, <fcntl.h>, <sys_stat.h>, <sys_types.h>
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 .