MKFIFO
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
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
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https://www.kernel.org/doc/man-pages/reporting_bugs.html .