NAME

mkfifo - make a FIFO special file

SYNOPSIS

#include <sys/stat.h>

int mkfifo(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 st_atime, st_ctime, and st_mtime fields of the file. Also, the st_ctime and st_mtime fields of the directory that contains the new entry shall be marked for update.

RETURN VALUE

Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned, no FIFO shall be created, and errno shall be set to indicate the error.

ERRORS

The mkfifo() function 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 the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.

[ENOENT]

A component of the path prefix specified by path does not name an existing directory or path is an empty string.

[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 is not a directory.

[EROFS]

The named file resides on a read-only file system.

The mkfifo() function may fail if:

[ELOOP]

More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.

[ENAMETOOLONG]

As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname string exceeded {PATH_MAX}.

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.

FUTURE DIRECTIONS

None.

SEE ALSO

umask(), the Base Definitions volume of IEEE Std 1003.1-2001, <sys/stat.h>, <sys/types.h>

CHANGE HISTORY

First released in Issue 3. Included for alignment with the POSIX.1-1988 standard.

Issue 6

In the SYNOPSIS, the optional include of the <sys/types.h> header is removed.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• The requirement to include <sys/types.h> has been removed. Although <sys/types.h> was required for conforming implementations of previous POSIX specifications, it was not required for UNIX applications.

• The [ELOOP] mandatory error condition is added.

• A second [ENAMETOOLONG] is added as an optional error condition.

The following changes were made to align with the IEEE P1003.1a draft standard:

• The [ELOOP] optional error condition is added.