mknod (3p) - Linux Manuals

mknod: make a directory, a special file, or a regular file

Command to display mknod manual in Linux: $ man 3p mknod

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

mknod - make a directory, a special file, or a regular file

SYNOPSIS

#include <sys/stat.h>

int mknod(const char *path, mode_t mode, dev_t dev);

DESCRIPTION

The mknod() function shall create a new file named by the pathname to which the argument path points.

The file type for path is OR'ed into the mode argument, and the application shall select one of the following symbolic constants:

Name	Description
S_IFIFO	FIFO-special
S_IFCHR	Character-special (non-portable)
S_IFDIR	Directory (non-portable)
S_IFBLK	Block-special (non-portable)
S_IFREG	Regular (non-portable)

The only portable use of mknod() is to create a FIFO-special file. If mode is not S_IFIFO or dev is not 0, the behavior of mknod() is unspecified.

The permissions for the new file are OR'ed into the mode argument, and may be selected from any combination of the following symbolic constants:

Name	Description
S_ISUID	Set user ID on execution.
S_ISGID	Set group ID on execution.
S_IRWXU	Read, write, or execute (search) by owner.
S_IRUSR	Read by owner.
S_IWUSR	Write by owner.
S_IXUSR	Execute (search) by owner.
S_IRWXG	Read, write, or execute (search) by group.
S_IRGRP	Read by group.
S_IWGRP	Write by group.
S_IXGRP	Execute (search) by group.
S_IRWXO	Read, write, or execute (search) by others.
S_IROTH	Read by others.
S_IWOTH	Write by others.
S_IXOTH	Execute (search) by others.
S_ISVTX	On directories, restricted deletion flag.

The user ID of the file shall be initialized to the effective user ID of the process. The group ID of the file shall be initialized to either the effective group ID of the process or the group ID of the parent directory. Implementations shall provide a way to initialize the file's group ID to the group ID of the parent directory. Implementations may, but need not, provide an implementation-defined way to initialize the file's group ID to the effective group ID of the calling process. The owner, group, and other permission bits of mode shall be modified by the file mode creation mask of the process. The mknod() function shall clear each bit whose corresponding bit in the file mode creation mask of the process is set.

If path names a symbolic link, mknod() shall fail and set errno to [EEXIST].

Upon successful completion, mknod() 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.

Only a process with appropriate privileges may invoke mknod() for file types other than FIFO-special.

RETURN VALUE

Upon successful completion, mknod() shall return 0. Otherwise, it shall return -1, the new file shall not be created, and errno shall be set to indicate the error.

ERRORS

The mknod() function shall fail if:

EACCES: A component of the path prefix denies search permission, or write permission is denied on the parent directory.
EEXIST: The named file exists.
EINVAL: An invalid argument exists.
EIO: An I/O error occurred while accessing the file system.
ELOOP: A loop exists in symbolic links encountered during resolution of the path argument.
ENAMETOOLONG: The length of a pathname 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.
EPERM: The invoking process does not have appropriate privileges and the file type is not FIFO-special.
EROFS: The directory in which the file is to be created is located on a read-only file system.

The mknod() function may fail if:

ELOOP: More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
ENAMETOOLONG: Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.

The following sections are informative.

EXAMPLES

Creating a FIFO Special File

The following example shows how to create a FIFO special 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>


dev_t dev;
int   status;
...
status  = mknod("/home/cnd/mod_done", S_IFIFO | S_IWUSR |
    S_IRUSR | S_IRGRP | S_IROTH, dev);

APPLICATION USAGE

The mkfifo() function is preferred over this function for making FIFO special files.

RATIONALE

The POSIX.1-1990 standard required that the group ID of a newly created file 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 file is created, or determine under what conditions the implementation will set the desired group ID.

FUTURE DIRECTIONS

None.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .