MKNOD(3P) POSIX Programmer's Manual MKNOD(3P)
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, mknodat -- make directory, special file, or regular file
SYNOPSIS
#include <sys/stat.h>
int mknod(const char *path, mode_t mode, dev_t dev);
int mknodat(int fd, 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 appli-
cation 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 unspec-
ified.
The permissions for the new file are OR'ed into the mode argument, and
may be selected from any combination of the following symbolic con-
stants:
+-----------+---------------------------------------------+
| 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 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.
Only a process with appropriate privileges may invoke mknod() for file
types other than FIFO-special.
The mknodat() function shall be equivalent to the mknod() function
except in the case where path specifies a relative path. In this case
the newly created directory, special file, or regular file is located
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 mknodat() 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 mknod().
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, the new file shall not 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.
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 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.
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 mknodat() 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 resolu-
tion of a symbolic link produced an intermediate result with a
length that 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 implementa-
tions provide a way to have the group ID be set to the group ID of the
containing directory, but did not prohibit implementations also sup-
porting a way to set the group ID to the effective group ID of the cre-
ating 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 condi-
tions the implementation will set the desired group ID.
The purpose of the mknodat() function is to create directories, special
files, or regular files 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 mknod(), resulting in
unspecified behavior. By opening a file descriptor for the target
directory and using the mknodat() function it can be guaranteed that
the newly created directory, special file, or regular file is located
relative to the desired directory.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod(), creat(), exec, fstatat(), mkdir(), mkfifo(), open(), umask()
The Base Definitions volume of POSIX.1-2008, <sys_stat.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 Electri-
cal 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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 MKNOD(3P)
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