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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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MKNOD(3P) POSIX Programmer's Manual MKNOD(3P)
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.
mknod, mknodat — make directory, special file, or regular file
#include <sys/stat.h>
int mknod(const char *path, mode_t mode, dev_t dev);
#include <fcntl.h>
int mknodat(int fd, const char *path, mode_t mode, dev_t dev);
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 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
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 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().
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.
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 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.
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 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.
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);
The mkfifo() function is preferred over this function for making
FIFO special files.
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.
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.
None.
chmod(3p), creat(3p), exec(1p), fstatat(3p), mkdir(3p),
mkfifo(3p), open(3p), umask(3p)
The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),
sys_stat.h(0p)
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 .
IEEE/The Open Group 2017 MKNOD(3P)
Pages that refer to this page: sys_stat.h(0p), chmod(3p), creat(3p), exec(3p), fchmod(3p), fstatat(3p), fstatvfs(3p), mkdir(3p), mkfifo(3p), open(3p), umask(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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