mknod(3p) — Linux manual page

PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

MKNOD(3P)               POSIX Programmer's Manual              MKNOD(3P)

PROLOG         top

       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         top

       mknod, mknodat — make directory, special file, or regular file

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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().

RETURN VALUE         top

       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         top

       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.

EXAMPLES         top

   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         top

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

RATIONALE         top

       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.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       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)

COPYRIGHT         top

       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)