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MKNOD(2) Linux Programmer's Manual MKNOD(2)


mknod, mknodat - create a special or ordinary file


#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int mknod(const char *pathname, mode_t mode, dev_t dev);
#include <fcntl.h> /* Definition of AT_* constants */ #include <sys/stat.h>
int mknodat(int dirfd, const char *pathname, mode_t mode, dev_t dev);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE


The system call mknod() creates a filesystem node (file, device special file, or named pipe) named pathname, with attributes specified by mode and dev.
The mode argument specifies both the file mode to use and the type of node to be created. It should be a combination (using bitwise OR) of one of the file types listed below and zero or more of the file mode bits listed in inode(7).
The file mode is modified by the process's umask in the usual way: in the absence of a default ACL, the permissions of the created node are (mode & ~umask).
The file type must be one of S_IFREG, S_IFCHR, S_IFBLK, S_IFIFO, or S_IFSOCK to specify a regular file (which will be created empty), character special file, block special file, FIFO (named pipe), or UNIX domain socket, respectively. (Zero file type is equivalent to type S_IFREG.)
If the file type is S_IFCHR or S_IFBLK, then dev specifies the major and minor numbers of the newly created device special file (makedev(3) may be useful to build the value for dev); otherwise it is ignored.
If pathname already exists, or is a symbolic link, this call fails with an EEXIST error.
The newly created node will be owned by the effective user ID of the process. If the directory containing the node has the set-group-ID bit set, or if the filesystem is mounted with BSD group semantics, the new node will inherit the group ownership from its parent directory; otherwise it will be owned by the effective group ID of the process.


The mknodat() system call operates in exactly the same way as mknod(), except for the differences described here.
If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by mknod() for a relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like mknod()).
If pathname is absolute, then dirfd is ignored.
See openat(2) for an explanation of the need for mknodat().


mknod() and mknodat() return zero on success, or -1 if an error occurred (in which case, errno is set appropriately).


The parent directory does not allow write permission to the process, or one of the directories in the path prefix of pathname did not allow search permission. (See also path_resolution(7).)
The user's quota of disk blocks or inodes on the filesystem has been exhausted.
pathname already exists. This includes the case where pathname is a symbolic link, dangling or not.
pathname points outside your accessible address space.
mode requested creation of something other than a regular file, device special file, FIFO or socket.
Too many symbolic links were encountered in resolving pathname.
pathname was too long.
A directory component in pathname does not exist or is a dangling symbolic link.
Insufficient kernel memory was available.
The device containing pathname has no room for the new node.
A component used as a directory in pathname is not, in fact, a directory.
mode requested creation of something other than a regular file, FIFO (named pipe), or UNIX domain socket, and the caller is not privileged (Linux: does not have the CAP_MKNOD capability); also returned if the filesystem containing pathname does not support the type of node requested.
pathname refers to a file on a read-only filesystem.
The following additional errors can occur for mknodat():
dirfd is not a valid file descriptor.
pathname is relative and dirfd is a file descriptor referring to a file other than a directory.


mknodat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.


mknod(): SVr4, 4.4BSD, POSIX.1-2001 (but see below), POSIX.1-2008.
mknodat(): POSIX.1-2008.


POSIX.1-2001 says: "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." However, nowadays one should never use mknod() for this purpose; one should use mkfifo(3), a function especially defined for this purpose.
Under Linux, mknod() cannot be used to create directories. One should make directories with mkdir(2).
There are many infelicities in the protocol underlying NFS. Some of these affect mknod() and mknodat().


mknod(1), chmod(2), chown(2), fcntl(2), mkdir(2), mount(2), socket(2), stat(2), umask(2), unlink(2), makedev(3), mkfifo(3), acl(5) path_resolution(7)


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2017-09-15 Linux