`mknod` (3) - glibc

Split from the chapter glibc: File System Interface into the per-function layout; see the method page and the glibc ledger for coverage status. Kernel side: mknod(2); FIFO-only portable subset: mkfifo-mknod-wrappers.md.

Field	Value
Header	`sys/stat.h`
Standards	SVr4, 4.4BSD; POSIX.1-2001 (portable use: FIFO only), POSIX.1-2008
Exported	`mknod@@GLIBC_2.33`, `mknodat@@GLIBC_2.33`; internal `__xmknod@@GLIBC_2.2.5`, `__xmknodat@@GLIBC_2.4` also visible (nm, host glibc 2.41)
Thread-safety	MT-Safe, AS-Safe, AC-Safe (manual safety annotations; `mknod.2` carries no ATTRIBUTES table in man-pages 5.14)
Ledger ID	GLIBC-mknod

Declaration and gates

#include <sys/stat.h>

int mknod(const char *pathname, mode_t mode, dev_t dev);
int mknodat(int dirfd, const char *pathname, mode_t mode, dev_t dev);

mknod sits in #if defined __USE_MISC || defined __USE_XOPEN_EXTENDED (/usr/include/x86_64-linux-gnu/sys/stat.h lines 403-407, prototype at 404), matching mknod.2’s _DEFAULT_SOURCE/_XOPEN_SOURCE >= 500 gate; mknodat (line 411) adds __USE_ATFILE. The wrapper reaches the kernel through the internal __xmknod versioned shim (nm; manual safety comment “we go through xmknod”).

Semantics

Creates a filesystem node named pathname: mode combines one file type bit – S_IFREG (zero type means regular, created empty), S_IFCHR, S_IFBLK, S_IFIFO, S_IFSOCK – with permission bits, which the umask then masks; dev is the makedev(3)-built device number, honored only for S_IFCHR/S_IFBLK (mknod.2, manual Making Special Files). EEXIST fires if the name exists, including as a symlink. Group ownership follows the parent directory’s set-group-ID bit or BSD group semantics. mknodat (kernel 2.6.16, glibc 2.4) resolves relative paths against dirfd.

Gotchas and portability

POSIX.1-2001’s own words: the only portable use is creating a FIFO; use mkfifo(3) even for that (mknod.2 NOTES, manual).
Directories cannot be made with mknod on Linux – mkdir(2) owns that (mknod.2 NOTES).
Privilege: creating device nodes needs CAP_MKNOD; since Linux 2.4 unprivileged callers may create FIFOs, sockets and regular files (comment in mknod.2 ERRORS). The manual’s Making Special Files still states the stricter “only the superuser can create special files” rule – quoted as the manual’s Hurd-flavored claim.
The manual’s FIXME list flags mknodat as undocumented in the glibc manual; the entry above follows the kernel man page.
musl 1.2.5: mknod prefers SYS_mknod and falls back to mknodat(AT_FDCWD, ...) on architectures without the old slot (src/stat/mknod.c).

Errors

EACCES, EBADF (mknodat), EDQUOT, EEXIST, EFAULT, EINVAL (bad type), ELOOP, ENAMETOOLONG, ENOENT, ENOMEM, ENOSPC, ENOTDIR, EPERM (unprivileged device node, or unsupported node type), EROFS (mknod.2).

References

glibc manual: filesys.texi, node Making Special Files
man2: mknod.2 (covers mknodat)
headers: /usr/include/x86_64-linux-gnu/sys/stat.h lines 403-413
symbols: nm -D --with-symbol-versions libc.so.6

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