openpty / forkpty (3) - glibc

Split from the chapter glibc: Low-Level Terminal Interface into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <pty.h>

int openpty(int *amaster, int *aslave, char *name,
            const struct termios *termp, const struct winsize *winp);
int forkpty(int *amaster, char *name,
            const struct termios *termp, const struct winsize *winp);

Both are declared unconditionally in /usr/include/pty.h (lines 33-42), after including <termios.h> and <sys/ioctl.h>. The man page synopsis still says “Link with -lutil”.

Semantics

openpty finds an available pseudoterminal and returns descriptors for the master in *amaster and the slave in *aslave; if name is not NULL the slave’s filename is stored there, and non-NULL termp/winp set the slave’s initial termios attributes and window size. On failure both functions return -1 with errno set; openpty fails with ENOENT when no terminals are available (openpty(3)). forkpty combines openpty, fork(2) and login_tty(3): the child gets the slave as controlling terminal and stdio and returns 0 there, the parent returns the child pid with the master descriptor in *amaster (openpty(3)).

Gotchas and portability

• libutil is gone as a real library: since glibc 2.34 openpty/ forkpty moved into libc proper – nm shows @@GLIBC_2.34 default versions with @GLIBC_2.2.5 compat twins, and host libutil.so.1 exports only __libutil_version_placeholder (nm, host). Linking -lutil still works through the stub.
• login_tty(3) (declared in utmp.h, not covered by a ledger row here) is the third function of the pty.h-family man page.
• musl 1.2.5: pty.h declares both and they live directly in libc (musl src/misc/openpty.c, forkpty.c) – no libutil ever existed there.

References

• glibc manual: terminal.texi (pty allocation)
• man3: openpty(3) (covers forkpty, login_tty); man4: pts(4)
• headers: /usr/include/pty.h
• symbols: nm -D --with-symbol-versions libc.so.6, nm -D libutil.so.1