posix_spawn and posix_spawnp - glibc

Split from the chapter glibc: Process, Threads and Signals into the per-function layout; see the method page and the glibc ledger for coverage status.

Semantics – spawn a child that runs path (posix_spawn) or searches PATH like execvp (posix_spawnp), performing a bounded, declarative set of pre-exec housekeeping. Return convention is unlike most libc: 0 or an error number (no errno), and only the clone step can fail at the parent; if any housekeeping or the exec itself fails in the child, the child exits with status 127 – the caller discovers this only via waitpid/WEXITSTATUS.

file_actions – posix_spawn_file_actions_init then addopen/addclose/adddup2 (POSIX) or addchdir_np/addfchdir_np/ addclosefrom_np/addtcsetpgrp_np (GNU, _MISC), executed in child in addition order, then FD_CLOEXEC descriptors close.

attributes (posix_spawnattr_*) – flag-gated via setflags: POSIX_SPAWN_SETSIGMASK, SETSIGDEF, SETPGROUP (pgroup 0 = new group of its own), SETSCHEDPARAM/SETSCHEDULER (before glibc 2.14 requesting the scheduler without parameters failed – bug 12052), RESETIDS, and GNU POSIX_SPAWN_SETSID (2.26), SETCGROUP (cgroup v2 fd), USEVFORK (no-op since 2.24). POSIX_SPAWN_SETSCHEDULER needs _GNU_SOURCE for SETSID/SETCGROUP/USEVFORK per spawn.h.

glibc layer / why preferred – since 2.24 the child is created with clone(CLONE_VM|CLONE_VFORK): no page-table copy, parent suspended until exec, and – unlike fork() – no pthread_atfork handlers run (POSIX leaves this unspecified; glibc 2.24+ never runs them). That makes it deterministic in libraries, and MMU-less systems can implement it. There is no posix_fexecve; pass a /proc/self/fd/N path (or use GNU pidfd_spawn/pidfd_spawnp, glibc 2.41 headers, bits/spawn_ext.h).

References – posix_spawn(3), spawn.h; syscall view: execve.