From 342cc934a3bf74ac618e2318d738f22ac93257ba Mon Sep 17 00:00:00 2001 From: Adhemerval Zanella Date: Mon, 14 Jun 2021 14:41:31 -0300 Subject: posix: Add terminal control setting support for posix_spawn Currently there is no proper way to set the controlling terminal through posix_spawn in race free manner [1]. This forces shell implementations to keep using fork+exec when launching background process groups, even when using posix_spawn yields better performance. This patch adds a new GNU extension so the creating process can configure the created process terminal group. This is done with a new flag, POSIX_SPAWN_TCSETPGROUP, along with two new attribute functions: posix_spawnattr_tcsetpgrp_np, and posix_spawnattr_tcgetpgrp_np. The function sets a new attribute, spawn-tcgroupfd, that references to the controlling terminal. The controlling terminal is set after the spawn-pgroup attribute, and uses the spawn-tcgroupfd along with current creating process group (so it is composable with POSIX_SPAWN_SETPGROUP). To create a process and set the controlling terminal, one can use the following sequence: posix_spawnattr_t attr; posix_spawnattr_init (&attr); posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP); posix_spawnattr_tcsetpgrp_np (&attr, tcfd); If the idea is also to create a new process groups: posix_spawnattr_t attr; posix_spawnattr_init (&attr); posix_spawnattr_setflags (&attr, POSIX_SPAWN_TCSETPGROUP | POSIX_SPAWN_SETPGROUP); posix_spawnattr_tcsetpgrp_np (&attr, tcfd); posix_spawnattr_setpgroup (&attr, 0); The controlling terminal file descriptor is ignored if the new flag is not set. This interface is slight different than the one provided by QNX [2], which only provides the POSIX_SPAWN_TCSETPGROUP flag. The QNX documentation does not specify how the controlling terminal is obtained nor how it iteracts with POSIX_SPAWN_SETPGROUP. Since a glibc implementation is library based, it is more straightforward and avoid requires additional file descriptor operations to request the caller to setup the controlling terminal file descriptor (and it also allows a bit less error handling by posix_spawn). Checked on x86_64-linux-gnu and i686-linux-gnu. [1] https://github.com/ksh93/ksh/issues/79 [2] https://www.qnx.com/developers/docs/7.0.0/index.html#com.qnx.doc.neutrino.lib_ref/topic/p/posix_spawn.html Reviewed-by: Carlos O'Donell Tested-by: Carlos O'Donell --- NEWS | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'NEWS') diff --git a/NEWS b/NEWS index 07e9eac..a9f25d3 100644 --- a/NEWS +++ b/NEWS @@ -129,6 +129,11 @@ Major new features: * On Linux, the epoll_pwait2 function has been added. It is similar to epoll_wait with the difference the timeout has nanoseconds resolution. +* The functions posix_spawnattr_tcsetpgrp_np and posix_spawnattr_tcgetpgrp_np + have been added, enabling posix_spawn and posix_spawnp to set the + controlling terminal in the new process in a race free manner. These + functions are GNU extensions. + Deprecated and removed features, and other changes affecting compatibility: * On x86-64, the LD_PREFER_MAP_32BIT_EXEC environment variable support -- cgit v1.1