path: root/src/process/posix_spawn.c
diff options
authorRich Felker <>2013-08-09 21:03:47 -0400
committerRich Felker <>2013-08-09 21:03:47 -0400
commit3c5c5e6f926feea7b823a96c2872885b100fe31f (patch)
treebec398659ee1f88f8150348940c2a7c843c395fe /src/process/posix_spawn.c
parent65d7aa4dfde9697b93d765f2f736e5f4a38bdfd5 (diff)
optimize posix_spawn to avoid spurious sigaction syscalls
the trick here is that sigaction can track for us which signals have ever had a signal handler set for them, and only those signals need to be considered for reset. this tracking mask may have false positives, since it is impossible to remove bits from it without race conditions. false negatives are not possible since the mask is updated with atomic operations prior to making the sigaction syscall. implementation-internal signals are set to SIG_IGN rather than SIG_DFL so that a signal raised in the parent (e.g. calling pthread_cancel on the thread executing pthread_spawn) does not have any chance make it to the child, where it would cause spurious termination by signal. this change reduces the minimum/typical number of syscalls in the child from around 70 to 4 (including execve). this should greatly improve the performance of posix_spawn and other interfaces which use it (popen and system). to facilitate these changes, sigismember is also changed to return 0 rather than -1 for invalid signals, and to return the actual status of implementation-internal signals. POSIX allows but does not require an error on invalid signal numbers, and in fact returning an error tends to confuse applications which wrongly assume the return value of sigismember is boolean.
Diffstat (limited to 'src/process/posix_spawn.c')
1 files changed, 21 insertions, 7 deletions
diff --git a/src/process/posix_spawn.c b/src/process/posix_spawn.c
index c6886526..eb98f9f6 100644
--- a/src/process/posix_spawn.c
+++ b/src/process/posix_spawn.c
@@ -20,29 +20,43 @@ struct args {
char *const *argv, *const *envp;
+void __get_handler_set(sigset_t *);
static int child(void *args_vp)
int i, ret;
- struct sigaction sa;
+ struct sigaction sa = {0};
struct args *args = args_vp;
int p = args->p[1];
const posix_spawn_file_actions_t *fa = args->fa;
const posix_spawnattr_t *restrict attr = args->attr;
+ sigset_t hset;
/* All signal dispositions must be either SIG_DFL or SIG_IGN
* before signals are unblocked. Otherwise a signal handler
* from the parent might get run in the child while sharing
- * memory, with unpredictable and dangerous results. */
+ * memory, with unpredictable and dangerous results. To
+ * reduce overhead, sigaction has tracked for us which signals
+ * potentially have a signal handler. */
+ __get_handler_set(&hset);
for (i=1; i<_NSIG; i++) {
- __libc_sigaction(i, 0, &sa);
- if (sa.sa_handler!=SIG_DFL && (sa.sa_handler!=SIG_IGN ||
- ((attr->__flags & POSIX_SPAWN_SETSIGDEF)
- && sigismember(&attr->__def, i) ))) {
+ if ((attr->__flags & POSIX_SPAWN_SETSIGDEF)
+ && sigismember(&attr->__def, i)) {
sa.sa_handler = SIG_DFL;
- __libc_sigaction(i, &sa, 0);
+ } else if (sigismember(&hset, i)) {
+ if (i-32<3U) {
+ sa.sa_handler = SIG_IGN;
+ } else {
+ __libc_sigaction(i, 0, &sa);
+ if (sa.sa_handler==SIG_IGN) continue;
+ sa.sa_handler = SIG_DFL;
+ }
+ } else {
+ continue;
+ __libc_sigaction(i, &sa, 0);
if (attr->__flags & POSIX_SPAWN_SETPGROUP)