What's going on here?


S

subnet

I'm trying to write a very simple program that uses a signal-based
synchronization between two processes. Here's it:
-----------------------------------------------
/* The world's simplest syncronization example */
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>

void handler(int sig);
void wait_signal();

int main() {
sigset_t curr_mask;
struct sigaction act;
pid_t pid;

act.sa_handler = handler;
sigaction(SIGUSR1, &act, NULL);

sigprocmask(0, NULL, &curr_mask);
sigaddset(&curr_mask, SIGUSR1);
sigprocmask(SIG_SETMASK, &curr_mask, NULL);

pid = fork();

/* Strict alternation */
if (pid > 0) {
while(1) {
printf("Parent\n");
/* Tell the child we're done */
kill(pid, SIGUSR1);
wait_signal();
}
}
else {
pid_t parent_pid = getppid();

while(1) {
wait_signal();
printf("Child\n");
/* Tell the parent we're done */
kill(parent_pid, SIGUSR1);
}
}
}

void handler(int sig) {
/* do nothing, just used for sync */
}

void wait_signal() {
sigset_t mask;
sigemptyset(&mask);
sigsuspend(&mask);
}
-----------------------------------------------

With a 2.6 linux kernel, this hangs miserably with this output:

Parent
Child
Parent

Doing an strace yelds the following result:
-------------------
execve("./myprog", ["./myprog"], [/* 56 vars */]) = 0
uname({sys="Linux", node="scooter", ...}) = 0
brk(0) = 0x501000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x2a9556b000
open("/etc/ld.so.preload", O_RDONLY) = -1 ENOENT (No such file or
directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=62108, ...}) = 0
mmap(NULL, 62108, PROT_READ, MAP_PRIVATE, 3, 0) = 0x2a9556c000
close(3) = 0
open("/lib/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0`\315\1\0"...,
640) = 640
lseek(3, 624, SEEK_SET) = 624
read(3, "\4\0\0\0\20\0\0\0\1\0\0\0GNU\0\0\0\0\0\2\0\0\0\6\0\0\0"...,
32) = 32
fstat(3, {st_mode=S_IFREG|0755, st_size=1232856, ...}) = 0
mmap(NULL, 2215816, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) =
0x2a9566b000
mprotect(0, 1085320, PROT_NONE) = -1 ENOMEM (Cannot allocate
memory)
mmap(0x2a9586b000, 102400, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED, 3, 0x100000) = 0x2a9586b000
mmap(0x2a95884000, 16264, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x2a95884000
close(3) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x2a95888000
arch_prctl(0x1002, 0x2a95888640) = 0
munmap(0x2a9556c000, 62108) = 0
open("/dev/urandom", O_RDONLY) = 3
read(3, "\304fd\200\34\2727f\333\316\335\370\334\251\332Pi\235-"...,
64) = 64
close(3) = 0
rt_sigaction(SIGUSR1, {0x400819, [HUP],
SA_RESTART|SA_ONESHOT|SA_NOCLDSTOP|0x555fa10}, NULL, 8) = 0
rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0
rt_sigprocmask(SIG_SETMASK, [USR1], NULL, 8) = 0
clone(child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
child_tidptr=0x2a958886d0) = 16791
fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x2a9556c000
write(1, "Parent\n", 7Parent
) = 7
kill(16791, SIGUSR1) = 0
rt_sigsuspend([]Child
<unfinished ...>
--- SIGUSR1 (User defined signal 1) @ 0 (0) ---
<... rt_sigsuspend resumed> ) = -1 EINTR (Interrupted system
call)
rt_sigreturn(0xa) = -1 EINTR (Interrupted system
call)
write(1, "Parent\n", 7Parent
) = 7
kill(16791, SIGUSR1) = 0
rt_sigsuspend([] <unfinished ...>
--- SIGCHLD (Child exited) @ 0 (0) ---
----------------------------------------
And that's why it hangs, of course. The call to rt_sigaction shows
that it's called with the flag (among the others) SA_ONESHOT.

The funny part comes now: if I modify line 11 from

sigset_t curr_mask;

to, say

sigset_t curr_mask, mask1, mask2; /* add two unused variables */

it runs just fine!!

Here's the strace output (only the beginning, of course)
-------------------------------------------------
execve("./myprog", ["./myprog"], [/* 56 vars */]) = 0
uname({sys="Linux", node="scooter", ...}) = 0
brk(0) = 0x501000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x2a9556b000
open("/etc/ld.so.preload", O_RDONLY) = -1 ENOENT (No such file or
directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=62108, ...}) = 0
mmap(NULL, 62108, PROT_READ, MAP_PRIVATE, 3, 0) = 0x2a9556c000
close(3) = 0
open("/lib/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0`\315\1\0"...,
640) = 640
lseek(3, 624, SEEK_SET) = 624
read(3, "\4\0\0\0\20\0\0\0\1\0\0\0GNU\0\0\0\0\0\2\0\0\0\6\0\0\0"...,
32) = 32
fstat(3, {st_mode=S_IFREG|0755, st_size=1232856, ...}) = 0
mmap(NULL, 2215816, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) =
0x2a9566b000
mprotect(0, 1085320, PROT_NONE) = -1 ENOMEM (Cannot allocate
memory)
mmap(0x2a9586b000, 102400, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED, 3, 0x100000) = 0x2a9586b000
mmap(0x2a95884000, 16264, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x2a95884000
close(3) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x2a95888000
arch_prctl(0x1002, 0x2a95888640) = 0
munmap(0x2a9556c000, 62108) = 0
open("/dev/urandom", O_RDONLY) = 3
read(3, "\235!\fl\347\370Y\20}$\4\232N97\263W\201\317\273\4\230"...,
64) = 64
close(3) = 0
rt_sigaction(SIGUSR1, {0x400819, [ILL TRAP ABRT FPE USR1 SEGV USR2
PIPE ALRM STKFLT CONT STOP TTIN URG XFSZ PROF IO RTMIN RT_2 RT_4
RT_6], SA_NOCLDSTOP|0x4000000}, NULL, 8) = 0
rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0
rt_sigprocmask(SIG_SETMASK, [USR1], NULL, 8) = 0
clone(child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
child_tidptr=0x2a958886d0) = 16892
fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x2a9556c000
--------------------------------------------------------
This shows that rt_sigaction is called with different arguments this
time (without SA_ONESHOT)!

What's going on here? This is driving me mad. The compiler is
gcc-3.3.3.

Thanks for any help.
 
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K

Keith Thompson

I'm trying to write a very simple program that uses a signal-based
synchronization between two processes. Here's it:
-----------------------------------------------
/* The world's simplest syncronization example */
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

So far, so good.
#include <unistd.h>

Sorry, you've just exceeded the bounds of comp.lang.c. The <unistd.h>
header is system-specific; it's not defined by the C standard. You
can probably get help in comp.unix.programmer.
 
S

subnet

Keith Thompson said:
Sorry, you've just exceeded the bounds of comp.lang.c. The <unistd.h>
header is system-specific; it's not defined by the C standard. You
can probably get help in comp.unix.programmer.

Ok, I apologize with all the readers of this newsgroup for wasting
your time. Also, thanks for your kind and polite answer (when you post
to the wrong newsgroup, you usually get much harsher answers).
Sorry.
 
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C

CBFalconer

subnet said:
Ok, I apologize with all the readers of this newsgroup for
wasting your time. Also, thanks for your kind and polite answer
(when you post to the wrong newsgroup, you usually get much
harsher answers). Sorry.

A breath of fresh air. He gets it, learns quickly, and will
doubtless be back with pertinent problems etc. without long
arguments about topicality.
 

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