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POLooking for guidance on a deadlock scenario
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  1. POLooking for guidance on a deadlock scenario
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    copied!<p>I have a program that spawns lots of children and runs for long periods of time. The program contains a SIGCHLD handler to reap defunct processes. Occasionally, this program freezes. I believe that pstack is indicating a deadlock scenario. Is that the proper interpretation of this output?</p> <pre><code>10533: ./asyncsignalhandler ff3954e4 lwp_park (0, 0, 0) ff391bbc slow_lock (ff341688, ff350000, 0, 0, 0, 0) + 58 ff2c45c8 localtime_r (ffbfe7a0, 0, 0, 0, 0, 0) + 24 ff2ba39c __posix_ctime_r (ffbfe7a0, ffbfe80e, ffbfe7a0, 0, 0, 0) + c 00010bd8 gettimestamp (ffbfe80e, ffbfe828, 40, 0, 0, 0) + 18 00010c50 sig_chld (12, 0, ffbfe9f0, 0, 0, 0) + 30 ff3956fc __sighndlr (12, 0, ffbfe9f0, 10c20, 0, 0) + c ff38f354 call_user_handler (12, 0, ffbfe9f0, 0, 0, 0) + 234 ff38f504 sigacthandler (12, 0, ffbfe9f0, 0, 0, 0) + 64 --- called from signal handler with signal 18 (SIGCLD) --- ff391c14 pthread_mutex_lock (20fc8, 0, 0, 0, 0, 0) + 48 ff2bcdec getenv (ff32a9ac, 770d0, 0, 0, 0, 0) + 1c ff2c6f40 getsystemTZ (0, 79268, 0, 0, 0, 0) + 14 ff2c4da8 ltzset_u (4ede65ba, 0, 0, 0, 0, 0) + 14 ff2c45d0 localtime_r (ffbff378, 0, 0, 0, 0, 0) + 2c ff2ba39c __posix_ctime_r (ffbff378, ffbff402, ffbff378, ff33e000, 0, 0) + c 00010bd8 gettimestamp (ffbff402, ffbff402, 2925, 29a7, 79c38, 10b54) + 18 00010ae0 main (1, ffbff4ac, ffbff4b4, 20c00, 0, 0) + 190 00010928 _start (0, 0, 0, 0, 0, 0) + 108 </code></pre> <p>I don't really fancy myself a C coder and am not familiar with the nuances of the language. I'm specifically using the re-entrant version of ctime(_r) in the program. Why is this still deadlocking?</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; #include &lt;time.h&gt; // import pid_t type #include &lt;sys/types.h&gt; // import _exit function #include &lt;unistd.h&gt; // import WNOHANG definition #include &lt;sys/wait.h&gt; // import errno variable #include &lt;errno.h&gt; // header for signal functions #include &lt;signal.h&gt; // function prototypes void sig_chld(int); char * gettimestamp(char *); // begin int main(int argc, char **argv) { time_t sleepstart; time_t sleepcheck; pid_t childpid; int i; unsigned int sleeptime; char sleepcommand[20]; char ctime_buf[26]; struct sigaction act; /* set stdout to line buffered for logging purposes */ setvbuf(stdout, NULL, _IOLBF, BUFSIZ); /* Assign sig_chld as our SIGCHLD handler */ act.sa_handler = sig_chld; /* We don't want to block any other signals */ sigemptyset(&amp;act.sa_mask); /* * We're only interested in children that have terminated, not ones * which have been stopped (eg user pressing control-Z at terminal) */ act.sa_flags = SA_NOCLDSTOP; /* Make these values effective. */ if (sigaction(SIGCHLD, &amp;act, NULL) &lt; 0) { printf("sigaction failed\n"); return 1; } while (1) { for (i = 0; i &lt; 20; i++) { /* fork/exec child program */ childpid = fork(); if (childpid == 0) // child { //sleeptime = 30 + i; sprintf(sleepcommand, "sleep %d", i); printf("\t[%s][%d] Executing /bin/sh -c %s\n", gettimestamp(ctime_buf), getpid(), sleepcommand); execl("/bin/sh", "/bin/sh", "-c", sleepcommand, NULL); // only executed if exec fails printf("[%s][%d] Error executing program, errno: %d\n", gettimestamp(ctime_buf), getpid(), errno); _exit(1); } else if (childpid &lt; 0) // error { printf("[%s][%d] Error forking, errno: %d\n", gettimestamp(ctime_buf), getpid(), errno); } else // parent { printf("[%s][%d] Spawned child, pid: %d\n", gettimestamp(ctime_buf), getpid(), childpid); } } // sleep is interrupted by SIGCHLD, so we can't simply sleep(5) printf("[%s][%d] Sleeping for 5 seconds\n", gettimestamp(ctime_buf), getpid()); time(&amp;sleepstart); while (1) { time(&amp;sleepcheck); if (difftime(sleepcheck, sleepstart) &lt; 5) { sleep(1); } else { break; } } } return(0); } char * gettimestamp(char *ctime_buf) { time_t now; time(&amp;now); // format the timestamp and chomp the newline ctime_r(&amp;now, ctime_buf); ctime_buf[strlen(ctime_buf) - 1] = '\0'; return ctime_buf; } /* * The signal handler function -- only gets called when a SIGCHLD * is received, ie when a child terminates. */ void sig_chld(int signo) { pid_t childpid; int childexitstatus; char ctime_buf[26]; while (1) { childpid = waitpid(-1, &amp;childexitstatus, WNOHANG); if (childpid &gt; 0) printf("[%s][%d] Reaped child, pid: %d, exitstatus: %d\n", gettimestamp(ctime_buf), getpid(), childpid, WEXITSTATUS(childexitstatus)); else return; } } </code></pre> <p>I'm running in a Solaris 9 environment. The program was compiled with Sun WorkShop 6 update 2 C 5.3 Patch 111679-15 2009/09/10 using the following syntax:</p> <pre><code>cc -o asyncsignalhandler asyncsignalhandler.c -mt -D_POSIX_PTHREAD_SEMANTICS </code></pre> <p>Is there a flaw in the program? Are there better ways to handle logging (with timestamps) from a signal handler?</p>
 

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