Core dump on Unix/Linux is a equivalence to minidump on Windows. As I’m very used to work with dump file as a way for post mortem debugging, frequently I feel sad with Crash Reporter on Mac OS X, which is an error report in pure text format and has limitation on the information it contains.
Mac OS X turns core dump off by default. Surely we can manually enable it by running a command in terminal, that is, "ulimit –c unlimited” and build the application with "-g". But unfortunately, kernel chooses to generate core dump for only a few signals. So what if we want to generate core dump when some other signal occurs. (And also it’s not possible (1) to generate core dump no matter whether there’s a signal or not; (2) there’s no API available for it.) Just find there’s a tool – gcore – that is available on most of the Unix BSD system, but it’s funny that Mac OS X does not. 😦
Anyway, author of book “Mac OS X internal” writes a Mac OS’s version of gcore, and source code download available here: http://osxbook.com/book/bonus/chapter8/core/download/gcore-1.3.tar.gz. This is for i386 version of Mac OS X, compilation pass with “gcc –o gcore gcore.c”. And I tested on my Snow Leopard, x86_64 by default, and it just works fine. So now here comes to how we can get core dump generated in our own code? Don’t puke 🙂 The work around is to fork/exec/wait.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
#include <string.h>
void _signal_handler(int signo) {
printf("SIGSEGV is caught…\n");
printf("Starting to generate core dump with gcore…\n");
char cmd[128] = { 0 };
sprintf(cmd, "%d", getpid());
printf("Pid is %s\n", cmd);
int ret;
pid_t _child_pid = fork();
if (_child_pid == 0) {
execlp("./gcore", "./gcore", "-c", "./core.test", cmd, NULL);
perror("Error when exec child process.\n");
exit(errno);
} else {
wait(&ret);
printf("gcore exited.\n");
}
_exit(1);
}
void foo(){
int * p = NULL;
*p = 1;
}
int main(void){
signal(SIGSEGV, _signal_handler);
foo();
return 0;
}
And run "gdb -c core.test" and followed by "bt" to get the stack trace. (It’s symbolized!!)
You should know that gcore can only run under root. (Use Directory Utilities to enable root account for Mac OS X.) The core dump file can only be read under root. Since we have source code of gcore, we may change this behavior. And also we can study into the mechanism and generate core dump within the crashed process directly, just like what we do on Windows via MiniDumpWriteDump API in dbghelp.dll. (But it takes time, I will not do it for now. :-))
Austin.D
I’m attending a project management training recently. haha~~
Yesterday is HISTORY, Tomorrow is MYSTERY. 