How to generate a core dump in Linux on a segmentation fault?

Question

This depends on what shell you are using. If you are using bash, then the ulimit command controls several settings relating to program execution, such as whether you should dump core. If you type

ulimit -c unlimited

then that will tell bash that its programs can dump cores of any size. You can specify a size such as 52M instead of unlimited if you want, but in practice this shouldn't be necessary since the size of core files will probably never be an issue for you.

In tcsh, you'd type

limit coredumpsize unlimited

As explained above the real question being asked here is how to enable core dumps on a system where they are not enabled. That question is answered here.

If you've come here hoping to learn how to generate a core dump for a hung process, the answer is

gcore <pid>

if gcore is not available on your system then

kill -ABRT <pid>

Don't use kill -SEGV as that will often invoke a signal handler making it harder to diagnose the stuck process

To check where the core dumps are generated, run:

sysctl kernel.core_pattern

or:

cat /proc/sys/kernel/core_pattern

where %e is the process name and %t the system time. You can change it in /etc/sysctl.conf and reloading by sysctl -p.

If the core files are not generated (test it by: sleep 10 & and killall -SIGSEGV sleep), check the limits by: ulimit -a.

If your core file size is limited, run:

ulimit -c unlimited

to make it unlimited.

Then test again, if the core dumping is successful, you will see “(core dumped)” after the segmentation fault indication as below:

Segmentation fault: 11 (core dumped)

See also: core dumped - but core file is not in current directory?

---

Ubuntu

In Ubuntu the core dumps are handled by Apport and can be located in /var/crash/. However, it is disabled by default in stable releases.

For more details, please check: Where do I find the core dump in Ubuntu?.

macOS

For macOS, see: How to generate core dumps in Mac OS X?

What I did at the end was attach gdb to the process before it crashed, and then when it got the segfault I executed the generate-core-file command. That forced generation of a core dump.

Maybe you could do it this way, this program is a demonstration of how to trap a segmentation fault and shells out to a debugger (this is the original code used under AIX) and prints the stack trace up to the point of a segmentation fault. You will need to change the sprintf variable to use gdb in the case of Linux.

#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <stdarg.h>

static void signal_handler(int);
static void dumpstack(void);
static void cleanup(void);
void init_signals(void);
void panic(const char *, ...);

struct sigaction sigact;
char *progname;

int main(int argc, char **argv) {
    char *s;
    progname = *(argv);
    atexit(cleanup);
    init_signals();
    printf("About to seg fault by assigning zero to *s\n");
    *s = 0;
    sigemptyset(&sigact.sa_mask);
    return 0;
}

void init_signals(void) {
    sigact.sa_handler = signal_handler;
    sigemptyset(&sigact.sa_mask);
    sigact.sa_flags = 0;
    sigaction(SIGINT, &sigact, (struct sigaction *)NULL);

    sigaddset(&sigact.sa_mask, SIGSEGV);
    sigaction(SIGSEGV, &sigact, (struct sigaction *)NULL);

    sigaddset(&sigact.sa_mask, SIGBUS);
    sigaction(SIGBUS, &sigact, (struct sigaction *)NULL);

    sigaddset(&sigact.sa_mask, SIGQUIT);
    sigaction(SIGQUIT, &sigact, (struct sigaction *)NULL);

    sigaddset(&sigact.sa_mask, SIGHUP);
    sigaction(SIGHUP, &sigact, (struct sigaction *)NULL);

    sigaddset(&sigact.sa_mask, SIGKILL);
    sigaction(SIGKILL, &sigact, (struct sigaction *)NULL);
}

static void signal_handler(int sig) {
    if (sig == SIGHUP) panic("FATAL: Program hanged up\n");
    if (sig == SIGSEGV || sig == SIGBUS){
        dumpstack();
        panic("FATAL: %s Fault. Logged StackTrace\n", (sig == SIGSEGV) ? "Segmentation" : ((sig == SIGBUS) ? "Bus" : "Unknown"));
    }
    if (sig == SIGQUIT) panic("QUIT signal ended program\n");
    if (sig == SIGKILL) panic("KILL signal ended program\n");
    if (sig == SIGINT) ;
}

void panic(const char *fmt, ...) {
    char buf[50];
    va_list argptr;
    va_start(argptr, fmt);
    vsprintf(buf, fmt, argptr);
    va_end(argptr);
    fprintf(stderr, buf);
    exit(-1);
}

static void dumpstack(void) {
    /* Got this routine from http://www.whitefang.com/unix/faq_toc.html
    ** Section 6.5. Modified to redirect to file to prevent clutter
    */
    /* This needs to be changed... */
    char dbx[160];

    sprintf(dbx, "echo 'where\ndetach' | dbx -a %d > %s.dump", getpid(), progname);
    /* Change the dbx to gdb */

    system(dbx);
    return;
}

void cleanup(void) {
    sigemptyset(&sigact.sa_mask);
    /* Do any cleaning up chores here */
}

You may have to additionally add a parameter to get gdb to dump the core as shown here in this blog here.

There are more things that may influence the generation of a core dump. I encountered these:

• the directory for the dump must be writable. By default this is the current directory of the process, but that may be changed by setting /proc/sys/kernel/core_pattern.
• in some conditions, the kernel value in /proc/sys/fs/suid_dumpable may prevent the core to be generated.

There are more situations which may prevent the generation that are described in the man page - try man core.