Question

Using a function like this:

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

void print_trace() {
    char pid_buf[30];
    sprintf(pid_buf, "--pid=%d", getpid());
    char name_buf[512];
    name_buf[readlink("/proc/self/exe", name_buf, 511)]=0;
    int child_pid = fork();
    if (!child_pid) {           
        dup2(2,1); // redirect output to stderr
        fprintf(stdout,"stack trace for %s pid=%s\n",name_buf,pid_buf);
        execlp("gdb", "gdb", "--batch", "-n", "-ex", "thread", "-ex", "bt", name_buf, pid_buf, NULL);
        abort(); /* If gdb failed to start */
    } else {
        waitpid(child_pid,NULL,0);
    }
}

I see the details of print_trace in the output.

What are other ways to do it?

Answer 1

You mentioned on my other answer (now deleted) that you also want to see line numbers. I’m not sure how to do that when invoking gdb from inside your application.

But I’m going to share with you a couple of ways to print a simple stacktrace with function names and their respective line numbers without using gdb. Most of them came from a very nice article from Linux Journal:

• Method #1:

The first method is to disseminate it
with print and log messages in order
to pinpoint the execution path. In a
complex program, this option can
become cumbersome and tedious even if,
with the help of some GCC-specific
macros, it can be simplified a bit.
Consider, for example, a debug macro
such as:

 #define TRACE_MSG fprintf(stderr, __FUNCTION__     \
                          "() [%s:%d] here I am\n", \
                          __FILE__, __LINE__)

You can propagate this macro quickly
throughout your program by cutting and
pasting it. When you do not need it
anymore, switch it off simply by
defining it to no-op.

• Method #2: (It doesn’t say anything about line numbers, but I do on method 4)

A nicer way to get a stack backtrace,
however, is to use some of the
specific support functions provided by
glibc. The key one is backtrace(),
which navigates the stack frames from
the calling point to the beginning of
the program and provides an array of
return addresses. You then can map
each address to the body of a
particular function in your code by
having a look at the object file with
the nm command. Or, you can do it a
simpler way–use backtrace_symbols().
This function transforms a list of
return addresses, as returned by
backtrace(), into a list of strings,
each containing the function name
offset within the function and the
return address. The list of strings is
allocated from your heap space (as if
you called malloc()), so you should
free() it as soon as you are done with
it.

I encourage you to read it since the page has source code examples. In order to convert an address to a function name you must compile your application with the -rdynamic option.

• Method #3: (A better way of doing method 2)

An even more useful application for
this technique is putting a stack
backtrace inside a signal handler and
having the latter catch all the “bad”
signals your program can receive
(SIGSEGV, SIGBUS, SIGILL, SIGFPE and
the like). This way, if your program
unfortunately crashes and you were not
running it with a debugger, you can
get a stack trace and know where the
fault happened. This technique also
can be used to understand where your
program is looping in case it stops
responding

An implementation of this technique is available here.

• Method #4:

A small improvement I’ve done on method #3 to print line numbers. This could be copied to work on method #2 also.

Basically, I followed a tip that uses addr2line to

convert addresses into file names and
line numbers.

The source code below prints line numbers for all local functions. If a function from another library is called, you might see a couple of ??:0 instead of file names.

#include <stdio.h>
#include <signal.h>
#include <stdio.h>
#include <signal.h>
#include <execinfo.h>

void bt_sighandler(int sig, struct sigcontext ctx) {

  void *trace[16];
  char **messages = (char **)NULL;
  int i, trace_size = 0;

  if (sig == SIGSEGV)
    printf("Got signal %d, faulty address is %p, "
           "from %p\n", sig, ctx.cr2, ctx.eip);
  else
    printf("Got signal %d\n", sig);

  trace_size = backtrace(trace, 16);
  /* overwrite sigaction with caller's address */
  trace[1] = (void *)ctx.eip;
  messages = backtrace_symbols(trace, trace_size);
  /* skip first stack frame (points here) */
  printf("[bt] Execution path:\n");
  for (i=1; i<trace_size; ++i)
  {
    printf("[bt] #%d %s\n", i, messages[i]);

    /* find first occurence of '(' or ' ' in message[i] and assume
     * everything before that is the file name. (Don't go beyond 0 though
     * (string terminator)*/
    size_t p = 0;
    while(messages[i][p] != '(' && messages[i][p] != ' '
            && messages[i][p] != 0)
        ++p;

    char syscom[256];
    sprintf(syscom,"addr2line %p -e %.*s", trace[i], p, messages[i]);
        //last parameter is the file name of the symbol
    system(syscom);
  }

  exit(0);
}


int func_a(int a, char b) {

  char *p = (char *)0xdeadbeef;

  a = a + b;
  *p = 10;  /* CRASH here!! */

  return 2*a;
}


int func_b() {

  int res, a = 5;

  res = 5 + func_a(a, 't');

  return res;
}


int main() {

  /* Install our signal handler */
  struct sigaction sa;

  sa.sa_handler = (void *)bt_sighandler;
  sigemptyset(&sa.sa_mask);
  sa.sa_flags = SA_RESTART;

  sigaction(SIGSEGV, &sa, NULL);
  sigaction(SIGUSR1, &sa, NULL);
  /* ... add any other signal here */

  /* Do something */
  printf("%d\n", func_b());
}

This code should be compiled as: gcc sighandler.c -o sighandler -rdynamic

The program outputs:

Got signal 11, faulty address is 0xdeadbeef, from 0x8048975
[bt] Execution path:
[bt] #1 ./sighandler(func_a+0x1d) [0x8048975]
/home/karl/workspace/stacktrace/sighandler.c:44
[bt] #2 ./sighandler(func_b+0x20) [0x804899f]
/home/karl/workspace/stacktrace/sighandler.c:54
[bt] #3 ./sighandler(main+0x6c) [0x8048a16]
/home/karl/workspace/stacktrace/sighandler.c:74
[bt] #4 /lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6) [0x3fdbd6]
??:0
[bt] #5 ./sighandler() [0x8048781]
??:0

---

Update 2012/04/28 for recent linux kernel versions, the above sigaction signature is obsolete. Also I improved it a bit by grabbing the executable name from this answer. Here is an up to date version:

char* exe = 0;

int initialiseExecutableName() 
{
    char link[1024];
    exe = new char[1024];
    snprintf(link,sizeof link,"/proc/%d/exe",getpid());
    if(readlink(link,exe,sizeof link)==-1) {
        fprintf(stderr,"ERRORRRRR\n");
        exit(1);
    }
    printf("Executable name initialised: %s\n",exe);
}

const char* getExecutableName()
{
    if (exe == 0)
        initialiseExecutableName();
    return exe;
}

/* get REG_EIP from ucontext.h */
#define __USE_GNU
#include <ucontext.h>

void bt_sighandler(int sig, siginfo_t *info,
                   void *secret) {

  void *trace[16];
  char **messages = (char **)NULL;
  int i, trace_size = 0;
  ucontext_t *uc = (ucontext_t *)secret;

  /* Do something useful with siginfo_t */
  if (sig == SIGSEGV)
    printf("Got signal %d, faulty address is %p, "
           "from %p\n", sig, info->si_addr, 
           uc->uc_mcontext.gregs[REG_EIP]);
  else
    printf("Got signal %d\n", sig);

  trace_size = backtrace(trace, 16);
  /* overwrite sigaction with caller's address */
  trace[1] = (void *) uc->uc_mcontext.gregs[REG_EIP];

  messages = backtrace_symbols(trace, trace_size);
  /* skip first stack frame (points here) */
  printf("[bt] Execution path:\n");
  for (i=1; i<trace_size; ++i)
  {
    printf("[bt] %s\n", messages[i]);

    /* find first occurence of '(' or ' ' in message[i] and assume
     * everything before that is the file name. (Don't go beyond 0 though
     * (string terminator)*/
    size_t p = 0;
    while(messages[i][p] != '(' && messages[i][p] != ' '
            && messages[i][p] != 0)
        ++p;

    char syscom[256];
    sprintf(syscom,"addr2line %p -e %.*s", trace[i] , p, messages[i] );
           //last parameter is the filename of the symbol
    system(syscom);

  }
  exit(0);
}

and initialise like this:

int main() {

  /* Install our signal handler */
  struct sigaction sa;

  sa.sa_sigaction = (void *)bt_sighandler;
  sigemptyset (&sa.sa_mask);
  sa.sa_flags = SA_RESTART | SA_SIGINFO;

  sigaction(SIGSEGV, &sa, NULL);
  sigaction(SIGUSR1, &sa, NULL);
  /* ... add any other signal here */

  /* Do something */
  printf("%d\n", func_b());

}