Section 7.4. glibc Debugging Support

7.4. glibc Debugging Support

The usual way to examine a back trace of a program is to use an external debugger such as GDB. Sometimes, however, it is useful to obtain back-trace information from within a running program for logging or debugging purposes. The GNU glibc provides support that enables the developer to produce back-trace printouts from within the application.

Example 7-4 shows how to use backtrace inside a signal handler and outside a signal handler. The API prototypes for back trace are declared in the header file execinfo.h and should be included in the application.

Example 7-4. Code Listing of sample_backtrace.c

1 #include <execinfo.h> 2 #include <stdio.h> 3 #include <signal.h> 4 5 void handler() 6 { 7   int i; 8   void *trace[128]; 9   int tr_size; 10   char **strings; 11 12   printf("\n########## start backtrace ##########\n\n"); 13   tr_size = backtrace(trace, sizeof(trace) / sizeof(void *)); 14   strings = backtrace_symbols(trace, tr_size); 15   printf("Number of elements in backtrace: %d\n", tr_size); 16   for (i = 0; i < tr_size; i++) { 17     printf("%s\n", strings[i]); 18   } 19   printf("\n########## end backtrace ##########\n"); 20 21   free(strings); 22 } 23 24 h() 25 { 26 #ifdef _handler_ 27   abort(); 28 #else 29   handler(); 30 #endif 31 } 32 33 void g() 34 { 35   h(); 36 } 37 38 void foo() 39 { 40   g(); 41 } 42 43 int main(int argc, char **argv) 44 { 45   struct sigaction sigact; 46 47    sigact.sa_sigaction = handler; 48    sigemptyset(&sigact.sa_mask); 49    sigact.sa_flags = SA_SIGINFO; 50    sigaction(SIGSEGV, &sigact, NULL); 51    sigaction(SIGABRT, &sigact, NULL); 52 53    foo(); 54 } 

Compile the code listing sample_backtrace.c with the _handler_ macro defined to simulate an application program that aborts in the middle of its run:

$ gcc sample_backtrace.c -o sample_backtrace -g -rdynamic -D_handler_ $./sample_backtrace ########## start backtrace ########## Number of elements in backtrace: 9 ./main1(handler+0x2d) [0x8048829] [0xffffe440] /lib/tls/libc.so.6(abort+0x129) [0x40051f79] ./main1(g+0) [0x80488d2] ./main1(g+0xb) [0x80488dd] ./main1(foo+0xb) [0x80488ea] ./main1(main+0x6a) [0x8048956] /lib/tls/libc.so.6(__libc_start_main+0xe0) [0x4003e500] ./main1 [0x8048761] ########## end backtrace ########## Aborted 

In the preceding example, back-trace information is produced through the abort signal handler routine called handler(). The same method can be implemented for other signals caught by the application.

In the following, Example 7-4 is compiled without the _handler_ macro defined. In this example, the handler() function is called explicitly by the application to produce a back-trace printout. This method can be used to create back-trace information at certain points of the application for debugging or monitoring reasons.

$ gcc sample_backtrace.c -o sample_backtrace -g -rdynamic $ ./sample_backtrace ########## start backtrace ########## Number of elements in backtrace: 7 ./main1(handler+0x2d) [0x80487f9] ./main1(h+0xb) [0x80488a2] ./main1(g+0xb) [0x80488af] ./main1(foo+0xb) [0x80488bc] ./main1(main+0x6a) [0x8048928] /lib/tls/libc.so.6(__libc_start_main+0xe0) [0x4003e500] ./main1 [0x8048731] ########## end backtrace ########## 

When compiling, notice that the rdynamic flag is used. The rdynamic flag is passed on to the linker to force inclusion of global symbol tables in the final executable.