c - How to set conditional breakpoint if malloc returns NULL via gdb -

sample source code:

#include        <stdio.h> #include        <stdlib.h> #include        <errno.h>  #define         gigabyte        1024*1024*1024  int main (void) {         void    *foo;         int     result;          foo = (void *) malloc (gigabyte*5);         result = errno;          if (foo != null) {                 return 2;         } else {                 fprintf (stderr, "error: %d\n", result);                 return 1;         }         return 0; } 

question:

• how instruct gdb (# gdb -silent ./huge_malloc) stop/halt execution, if malloc() returns 0x0, without checking if foo 0x0

you identify exit point of malloc , put conditional breakpoint there. such as:

(gdb) tbreak main breakpoint 1 @ 0x4005c4: file t.c, line 13. (gdb) r starting program: /var/tmp/a.out  main () @ t.c:13 13          foo = malloc (64); (gdb) br *__libc_malloc+211 if $rax==0 breakpoint 2 @ 0x7f26d143ea93 (gdb) n 14          foo = malloc (gigabyte*64); (gdb) p foo $1 = (void *) 0x21dc010 (gdb) n  breakpoint 2, 0x00007f26d143ea93 in malloc () /lib/libc.so.6 

note, have added malloc call succeeds first, illustrate breakpoint triggers null return value. breakpoint address may vary libc versions, found stepping through malloc nexti until hit ret instruction.