strcmp in gdb giving odd results

Asked Mar 07 '12 at 23:42

Active Mar 14 '13 at 07:48

Viewed 1,715 times

In GDB (gnu v 7.1-ubuntu) I am getting really weird results when I try to use strcmp to determine if two strings are equal. p strcmp("hello","hello") is giving me the result -145947168.

Everything I try with strcmp or strncmp is returning -145947168 in gdb. What am I doing wrong?

EDIT (thanks to Carl for the pointer to a related answer in the comments): See the answer to this question: How to evaluate functions in GDB?

Apparently sometimes the compiler optimizes out functions called from external libraries, and defining a function in code that calls the function of the external library you want access to in GDB will make it available.

I added this to my code:

#ifdef DEBUG
int mystrcmp(char *a, char *b){
        return strcmp(a,b);
}
int mystrncmp(char *a, char *b, int n){
        return strncmp(a,b,n);
}
#endif

and then re-made with -DDEBUG -g to enable the compilation of those helper functions for my gdb debugging.

(gdb) p mystrcmp("hello","hello")
$1 = 0
(gdb) p strcmp("hello","hello")
$2 = -145947168

edited May 23 '17 at 11:45

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asked Mar 07 '12 at 23:42

John St. John

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Related? http://stackoverflow.com/questions/1354731/gdb-evaluation-of-a-function – Carl Norum Mar 07 '12 at 23:48
Please also mention which version of GDB you're using. – FatalError Mar 07 '12 at 23:52
It looks like you are missing the '\0' at the end of each string hence the reason gdb is providing garbage. – darren102 Mar 08 '12 at 00:03
Yes Carl, thanks, it is related to that issue. The compiler was optimizing out strcmp, so when I added in a function to call strcmp in my code, and called that function from gdb, it works! – John St. John Mar 08 '12 at 00:11

strcmp in gdb giving odd results

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