I can't understand the output of gdb

Question

I'm just trying to learn to use gdb at this point. The program I'm using it on works perfectly fine; I'm not trying to debug it or anything; I'm just testing the functionality of gdb. Here's the source code:

#include <stdio.h>
#include <ctype.h>
#include <string.h>

int main( int argc, char **argv ){
        int wordcount = 0;
        int len = strlen( argv[1] );
        for( int i = 0; i < len; i++ ){
                if( isspace( argv[1][i] ) && !isspace( argv[1][i-1] ) ) 
                        wordcount++;
        }   
        if( !isspace( argv[1][len-1] ) ) wordcount++;
        if( wordcount == 0 && len > 0 ) // if all characters were non-whitespace,
                wordcount = 1;          // then there was exactly one word
        printf( "%d\n", wordcount );
        return 0;
}   

I started gdb and ran the program, setting breakpoints at lines 7 and 9. I used the backtrace and step commands, and I don't understand their output at all. When I typed "backtrace full" gdb gave me this:

#0  _start () at ../sysdeps/i386/elf/start.S:65
No locals.

What exactly does this mean? What is _start? What is ../sysdeps/i386/elf/start.S:65? And how can there be no locals, when clearly I have declared wordcount and len? I have tried Google, but every tutorial I can find on gdb shows it producing completely different (more detailed) output from what I got. When I Google the string I got, I get a bunch of results on the ARM architecture.

Answer 1

The program I'm using it on works perfectly fine;

For some definition of works. Your program has at least 2 bugs.

When I typed "backtrace full" gdb gave me this: ... What exactly does this mean?

It's hard to tell without knowing which commands you used before reaching this point. Most likely you did next until you returned from main , and thus landed in _start (which is the routine that calls main , and which is where the execution of any program usually starts).

What is ../sysdeps/i386/elf/start.S:65?

The _start routine is part of GLIBC, and is defined in sysdeps/i386/elf/start.S source file.

how can there be no locals, when clearly I have declared wordcount and len?

You are not inside main , so whatever locals are present in main is irrelevant: they are only active while main is executing, and it's not (either not yet, or not any longer).