segmentation fault assistance

**mack99** · 08-07-2012

I got the following when I ran gdb on a program, SimpleDemo, which indicated a segmentation fault:

(gdb) run
Starting program: /home/bill/asm/SimpleDemo

Program received signal SIGSEGV, Segmentation fault.
0xb7e56590 in ?? () from /lib/i386-linux-gnu/libc.so.6

I was expecting to get info on where the program created the fault but it seems it was caused in one of the c libraries. Not sure where to go with this as I was following along with a debug tutorial and am now in (for me) uncharted waters.

SimpleDemo.c:

Code:

#include<stdio.h>
#include<stdlib.h>


int add(int x, int y)
{
        int z =10;


        z = x + y;
        return z;
}


main(int argc, char **argv)
{
        int a = atoi(argv[1]);
        int b = atoi(argv[2]);
        int c;
        char buffer[100];


        gets(buffer);
        puts(buffer);


        c = add(a,b);


        printf("Sum of %d+%d = %d\n",a, b, c);


        exit(0);


}

Thanks,

**laserlight** · 08-07-2012

I noticed:

You did not check that argv[1] and argv[2] existed, e.g., by checking argc.
You used gets, which is vulnerable to buffer overflow. You should use say, fgets instead.

**mack99** · 08-07-2012

Hello laserlight,

Thanks for your response. I'm not sure what you mean when you say "check argc".

**oogabooga** · 08-07-2012

You need to run this program with two command-line arguments:
> prog 10 25

In the program, you should check main's argc ("argument count") parameter. argv[0] holds the program name, so the arguments start at argv[1]. If you're expecting 2 arguments, then you want an argc of 3.

Code:

int main(int argc, char **argv)
{
    if (argc != 3) {
        printf("Usage: prog n m\n");
        exit(1);
    }
    // ...
}

**AndiPersti** · 08-08-2012

Originally Posted by mack99

I was expecting to get info on where the program created the fault but it seems it was caused in one of the c libraries. Not sure where to go with this as I was following along with a debug tutorial and am now in (for me) uncharted waters.

Use the bt (backtrace) command in gdb:

Code:

$ gdb -q ./test
Reading symbols from test...done.
(gdb) r
Starting program: test 

Program received signal SIGSEGV, Segmentation fault.
0x00167590 in __GI_____strtol_l_internal (nptr=0x0, endptr=0x0, base=10, 
    group=0, loc=0x2d48c0) at strtol_l.c:298
298    strtol_l.c: No such file or directory.
(gdb) bt
#0  0x00167590 in __GI_____strtol_l_internal (nptr=0x0, endptr=0x0, base=10, 
    group=0, loc=0x2d48c0) at strtol_l.c:298
#1  0x00167347 in __GI_strtol (nptr=0x0, endptr=0x0, base=10) at strtol.c:110
#2  0x0016464f in atoi (nptr=0x0) at atoi.c:28
#3  0x080484e4 in main (argc=1, argv=0xbffff304) at test.c:17
(gdb) help bt
Print backtrace of all stack frames, or innermost COUNT frames.
With a negative argument, print outermost -COUNT frames.
Use of the 'full' qualifier also prints the values of the local variables.

So your program crashes on line 17.
You're are only interested in the innermost frame of your own code, which in this case is frame #3.

See also Debugging with GDB - Examining the Stack

Bye, Andreas

**rags_to_riches** · 08-08-2012

Also, to set the arguments in gdb, after starting gdb at the prompt type

set args arg1 arg2

**Salem** · 08-08-2012

> #2 0x0016464f in atoi (nptr=0x0) at atoi.c:28
You can also tell from the call to atoi() that you passed it a NULL pointer.

**mack99** · 08-08-2012

Thanks everyone for the advice and help with this. Greatly appreciated !

**mack99** · 08-09-2012

Thanks again. I understand now why the program didn't work. To learn more I found an exercise that gives an example (show_args.c).

Code:

#include<stdio.h>
#include<stdlib.h>


int main(int argc, char **argv)
{
	while(argc--)
		printf("%s\n", *argv++);
	
	exit(EXIT_SUCCESS);


}

When I ran:

show_args abcde text hello

I got:

show_args:command not found.

I also tried ./show_args abcde text hello and got the same response.

When I tried: echo $PATH I got:

/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

Not sure what is going on ?

Thanks.

**Salem** · 08-09-2012

Did you compile it?

What executable name did you specify when you compiled it?

gcc -o myprogname source.c

Then you would type in
./myprogname

The default is usually a.out

**mack99** · 08-10-2012

Thanks Salem. I must have made a mistake in compiling. This works.

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