C/Assembly Subprogram Segmentation fault - c

Question

Trying to run through GDB and keep getting a segmentation fault once the C main program enters the main function.
GDB Error:
Breakpoint 1, main () at binom_main.c:7
7 n=10;
(gdb) s
10 0;
(gdb) s
12 +){
(gdb) s
Program received signal SIGSEGV, Segmentation fault.
0x00000000004005c4 in otherwise ()
(gdb)
I compiled the code as such:
as binom.s -o binom.o
gcc -S -Og binom_main.c
gcc -c binom_main.s
gcc binom_main.o binom.o -o runtimes
I'm trying to learn how to use GDB more efficiently here but segfaults like these are pretty ambiguous and limiting. Why is this segfault being caused the moment the function begins? Have I linked the two files incorrectly?
Main :
#include <stdio.h>
unsigned int result,m,n,i;
unsigned int binom(int,int);
int main(){
n=10;
i=0;
for (i=1; i<2;i++){
result = binom(n,i);
printf("i=%d | %d \n", i, result );
}
return 0;
}
Sub:
.text
.globl binom
binom:
mov $0x00, %edx #for difference calculation
cmp %edi, %esi #m=n?
je equalorzero #jump to equalorzero for returning of value 1
cmp $0x00, %esi #m=0?
je equalorzero
cmp $0x01, %esi #m=1?
mov %esi,%edx
sub %edi, %edx
cmp $0x01, %edx # n-m = 1 ?
je oneoronedifference
jmp otherwise
equalorzero:
add $1, %eax #return 1
call printf
ret
oneoronedifference:
add %edi, %eax #return n
ret
otherwise:
sub $1, %edi #binom(n-1,m)
call binom
sub $1, %esi #binom(n-1,m-1)
call binom
ret

Answer 1

When you use gdb to debug asm, look at the disassembly window as well as the source window. (e.g. layout asm / layout reg, and layout next until you get the combo of windows that you want.) See the bottom of the x86 tag wiki for some more tips and a link to docs.
You can use stepi (si) to step by instructions, not by C statements, while investigating a crash outside your asm, caused by it corrupting something before returning.
This looks like a bug:
sub $1, %edi #binom(n-1,m)
call binom
# at this point, %edi no longer holds n-1, and %esi no longer holds m.
# because binom clobbers them. (This is normal)
# as Jester points out, you also don't save the return value (%eax) from the first call anywhere.
sub $1, %esi #binom(n-1,m-1)
call binom
Another (minor?) bug is:
cmp $0x01, %esi #m=1?
# but then you never read the flags that cmp set
Another serious bug:
equalorzero:
add $1, %eax #return 1 # wrong: nothing before this set %eax to anything.
# mov $1, %eax # You probably want this instead
ret