Assembly Language for Beginners

1.9. SCANF()

j $31

addiu $sp,$sp,40 ; branch delay slot

IDA displays the stack layout as follows:

Listing 1.72: Optimizing GCC 4.4.5 (IDA)

.text:00000000 main:
.text:00000000
.text:00000000 var_18 = -0x18
.text:00000000 var_10 = -0x10
.text:00000000 var_4 = -4
.text:00000000
; function prologue:
.text:00000000 lui $gp, (gnu_local_gp >> 16)
.text:00000004 addiu $sp, -0x28
.text:00000008 la $gp, (gnu_local_gp & 0xFFFF)
.text:0000000C sw $ra, 0x28+var_4($sp)
.text:00000010 sw $gp, 0x28+var_18($sp)
; call puts():
.text:00000014 lw $t9, (puts & 0xFFFF)($gp)
.text:00000018 lui $a0, ($LC0 >> 16) # "Enter X:"
.text:0000001C jalr $t9
.text:00000020 la $a0, ($LC0 & 0xFFFF) # "Enter X:" ; branch delay slot
; call scanf():
.text:00000024 lw $gp, 0x28+var_18($sp)
.text:00000028 lui $a0, ($LC1 >> 16) # "%d"
.text:0000002C lw $t9, (__isoc99_scanf & 0xFFFF)($gp)
; set 2nd argument of scanf(), $a1=$sp+24:
.text:00000030 addiu $a1, $sp, 0x28+var_10
.text:00000034 jalr $t9 ; branch delay slot
.text:00000038 la $a0, ($LC1 & 0xFFFF) # "%d"
; call printf():
.text:0000003C lw $gp, 0x28+var_18($sp)
; set 2nd argument of printf(),
; load word at address $sp+24:
.text:00000040 lw $a1, 0x28+var_10($sp)
.text:00000044 lw $t9, (printf & 0xFFFF)($gp)
.text:00000048 lui $a0, ($LC2 >> 16) # "You entered %d...\n"
.text:0000004C jalr $t9
.text:00000050 la $a0, ($LC2 & 0xFFFF) # "You entered %d...\n" ; branch delay⤦
Çslot
; function epilogue:
.text:00000054 lw $ra, 0x28+var_4($sp)
; set return value to 0:
.text:00000058 move $v0, $zero
; return:
.text:0000005C jr $ra
.text:00000060 addiu $sp, 0x28 ; branch delay slot

1.9.2 Popular mistake

It’s a very popular mistake (and/or typo) to pass value ofxinstead of pointer tox:

#include <stdio.h>

int main()
{
int x;
printf ("Enter X:\n");

scanf ("%d", x); // BUG

printf ("You entered %d...\n", x);

return 0;
};