👌🏾 🦐 🦌 Disassembling loops written in C 🤚🏻 🧑🏾 👵🏿

Good day.

Today we will look at the disassembled code of instructions if, for, while, switchthat are written in the C language.

If statement

This instruction is fairly easy to distinguish in disassembled form from other instructions. Its distinguishing feature is the single conditional jump instructions je, jne, and other jump commands.

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#include <stdio.h>

void main() {
    int x = 1;
    int y = 2;

    if(x == y) {
        printf("x = y\n");
    }
    else{
        printf("x != y\n");
    }
}

gcc. gcc -m32 prog_if.c -o prog_if. -m32 , x86.

radare2, r2 prog_if. aaa main s main. pdf.

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( int x; int y ), 1 var_ch ( x) 2 var10h ( y). (cmp) 1 2 (cmp edx, dword [var_10h]). . jne ( jump if noe equal) 0x000011e1. if ( VV radare2 IDA).

. . .

#include <stdio.h>

void main() {
    int x = 0;
    int y = 1;
    int z = 2;

    if(x == y) {
        if(z == 0) {
        printf("z = 0; x = y\n");
        }
        else{
            printf("z = 0; x != y\n");
        }
    }
    else {
        if(z == 0) {
            printf("z = zero and x != y.\n");
        } else {
            printf("z non-zero and x != y.\n");
        }
    }
}

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for

for : , , /. for .

#include <stdio.h>

void main() {
    int x;

    for(x = 0; x < 100; x++) {
        printf("x = %d", x);
    }
}

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1 - var_ch (x = 0)

2 - , jle. ( x 2, .)

3 - (printf)

4 - var_ch (++x)

while

while , - , . while for, . for, .

#include <stdio.h>

int func_1(int x);
int change_status();

int main() {
    int status = 0;

    while(status == 0) {
        printf("int e = %d", func_1(5) );
        status = change_status();
    }

    return 0;
}

int change_status() {
    return 1;
}

int func_1(int x) {
    int c;
    int e;
    int l;

    c = 1 + 2;
    e = x / 5;
    l = 4 - 2;

    return e;
}

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1 - var_4h (status = 0)

2 - , je. ( x 0, .)

3 - (func1, printf, change_status)

switch

switch : .

#include <stdio.h>

int main() {
    int i = 3;

    switch(i) {
        case 1:
            printf("CASE_1 i = %d", i+4);
            break;
        case 2:
            printf("CASE_2 i = %d", i+9);
            break;
        case 3:
            printf("CASE_3 i = %d", i+14);
            break;
    }
    return 0;
}

radare2

1 - var_4h (i = 3)

2 - (add, printf)

"case" , (cmp, je, jne) i case.

, ( ) , — switch if . , cmp je jne.

switch. case 4 .

#include <stdio.h>

int main() {
    int i = 3;

    switch(i) {
        case 1:
            printf("CASE_1 i = %d", i+4);
            break;
        case 2:
            printf("CASE_2 i = %d", i+9);
            break;
        case 3:
            printf("CASE_3 i = %d", i+14);
            break;
        case 4:
            printf("CASE_3 i = %d", i+19);
            break;
        default:
            break;
    }
    return 0;
}