inline assembly scanf | Together C & C++ | Page 1

glacial helm Dec 25, 2022, 3:34 PM

#

i was trying to do scanf in the inline assembly function but it kept giving me 0 or segfault. any ideas? i want to read the %d from rax, put the result of the scanf into rcx, and then it will assign rcx to out via the =c

int bruh() {
  int out;
  char * x = "%d";
  asm(
"mov %%rax, %%rdi;"
"mov %%rcx, %%rsi;"
"mov $0, %%al;"
"call scanf;"

: "=c" (out)
: "a" (x)
  );
  return out;
}

void main() {
  printf("val: %d\n", bruh());
}
```i expect the output ```
val: 32
``` given the stdin of `32`
here is the tio link https://tio.run/##PY9BCoMwEEX3OcWQNpAUW0q709qTdDOdaA3EWGIUoXh2m6C4mnkPhv@Hzh@iZTkYR3bQFTz6oE13aZ6MGRfg7YdGKvgxgITdEIq4UoMeTjBBCVxonhT2rWS87UYQwuOUpaFNwXdHq@t3d7wmgTYxobXQE7o6EsuBl8RBxjSVAOM@qRiiUpKvwuDdWmVmbOyMhhaN22p@fSxaSz6izUHol@PZ9kW8npflfvsD

keen lightBOT Dec 25, 2022, 3:34 PM

#

When your question is answered use !solved to mark the question as resolved.

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glacial helm Dec 25, 2022, 3:38 PM

#

i have a similar working code using printf ```c
int bruh() {
int out;
char * x = "val: %d\n";

asm(
"mov %%rax, %%rbx;"
"mov %%rax, %%rdi;"
"mov $5, %%rsi;"
"mov $0, %%al;"
"call printf;"

: "=b" (out)
: "a" ("val: %d\n")
);
return out;
}

void main() {
printf("ebx: %d\n", bruh());
printf(bruh(), 32);
}

remote path Dec 25, 2022, 10:39 PM

#

if you would use scanf("%d", ...) in normal C it expects an pointer to int as second argument (so you'd do scanf("%d", &out); to read an integer to out). When translating this to assembly you want to pass the address of"%d" (x) in %rdi and &out in %rsi. Both are just input values because the actual output is going to be written to the address %rsi is pointing to. So the code would look like this: ```c
asm(
"mov %%rax, %%rdi;"
"mov %%rcx, %%rsi;"
"mov $0, %%al;"
"call scanf;"
: /* no output variables */
: "a" (x), "c" (&out)
);

You could also let the compiler put all the values to the right registers: ```c
asm(
  "call scanf;"
  : /* no output variables */
  : "D" (x), "S" (&out), "a" (0) /* "D" means di register and "S" si register */
);

#

@glacial helm

glacial helm Dec 25, 2022, 10:47 PM

#

thanks it works. is it possible to grab address rsi is pointing to as an output or use the out inside of the asm after scanf @remote path

keen lightBOT Dec 25, 2022, 10:52 PM

#

@glacial helm Has your question been resolved? If so, run !solved :)

glacial helm Dec 26, 2022, 1:23 AM

#

hm i copied some code from godbolt and i think it worked ```c
int main() {
int x;
int out;
asm(
"movl %%eax, -12(%%rbp);"
"leaq -12(%%rbp), %%rax;"
"movq %%rax, %%rsi;"
"movl %%ecx, %%edi;"
"movl $0, %%eax;"
"call scanf;"
"movl -12(%%rbp), %%eax;"
"addl $69, %%eax;"

: "=a" (out)
: "a" (x), "c" ("%d")
);
printf("%d", out);
}

keen lightBOT Dec 26, 2022, 1:46 PM

#

This question thread is being automatically closed. If your question is not answered feel free to bump the post or re-ask. Take a look at !howto ask for tips on improving your question.

glacial helm Dec 26, 2022, 2:21 PM

#

int main() {
asm(
  "lea -800(%%rbp), %%rax;"
  "mov %%rax, -800(%%rbp);" 
  "mov %%rcx, -1600(%%rbp);"
  "mov -1600(%%rbp), %%rdi;"
  "mov -800(%%rbp), %%rsi;"
  "mov $0, %%al;"
  "call scanf;"
  "mov -800(%%rbp), %%rax;"
  "add $69, %%rax;"
  "mov %%rax, -800(%%rbp);"
  "mov -1600(%%rbp), %%rdi;"
  "mov -800(%%rbp), %%rsi;"
  "mov $0, %%al;"
  "call printf;"
  :
  : "c" ("%d")
);
}
``` is a full program if anyone is curious

#

!solved

#inline assembly scanf