System calls

syscall.c

/*
 * Description: output/input strings from stdout/stdin over Lunux system call
 *
 * Author: Taihsiang Ho (tai271828)
 *
 * Usage: gcc -g --static syscall.c
 * 
 * Original source: shiwulo's os hw 3
 *
 * Also refer to the system call summary of Linux:
 * http://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/
 *
 */
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char** argv) {
    char* str_to_output = "Hello, world!!\n";
    long len_to_output = strlen(str_to_output) + 1; // long for int in 64-bit
    long ret_output;
  
    __asm__ volatile ( 
        "mov $1, %%rax\n"  // system call number, this is sys_write
        "mov $2, %%rdi\n"  // file descriptor number: stderr
        "mov %1, %%rsi\n"  //
        "mov %2, %%rdx\n"
        "syscall\n"
        "mov %%rax, %0"
        : "=m"(ret_output)
        : "g" (str_to_output), "g" (len_to_output));
    printf("return value: %ld\n", ret_output); // will be the number of all char output

    // read 2 char from stdin
    long len_from_stdin = 2;
    char* str_from_stdin = malloc(sizeof(len_from_stdin));
    long ret_input;

    __asm__ volatile ( 
        "mov $0, %%rax\n" // system call number, this is sys_read
        "mov $0, %%rdi\n" // fd: stdin
        "mov %1, %%rsi\n"
        "mov %2, %%rdx\n"
        "syscall\n"
        "mov %%rax, %0"
        : "=m"(ret_input)
        : "g" (str_from_stdin), "g" (len_from_stdin));
    printf("return value: %ld\n", ret_input); // will be the number of all char input
}