Lab1 - System call

ตัวอย่าง System Call ที่เรียกผ่าน Library ของภาษา C

#include <stdio.h>
int main(void) {
  printf("hello, world!\n"); //จะไปเรียก systemcall
  return 0;
}

ตัวอย่าง 1 - ใช้ได้เฉพาะ Linux อันนี้จะขึ้นอยู่กับระบบปฏิบัติการ unix เพียงอย่างเดียว

#include <unistd.h>
int main(void) {
  write(1, "hello, world!\n", 14);
  return 0;
}

ตัวอย่าง 2 - ใช้ได้เฉพาะ Linux อันนี้เป็นการเรียกใช้ system call โดยตรง แต่ข้อเสียคือ number ของ system call ที่เรียกใช้จะขึ้นอยู่กับ OS ที่เป็น unix

#include <unistd.h>
#include <sys/syscall.h>
int main(void) {
  syscall(SYS_write, 1, "hello, world!\n", 14);
  return 0;
}

ตัวอย่าง 3 - อ่านค่าจาก keyboard

#include <unistd.h>
#include <sys/syscall.h>

int main() {
    char buffer[50];
    long bytes;

    // Display prompt
    char *prompt = "Enter your name: ";
    syscall(SYS_write, 1, prompt, 17);

    // Read input
    bytes = syscall(SYS_read, 0, buffer, 50);

    // Display greeting
    char *greeting = "Hello, ";
    syscall(SYS_write, 1, greeting, 7);

    // Echo name
    syscall(SYS_write, 1, buffer, bytes);

    return 0;
}

ตัวอย่างที่ 4 - อ่านค่าจาก File

#include <unistd.h>
#include <sys/syscall.h>
#include <fcntl.h>

int main() {
    char buffer[100];
    char ch;
    long fd;
    long bytes;
    int pos = 0;
    int line_num = 1;

    fd = syscall(SYS_open, "lines.txt", O_RDONLY);

    if (fd < 0) {
        syscall(SYS_write, 2, "Error opening file\n", 19);
        return 1;
    }

    // Read character by character
    while ((bytes = syscall(SYS_read, fd, &ch, 1)) > 0) {
        if (ch == '\n') {
            // Display line number
            char num_str[10];
            int n = line_num;
            int digit_pos = 0;

            if (n == 0) {
                num_str[digit_pos++] = '0';
            } else {
                char digits[10];
                int dc = 0;
                while (n > 0) {
                    digits[dc++] = '0' + (n % 10);
                    n /= 10;
                }
                for (int i = dc - 1; i >= 0; i--) {
                    num_str[digit_pos++] = digits[i];
                }
            }
            num_str[digit_pos++] = ':';
            num_str[digit_pos++] = ' ';

            syscall(SYS_write, 1, num_str, digit_pos);
            syscall(SYS_write, 1, buffer, pos);
            syscall(SYS_write, 1, "\n", 1);

            pos = 0;
            line_num++;
        } else {
            buffer[pos++] = ch;
        }
    }

    // Last line (if no newline at end)
    if (pos > 0) {
        syscall(SYS_write, 1, buffer, pos);
        syscall(SYS_write, 1, "\n", 1);
    }

    syscall(SYS_close, fd);

    return 0;
}

ตัวอย่างที่ 6 - เขียนข้อมูลลงไฟล์

#include <unistd.h>
#include <sys/syscall.h>
#include <fcntl.h>

int main() {
    long fd;

    fd = syscall(SYS_open, "lines.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);

    if (fd < 0) {
        syscall(SYS_write, 2, "Cannot create file\n", 19);
        return 1;
    }

    // Write multiple lines
    syscall(SYS_write, fd, "Line 1\n", 7);
    syscall(SYS_write, fd, "Line 2\n", 7);
    syscall(SYS_write, fd, "Line 3\n", 7);

    syscall(SYS_close, fd);

    syscall(SYS_write, 1, "Three lines written\n", 20);

    return 0;
}

ตัวอย่าง 7 - ใช้ inline assembly ใน linux เพื่อเรียกใช้ system call และ interrupt

#include <unistd.h>

int main() {
    const char *message = "Hello from inline assembly!\n";
    int len = 29;

    // System call: write(stdout, message, length)
    // syscall number 1 (write) on x86-64
    asm volatile (
        "movq $1, %%rax\n"      // syscall number (write)
        "movq $1, %%rdi\n"      // file descriptor (stdout)
        "movq %0, %%rsi\n"      // buffer address
        "movq %1, %%rdx\n"      // count (length)
        "syscall\n"             // interrupt
        :
        : "r" (message), "r" ((long)len)
        : "rax", "rdi", "rsi", "rdx"
    );

    return 0;
}

ตัวอย่าง 8 - การอ่านค่าจาก keyboard

int main() {
    char buffer[100];
    long bytes_read;

    const char *prompt = "Enter text: ";

    asm volatile (
        "movq $1, %%rax\n"
        "movq $1, %%rdi\n"
        "movq %0, %%rsi\n"
        "movl %1, %%edx\n"      // Use movl for 32-bit immediate
        "syscall\n"
        :
        : "r" (prompt), "i" (12)  // "i" for immediate constant
        : "rax", "rdi", "rsi", "rdx", "rcx", "r11"
    );

    asm volatile (
        "movq $0, %%rax\n"
        "movq $0, %%rdi\n"
        "movq %1, %%rsi\n"
        "movq $100, %%rdx\n"
        "syscall\n"
        "movq %%rax, %0\n"
        : "=r" (bytes_read)
        : "r" (buffer)
        : "rax", "rdi", "rsi", "rdx", "rcx", "r11", "memory"
    );

    asm volatile (
        "movq $1, %%rax\n"
        "movq $1, %%rdi\n"
        "movq %0, %%rsi\n"
        "movq %1, %%rdx\n"
        "syscall\n"
        :
        : "r" (buffer), "r" (bytes_read)
        : "rax", "rdi", "rsi", "rdx", "rcx", "r11"
    );

    return 0;
}

ตัวอย่าง 9 - การอ่านค่าจาก File

int main() {
    char *filename = "data.txt";
    char buffer[200];
    long fd, bytes;

    // Open
    asm volatile (
        "syscall"
        : "=a" (fd)
        : "a" (2), "D" (filename), "S" (0)
        : "rcx", "r11"
    );

    // Read
    asm volatile (
        "syscall"
        : "=a" (bytes)
        : "a" (0), "D" (fd), "S" (buffer), "d" (200)
        : "rcx", "r11", "memory"
    );

    // Display
    asm volatile (
        "syscall"
        :
        : "a" (1), "D" (1), "S" (buffer), "d" (bytes)
        : "rcx", "r11"
    );

    // Close
    asm volatile (
        "syscall"
        :
        : "a" (3), "D" (fd)
        : "rcx", "r11"
    );

    return 0;
}

ตัวอย่าง 10 - การอ่านค่าจาก File

int main() {
    char *filename = "test.txt";
    char *message = "Testing 123\n";
    long fd;

    // Open/Create
    asm volatile (
        "syscall"
        : "=a" (fd)
        : "a" (2), "D" (filename), "S" (0101), "d" (0644)
        : "rcx", "r11"
    );

    // Write
    asm volatile (
        "syscall"
        :
        : "a" (1), "D" (fd), "S" (message), "d" (12)
        : "rcx", "r11"
    );

    // Close
    asm volatile (
        "syscall"
        :
        : "a" (3), "D" (fd)
        : "rcx", "r11"
    );

    return 0;
}

Lab1 - System call

ตัวอย่างของ System Call

ตัวอย่างของ Inline Assembly