Dive into Systems Programming: A Beginner's Guide to C

Exploring Systems Programming: A Beginner’s Guide to C

Systems programming involves interacting with the underlying hardware and software of a computer. C is one of the preferred languages ​​for systems programming because of its ability to directly access hardware resources. This guide will take you on a system programming journey, from the basics of C language to practical application cases.

C language basics

Variables and data types:
Variables are used to store data. In C, variables must declare their data type, for example:

int age; // 声明一个整型变量 "age"

Function:
A function is a reusable block of code that performs a specific task. The syntax of the C function is as follows:

int add(int x, int y) {
  return x + y;
}

Pointer:
Pointer variables point to the memory address of other variables. They are used to dynamically manage memory.

int num = 3;
int *ptr = # // ptr 指向变量 num 的地址

System Programming Concepts

Memory Management:
System programming involves the direct management of memory. You need to know how to allocate and free memory space.

// 分配 sizeof(int) 字节的内存
int *p = malloc(sizeof(int));

// 释放内存
free(p);

File I/O:
File I/O operations are crucial for system programming. Files can be manipulated using the fopen(), fread() and fwrite() functions.

// 打开 "test.txt" 文件
FILE *file = fopen("test.txt", "r");

// 读取文件内容
char buffer[1024];
fread(buffer, 1, 1024, file);

// 关闭文件
fclose(file);

Operating system interaction:
C language provides system calls to interact with the operating system. These calls allow a program to perform specific tasks, such as creating or terminating a process.

// 创建一个子进程
pid_t pid = fork();

Practical Example: Creating a Simple Shell

Creating a simple shell is a great example of systems programming in action. Here are the steps:

1. Initialize variables: Including standard input, output and error streams.
2. Read user input: Use the fgets() function.
3. Parse the command: Split the command string into words and parameters.
4. Execute the command: Use the execv() function.
5. Error handling: Handle command execution errors.

The complete code is as follows:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
  char input[1024]; // 存储用户输入
  char *words[512]; // 存储命令和参数
  pid_t pid; // 子进程 ID

  while (1) {
    // 读取用户输入
    printf("> ");
    fgets(input, 1024, stdin);

    // 解析命令
    int num_words = tokenize(input, words);

    // 执行命令
    pid = fork();
    if (pid == 0) {
      // 子进程
      execv(words[0], words);
      perror("execv failed");
      exit(1);
    } else if (pid > 0) {
      // 父进程
      waitpid(pid, NULL, WUNTRACED);
    } else {
      // fork 失败
      perror("fork failed");
    }
  }

  return 0;
}

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