【Linux 进程控制】—— 进程亦生生不息：起于鸿蒙，守若空谷，归于太虚

NAME
       fork - create a child process

SYNOPSIS
       #include <sys/types.h>
       #include <unistd.h>

       pid_t fork(void);

int main(void)
{
    pid_t pid;
    printf("Before: pid is %d\n", getpid());
    if ((pid = fork()) == -1)
        perror("fork()"), exit(1);
    printf("After:pid is %d, fork return %d\n", getpid(), pid);
    sleep(1);
    return 0;
}

Before: pid is 40176
After:pid is 40176, fork return 40177
After:pid is 40177, fork return 0

if (pid == -1) {
    perror("fork failed"); // 输出类似 "fork failed: Resource temporarily unavailable"
}

#include<sys/types.h>
#include<sys/wait.h>
pid_t wait(int* status);

#include <sys/wait.h>
pid_t waitpid(pid_t pid, int *status, int options);

低 16 位结构：
| 15 14 13 12 11 10 9 8 | 7 | 6 5 4 3 2 1 0 |
---------------------------------------------
正常终止 → [ 退出状态（高8位） ]  0  [ 未使用 ]
被信号终止 → [   未使用（全0）   ] c  [ 信号编号 ]

int main()
{
    pid_t pid = fork();
    if (pid == 0)
    { // 子进程
        printf("子进程运行中... PID=%d\n", getpid());
        // 1. 正常退出：调用 exit(42)
        exit(42);
    }
    else
    { // 父进程
        int status;
        waitpid(pid, &status, 0); // 等待子进程结束
        if (WIFEXITED(status))
        { // 正常退出
            printf("子进程正常退出，退出码: %d\n", WEXITSTATUS(status));
        }
        else if (WIFSIGNALED(status))
        { // 被信号终止
            printf("子进程被信号终止，信号编号: %d\n", WTERMSIG(status));
        }
    }
    return 0;
}

子进程运行中... PID=56153
子进程正常退出，退出码: 42

int main()
{
    pid_t pid = fork();
    if (pid == 0)
    { // 子进程
        printf("子进程运行中... PID=%d\n", getpid());
        int *p = NULL;
        *p = 100;  // 对空指针解引用，触发 SIGSEGV 被信号终止
    }
    else
    { // 父进程
        int status;
        waitpid(pid, &status, 0); // 等待子进程结束
        if (WIFEXITED(status))
        { // 正常退出
            printf("子进程正常退出，退出码: %d\n", WEXITSTATUS(status));
        }
        else if (WIFSIGNALED(status))
        { // 被信号终止
            printf("子进程被信号终止，信号编号: %d\n", WTERMSIG(status));
        }
    }
    return 0;
}

子进程运行中... PID=56203
子进程被信号终止，信号编号: 11

pid_t waitpid(pid_t pid, int *status, 0);  // options 参数为 0

int main()
{
    int status;
    pid_t child_pid = fork();
    if (child_pid == 0)
    {
        // 子进程执行任务
        exit(10);
    }
    else
    {
        // 父进程阻塞等待子进程结束
        waitpid(child_pid, &status, 0);
        if (WIFEXITED(status))
        {
            printf("子进程退出码: %d\n", WEXITSTATUS(status));
        }
    }
}

pid_t waitpid(pid_t pid, int *status, WNOHANG);

int main()
{
    int status;
    pid_t child_pid = fork();
    if (child_pid == 0)
    {
        sleep(3); // 子进程休眠 3 秒后退出
        exit(10);
    }
    else
    {
        while (1)
        {
            pid_t ret = waitpid(child_pid, &status, WNOHANG);
            if (ret == -1)
            {
                perror("waitpid");
                break;
            }
            else if (ret == 0)
            {
                printf("子进程未结束，父进程继续工作...\n");
                sleep(1); // 避免频繁轮询消耗 CPU
            }
            else
            {
                if (WIFEXITED(status))
                {
                    printf("子进程退出码: %d\n", WEXITSTATUS(status));
                }
                break;
            }
        }
    }
}

int sum(int a, int b)
{
    return a + b;
}

int main()
{
    int result = sum(3, 5);
    printf("Result: %d\n", result); // 输出 8，结果正确
    return 0;
}

Result: 8

// 错误实现：本应计算阶乘，但初始值错误
int factorial(int n)
{
    int result = 0; // 错误！应为 result = 1
    for (int i = 1; i <= n; i++)
    {
        result *= i;
    }
    return result;
}

int main()
{
    printf("5! = %d\n", factorial(5)); // 输出 0，结果错误
    return 0;
}

int main()
{
    int *ptr = NULL;
    *ptr = 42;  // 对空指针解引用，触发段错误
    printf("Value: %d\n", *ptr);
    return 0;
}

Segmentation fault

int main()
{
    int a = 10;
    int b = a / 0; // 程序除零异常
    printf("Value: %d\n", b);
    return 0;
}

Floating point exception

#include <errno.h>

int main()
{
    for (int i = 0; i < 135; ++i)
    {
        printf("%d-> %s\n", i, strerror(i));
    }
    return 0;
}

0-> Success
1-> Operation not permitted
2-> No such file or directory
3-> No such process
4-> Interrupted system call
5-> Input/output error
6-> No such device or address
7-> Argument list too long
8-> Exec format error
9-> Bad file descriptor
10-> No child processes
11-> Resource temporarily unavailable
12-> Cannot allocate memory
13-> Permission denied
14-> Bad address
15-> Block device required
16-> Device or resource busy
17-> File exists
18-> Invalid cross-device link
19-> No such device
20-> Not a directory
21-> Is a directory
22-> Invalid argument
23-> Too many open files in system
24-> Too many open files
25-> Inappropriate ioctl for device
26-> Text file busy
27-> File too large
28-> No space left on device
29-> Illegal seek
30-> Read-only file system
31-> Too many links
32-> Broken pipe
33-> Numerical argument out of domain
34-> Numerical result out of range
35-> Resource deadlock avoided
36-> File name too long
37-> No locks available
38-> Function not implemented
39-> Directory not empty
40-> Too many levels of symbolic links
41-> Unknown error 41
42-> No message of desired type
43-> Identifier removed
44-> Channel number out of range
45-> Level 2 not synchronized
46-> Level 3 halted
47-> Level 3 reset
48-> Link number out of range
49-> Protocol driver not attached
50-> No CSI structure available
51-> Level 2 halted
52-> Invalid exchange
53-> Invalid request descriptor
54-> Exchange full
55-> No anode
56-> Invalid request code
57-> Invalid slot
58-> Unknown error 58
59-> Bad font file format
60-> Device not a stream
61-> No data available
62-> Timer expired
63-> Out of streams resources
64-> Machine is not on the network
65-> Package not installed
66-> Object is remote
67-> Link has been severed
68-> Advertise error
69-> Srmount error
70-> Communication error on send
71-> Protocol error
72-> Multihop attempted
73-> RFS specific error
74-> Bad message
75-> Value too large for defined data type
76-> Name not unique on network
77-> File descriptor in bad state
78-> Remote address changed
79-> Can not access a needed shared library
80-> Accessing a corrupted shared library
81-> .lib section in a.out corrupted
82-> Attempting to link in too many shared libraries
83-> Cannot exec a shared library directly
84-> Invalid or incomplete multibyte or wide character
85-> Interrupted system call should be restarted
86-> Streams pipe error
87-> Too many users
88-> Socket operation on non-socket
89-> Destination address required
90-> Message too long
91-> Protocol wrong type for socket
92-> Protocol not available
93-> Protocol not supported
94-> Socket type not supported
95-> Operation not supported
96-> Protocol family not supported
97-> Address family not supported by protocol
98-> Address already in use
99-> Cannot assign requested address
100-> Network is down
101-> Network is unreachable
102-> Network dropped connection on reset
103-> Software caused connection abort
104-> Connection reset by peer
105-> No buffer space available
106-> Transport endpoint is already connected
107-> Transport endpoint is not connected
108-> Cannot send after transport endpoint shutdown
109-> Too many references: cannot splice
110-> Connection timed out
111-> Connection refused
112-> Host is down
113-> No route to host
114-> Operation already in progress
115-> Operation now in progress
116-> Stale file handle
117-> Structure needs cleaning
118-> Not a XENIX named type file
119-> No XENIX semaphores available
120-> Is a named type file
121-> Remote I/O error
122-> Disk quota exceeded
123-> No medium found
124-> Wrong medium type
125-> Operation canceled
126-> Required key not available
127-> Key has expired
128-> Key has been revoked
129-> Key was rejected by service
130-> Owner died
131-> State not recoverable
132-> Operation not possible due to RF-kill
133-> Memory page has hardware error
134-> Unknown error 134

int main()
{
    FILE *fp = fopen("invalid.txt", "r");//以只读方式打开不存在的文件会出错
    if (fp == NULL)
    {
        // 使用 strerror 获取错误描述
        printf("%d->%s\n", errno,strerror(errno));            
        return 1; //退出码设为1
    }
    return 0;
}

2->No such file or directory

#include <unistd.h>
void _exit(int status);

#include <stdlib.h>
void exit(int status);  // C 

#include <cstdlib>
void exit(int status);  // C++ 

int main()
{
    printf("hello");
    exit(0);
}

[root@localhost linux]# ./a.out
hello[root@localhost linux]#

int main()
{
    printf("hello");
    _exit(0);
}

[root@localhost linux]# ./a.out
[root@localhost linux]#

int main()
{
    return 42;  // 等价于 exit(42);
}