Create thread

1. Overview of creating threads

In general ,main The thread on which the function is located is called the main thread （main Threads ）, The remaining threads created are called Sub thread .

In the program There is only one process by default ,fork() function After calling , There is 2 Threads in progress

In the program There is only one thread by default （main Threads , The main thread ）,pthread_create() Function call , There is 2 Threads .

2. Create thread api function

#include <pthread.h>
    int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg);

        -  function ： Create a child thread 
        -  Parameters ：
            - thread： Out parameter , After the thread is created successfully , The thread of a child thread ID Is written to this variable .
            - attr :  Set thread properties , The default value is generally used ,NULL
            - start_routine :  A function pointer , This function is the logical code that the sub thread needs to process 
            - arg :  Use... For the third parameter , The ginseng 
        -  Return value ：
             success ：0
             Failure ： Return error number . This error number is the same as before errno Not quite the same. .
             Get information about the error number ：  char * strerror(int errnum);

3. Case study —— Create a thread

①

stay vscode Created in pthread_create.c file . The attached code is as follows ：

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

//void* It's a universal pointer , The return value and parameters of the callback function are function pointers 
void* callback(void *arg) {
    
    printf(" Sub thread ");
    return NULL;
}

int main() {
    

    pthread_t tid; // The thread number used to receive the created sub thread 

    // Create a child thread 
    int ret = pthread_create(&tid, NULL, callback, NULL);

    //ret = 0 Create success ,！= Print error number 
    if (ret != 0) {
    
        char *errstr = strerror(ret);
        printf("error : %s\n", errstr);
    }

    for (int i = 0; i < 5; ++i) {
    
        printf("%d\n", i);
    }

    return 0; //exit(0), Withdraw from the process 
}

stay xshell Input in gcc pthread_create.c -o p -lpthread Create executable p.（ Because threads are not standard system libraries , Need to add -lpthread Represents the specified library pthread）.

Input ./p Execution procedure , The results are as follows ：

②

We found that the code in the sub thread did not execute , This is because The sub thread has not been created or has not been executed yet , The main thread runs Yes return 0 Quit the whole process , As a result, the sub thread did not have time to execute .

We make the following changes , stay return 0 And before sleep(1), Let the main thread sleep 1 second , Allow time to execute child threads .

// ditto 
	for (int i = 0; i < 5; ++i) {
    
        printf("%d\n", i);
    }
    sleep(1);
    return 0; //exit(0), Withdraw from the process 
}

It can be seen that the contents of the sub thread have been successfully executed .