Pthread_join Two-level pointer parameter analysis for thread wait function

The function prototype for the thread creation function pthread_create is as follows:

int pthread_create (pthread_t *restrict thread,
Const pthread_attr_t *restrict attr,
void * (*start_routine) (void*), void *restrict arg);

Where thread is the threading to be created;

Attr the properties of the specified thread, the default value is null;

Start_routine is the function that the thread executes;

ARG is a parameter passed to a function executed by a thread;

Note that: Start_routine is a function pointer

The function prototype for the thread wait function pthread_create is as follows:

int Pthread_join (pthread_t thread, void **value_ptr);

Where thread is the name of the threads to wait for;

VALUE_PTR: pointer *value_ptr points to the parameters returned by the thread

Note When using: For VALUE_PTR, you should declare a first-level pointer, and then pass the address to the Pthread_create function, instead of directly defining a level two pointer, the level two pointer is passed directly to Pthread_create.

Such as:

Correct method of delivery:

void *ret;

Pthread_join (thread, &ret);

Error Delivery Method:

void **ret;

Pthread_join (thread, ret);

Cause: There is a sentence of type such as Pthread_join: (* value_ptr) = arg; If you pass the parameter in the correct way, the statement on the left actually completes the operation: ret = arg; However, when parameters are passed in the wrong way, a fatal error occurs: (* value_ptr), which takes a value operation on a pointer that has not yet been initialized, which is not allowed by the system, at which point (* value_ptr) is equivalent to *ret,*ret or a pointer, is a level two pointer, but *ret is a wild pointer!

Watch out! You return is a first-level pointer, then the storage can only be stored with * &ret (hope you understand, is the pointer demotion) =ret;

Then the output is naturally ret.

Here is an example:

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

#define N 64

int a = 10;
void * Handler (void *arg)
{
printf ("a=%d,%s\n", A, (char *) arg);
strcat ((char *) arg, "...");
Pthread_exit (ARG);
}

int main ()
{
pthread_t Tid;
Char Buf[n] = {"Welcome"};
void *result;

if (Pthread_create (&tid, NULL, Handler, (void *) buf)! = 0)
Exit (-1);
printf ("*\n");
Pthread_join (Tid, &result);
printf ("ret:%s\n", (char *) result);

return 0;
}

Pthread_join Two-level pointer parameter analysis for thread wait function