在C语言中，我很难获取成员struct（双指针）的指针值。 在分配值的函数之外，该值丢失

Question

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

typedef struct {
  double *presult;
} SomeData;

//Fonction that assigns the value to be pointed
void  *assignValue(void *data) {
    SomeData *aData = (SomeData*)data;
    double valeurTotal = 45.50;

    aData->presult = &valeurTotal; //Make the pointer point to the value

    printf("%10.3f \n",*aData->presult); //Here it prints the right answer L 45.50
    pthread_exit(NULL);
}

int main(int argc, char *argv[]) {
    SomeData myData; // The struct
    pthread_t onethread; 
    pthread_create(&onethread, NULL, assignValue,(void *)&myData); 
    pthread_join(onethread, NULL);

    printf("**************************************** \n");
    printf("%10.3f \n", (myData.presult)); // prints: 0
    printf("%10.3f \n", *(myData.presult));// prints: 0

    exit(0);
}

Question might be confusing, so hopefully this simplified version of my code can explain better. So basically, I've created a thread that modifies the values of a struct.

Inside the thread function, the struct is passed-on as a pointer. One of the struct's member is a double pointer 'presult'. The thread function makes the 'presult' point to a value and it seems to work, because the print works.

However, back into the main function, I try to print the value of 'presult' again, but it doesn't print 45.50, but 0.0 instead.

In my full code, I actually get Segmentation fault with the last print. But even in this simplified code, it doesn't work. It doesn't print 45.50.

Output is as followed:

45.50
****************************************
0.000
0.000

Any help is appreciated. Thank you.

Answer 1

double valeurTotal = 45.50;
aData->presult = &valeurTotal; //Make the pointer point to the value

The memory location assigned to valeurTotal is going to be reused when assignValue goes out scope.

This is most likely the case at

printf("%10.3f \n", *(myData.presult)); // prints: 0

Also at this line you're trying to print a pointer as a floating number, which is wacky.

printf("%10.3f \n", (myData.presult)); // prints: 0

You need to actually store the value in your structure, which you can do by

typedef struct {
  double presult; // will copy into this when its assigned.
} SomeData;

Obviously, if you only wanted one double value you'd just pass that double as a pointer rather than a strucutre.