将void *变量分配给struct变量

Question

I have a void* variable which I am getting through a socket connection. I need to cast this to a struct type, wich is defined both on the client and server side.

I have provided a sample of the assignment in question in the code below. In the example I am leaving out the networking code for the sake of briefness.

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


typedef struct
{
    int x;
    int y;
} testStructType;


void someFunction(void* p)
{
    //some processing goes here.


}


int main(void) {

    testStructType testStruct;
    void* p;

    p=malloc(sizeof(testStructType));

    someFunction(p);


    testStruct=p;


    printf("%i ,%i",testStruct.x,testStruct.y);

    return EXIT_SUCCESS;
}

The problem is that I get error incompatible types when assigning to type 'testStructType' from 'void*'

What am I doing wrong? Can somebody please help me?

Answer 1

You mean

testStruct = *(testStructType *)p;

To get at the contents of what a pointer points at, you need to dereference it.

This makes a copy of the struct. If you don't actually need a copy then you can access the original directly:

testStructType *pt = p;
printf("%i ,%i", pt->x, pt->y);

Note that you are not "getting a void* variable through a socket connection". You are getting some data through a socket connection, and the void* variable is pointing at that data.

Answer 2

It's a pointer, so the line that reads

    testStructType testStruct;

should read

    testStructType *testStruct;

then the following code needs to change to:

    testStruct = (testStructType *)p;

    printf("%d, %d", testStruct->x, testStruct->y);