指向结构的参数，函数中引用的成员产生奇怪的值

Question

I'm tasked with adding the quick-fit allocation algorithm to MINIX, and as such need to use structures a lot. However, I came accross something for which I cannot find any resources on a solution.

The below code is a quick example I made to demonstrate the issue I have met with.

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

struct some_struct {
    int h_count;
};

void why();

struct some_struct * myStruct;

int main(){ 
    myStruct = (struct some_struct *) malloc(sizeof(struct some_struct));
    myStruct->h_count=0;

    printf("%d\n", myStruct->h_count);
    why(&myStruct);

    return 0;
}

void why(struct some_struct * t_some_struct){
    printf("%d\n", t_some_struct->h_count);
}

The output of the printf in the function why is different from the output given in main. The value printed by why is a seemingly random number that changes every time the program is ran.

Output:

0
7870280
Press any key to continue . . .`

The 7870280 changes on every run. I would assume it is printing some kind of memory value or something, but that's just a guess as I have no idea really.

As such, I thought I'd ask here for the benefit of others alongside myself:

• Why does this occur?
• Is my syntax the issue here?
• How do I use a pointer to a structure as a parameter for some function, then reference the given structure's members in the scope of the function? (I'd like to use h_count for t_some_struct as 0)

Thanks

Answer 1

The function why expects a pointer whereas you are passing a pointer to pointer. Call as why(myStruct); (by the way, myStruct is a global variable, so passing is a bit superfluous here). Rest of your accessing the struct member is fine.

Not directly related but some general suggestions:

1) Use standard prototype for main() such as int main(int argc, char*argv[])
2) Casting the result of malloc is needless and error-prone.