为什么这个 C 程序的输出是 0？

Question

In this code I am trying to pass a pointer to the structure and then use this pointer to work on the structure members, but why is the output always showing 0?

#include<stdio.h>
#include<string.h> 
struct student
{
    int go;
} s;
void m();
void main()
{
    m(&s);  
    printf("%d",s.go);
}

void m(struct student *ptr)  
{
    ptr->go;
}

Answer 1

struct student
{
   int go;
}s;

creates a global variable. Global variables are zero initialized unless they are initialized explicitly. That statement is equivalent to:

struct student
{
   int go;
};

struct student s = {0};

Hence the value of s.go is 0 . The call to m does not change the values of any variables. It has no effect on the output. The output will be 0 even if the call to m(&s) is removed.

Answer 2

The global variable s is initialized with all members 0 .

Nothing changes the value of s.go , so the output is 0 .

Answer 3

• When you declare your structure as global , their members are always initialize with its default value, if int than '0' and if char or string than '\\0' . So you are getting Value 0 .

 struct student { int go; } s;

Answer 4

Your question has all sorts of errors, here they are, in no particular order:

• you don't need #include <string.h> if you aren't using any "string" functions
• you shouldn't use global variables (if you did, it would eliminate the need to pass the pointer to s to functions to access the struct); you should remove the global variable (ie, make it local) and continue to pass the pointer-to-struct as you are to m() in order to be able to access it outside of main() (where you should declare it)
• your signature for main() is incorrect as I pointed out in a comment to OP, and naturally you are missing the return 0; statement in main() because of this
• you are missing a newline in your printf()
• you aren't actually doing anything with ptr->go in m() ; you aren't assigning anything to it or otherwise using it. It is printing zero because, as others have pointed out, global variables (because they are static ,) are (default-)initialized

Here is an example with corrections (note you can initialize s as described by others if you wish to use it's value before you modify/set it):

#include <stdio.h>

struct student
{
        int go;
};

void m();

int main(void)
{
        struct student s;

        m(&s);
        printf("%d\n", s.go);

        return 0;
}

void m(struct student *ptr)  
{
        ptr->go = 5;
}