无法访问C中的结构的值

Question

I am currently learning to code C, but I am stuck at creating a struct on the heap. I have got the following code:

#include <stdio.h>

typedef struct{
    int a;
    int b;
} some_struct;

int main(int argc, char ** argv) {
    printf("%i", sizeof(some_struct));
    some_struct * p_struct = malloc(sizeof(some_struct));
    p_struct->a = 600;
    p_struct->b = 100;
    return 0;
}

But when executed Visual Studio tells me:

Unhandled exception at 0x00007FF6286C1E39 in test.exe: 0xC0000005: Access violation writing location 0xFFFFFFFFAFBE4440.

Also the debugger tells me the memory of both a and b is unable to be read. I understand that the two integers should be uninitialized, but why is their memory seemingly unreadable?

Answer 1

Looking at your address, it looks like the lack of prototype for malloc() is the issue. Do:

#include <stdlib.h>

The implicit int causes the value returned by malloc() to be truncated (assuming 64 bit addresses and 32 bit ints). Hence, thus producing an invalid address.

In pre-C99, if a protoype is not found for a function, compiler implicitly declares a prototype with int . But this is no longer valid since C99.

Answer 2

You should #include <stdlib.h> which contains malloc . Doesn't Visual Studio give you any warnings?

When you don't include <stdlib.h> , malloc is given an implicit return type of int . This might work on a platform where sizeof(int) == sizeof(some_struct*) , but if they're of different size (eg 32 bit ints and 64 bit pointers), the value returned by malloc is truncated before assignment.

The value in the error message ( 0xFFFFFFFFAFBE4440 ) suggests that this is the case: malloc returned a pointer with a value of 0x????????AFBE4440 , which was truncated to 0xAFBE4440 , which was sign extended to 0xFFFFFFFFAFBE4440 on assignment.

Answer 3

It is happend because you call the function malloc but you forgot (or you did not know) to include stdlib.h.

Here is the working code:

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

typedef struct{
    int a;
    int b;
} some_struct;

int main(void) {
    printf("%lu", sizeof(some_struct));
    some_struct * p_struct = malloc(sizeof(some_struct));
    p_struct->a = 600;
    p_struct->b = 100;
    return 0;
}

You probably noticed that i changed:

printf("%i", sizeof(some_struct));

With:

printf("%lu", sizeof(some_struct));

Becouse if you turn on your compiler settings you will get:

format '%i' expects argument of type 'int', but argument 2 has type 'long unsigned int' [-Werror=format=]|

NOTE: Using %i instead of %d its ok in here but will not be the same with scanf.