Pointer to struct within the nested structs

Question

I'm trying to run the following code(in gcc 4.3 on fedora 11 i586 ):

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

struct s_smallstruct{
  int smallstruct;
};                      

struct s_test2{
        char * test2;
        struct s_smallstruct* smallstruct;
};

struct s_test3{
        char * test3;
        struct s_smallstruct * smallstruct;
};

struct s_test1{
        char * test1;
        struct s_test2 * test2;
        struct s_test3 * test3;
};


int main(){
        struct s_test1 *test1 = (struct s_test1 *) malloc( sizeof test1 );
        test1->test2[0].smallstruct[0].smallstruct = 123;
        int num = test1->test2[0].smallstruct[0].smallstruct;
//      struct s_smallstruct * smallstruct = (struct s_smallstruct *) malloc( sizeof smallstruct );
//      smallstruct[0].smallstruct =12;
//      int num =  smallstruct[0].smallstruct;
        printf( "%d\n" , num );
        return EXIT_SUCCESS;
}

But I got a segfault at test1->test2[0].smallstruct[0].smallstruct = 123; . Commented part is running without error. What is the reason of this behaviour. I'm not very proficient in C , so I'd appreciate any kind of help.

Answer 1

There are three problems with your code that I can see:

1. sizeof only tells you the size of the pointer, which is 4 for 32-bit pointers, not the size of the structure that is pointed to,
2. and even if you change sizeof to tell you the size of the structure, malloc will only allocate memory for the s_test1 structure, not for the structures that are pointed to from within it,
3. and finally, the pointers in test1, test2 etc. have to be initialized.

Here is something that works:

const int length = 2;    
struct s_test1 *test1 = malloc( length * sizeof *test1 );
test1->test2 = malloc( length * sizeof *test1->test2 );
test1->test2->smallstruct = malloc( length * sizeof *test1->test2->smallstruct );
test1[1].test2[0].smallstruct[1].smallstruct = 123;
int num = test1[1].test2[0].smallstruct[1].smallstruct;

Answer 2

Try changing:

struct s_test1 *test1 = (struct s_test1 *) malloc( sizeof test1 );

to

struct s_test1 *test1 = (struct s_test1 *) malloc( sizeof struct s_test1 );

test1 is a pointer, which in 32bit environments is 4 bytes.