I am attempting to create a structure (2nd) in a structure (1st) with dynamic memory allocation. For the sequence of steps below (case 1), where I call malloc for a pointer to the 2nd structure before calling realloc for the 1st structure, I see garbage when printing the value of the member (str[0].a1) in the first structure. If I place the same realloc code before malloc (case 2), there are no issues.
#include<stdio.h>
#include<stdlib.h>
typedef struct string2_t {
int a2;
} string2;
typedef struct string1_t {
int a1;
string2 * b;
} string1;
string1 * str = NULL;
int size = 0;
string1 test(int val){
// case 2
//str = (string1 *) realloc(str, size*sizeof(string1));
string1 S;
S.a1 = val;
size = 1;
S.b = (string2 *) malloc(2*sizeof(string2));
S.b[0].a2 = 11;
S.b[1].a2 = 12;
// case1
str = (string1 *) realloc(str, size*sizeof(string1));
return S;
}
int main() {
size = 1;
str = (string1 *) malloc(size*sizeof(string1));
str[0] = test(10);
printf("value of a:%d\n",str[0].a1);
str[1] = test(20);
printf("value of a:%d\n",str[0].a1);
return(0);
}
I can see that memory location changes for case 1, but I don't quite understand why doesn't pointer point to the right contents (str[0].a1 shows junk). I haven't returned the address of 1st structure before realloc, so I feel it should have worked. Could someone explain why it's pointing to garbage?
CodePudding user response:
As has been stated by several in the comments, lines like this cause undefined behavior:
str[0] = test(10);
The test()
function reallocates str
, and this is not sequenced with retrieving the address of str[0]
as the location to store the result. It's very likely to be compiled as if it had been written
string1 *temp = str;
temp[0] = test(10);
temp
caches the old address of str
from before the realloc()
, and this is invalid after the function returns.
Change your code to:
string1 temp = test(10);
str[0] = temp;
Better design would be to reorganize the code so that all maintenance of the str
array is done in one place. test()
should be a black box that just allocates the string1
object, while main()
allocates and reallocates str
when assigning to it.