In C programming, managing dynamically allocated memory within struct pointers can easily introduce subtle bugs if pointer semantics and string sizing rules aren't strictly followed. A classic issue occurs when attempting to assign a string parameter (a char *) to a struct member inside an array of struct pointers, resulting in unexpected empty output, silent failures, or segmentation faults.

Understanding the Problem

Let's look at the original code structure and identify why compiler warnings were missed while execution still failed:

typedef struct mystruct {
    char *mychar;
} *p_mystruct;

p_mystruct the_data[2]; // Array of POINTERS to struct

void insert(char *pstring, int index) {
    the_data[index]->mychar = malloc(sizeof(pstring) + 1);
    strlcpy(the_data[index]->mychar, pstring, sizeof(pstring) + 1);
}

The Root Causes

1. sizeof(pointer) vs. strlen(string)

When you pass a string to a function as a parameter like char *pstring, the identifier pstring is a pointer variable. Calling sizeof(pstring) evaluates the size of the pointer itself (typically 4 or 8 bytes depending on your system architecture), not the length of the string character sequence.

To calculate the correct memory required for a string, you must use strlen(pstring) to get the character count and add 1 for the null terminator ('\0').

2. Dereferencing Unallocated Struct Pointers

Because of the typedef struct mystruct { ... } *p_mystruct; syntax, p_mystruct the_data[2]; declares an array of pointers to mystruct, not actual struct instances. Since the_data is defined globally, the pointers in the array default to NULL.

Inside insert(), calling the_data[index]->mychar attempts to dereference a NULL pointer before allocating memory for the mystruct object itself. This triggers Undefined Behavior (UB).

3. Hiding Pointers Behind typedef

Hiding pointer indirection behind a typedef (e.g., typedef struct mystruct *p_mystruct;) is generally considered a bad practice in C. It obscures whether a variable holds object data or a pointer address, making memory allocation bugs much harder to spot during code reviews.

The Solution: Corrected Implementation

To fix these issues, explicit memory allocation must be performed for both the struct instance and its dynamic string member. Below is the fully refactored, robust implementation:

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

typedef struct mystruct {
    char *mychar;
} mystruct;

// Array of explicit pointers to struct
mystruct *the_data[2];

void insert(const char *pstring, int index) {
    // 1. Allocate memory for the struct instance
    the_data[index] = malloc(sizeof(mystruct));
    if (the_data[index] == NULL) {
        perror("Failed to allocate memory for struct");
        return;
    }

    // 2. Allocate memory for the string (+1 for null-terminator)
    size_t len = strlen(pstring);
    the_data[index]->mychar = malloc(len + 1);
    if (the_data[index]->mychar == NULL) {
        perror("Failed to allocate memory for string");
        return;
    }

    // 3. Copy string data securely
    memcpy(the_data[index]->mychar, pstring, len + 1);
}

int main() {
    insert("test", 0);

    if (the_data[0] && the_data[0]->mychar) {
        printf("%s\n", the_data[0]->mychar); // Output: test
        
        // Clean up memory in reverse order
        free(the_data[0]->mychar);
        free(the_data[0]);
    }

    return 0;
}

Key Takeaways

  • Never use sizeof() on function pointer parameters to determine string length. Always use strlen().
  • Ensure container structs are allocated before assigning members to them.
  • Avoid typedef-ing pointer types to keep pointer indirection explicit and clear.
  • Always verify memory allocations by checking for NULL returns from malloc().