kindly do not post solutions like this, as it goes to the main comment thread where nobody wants to see it

#include <stddef.h>
#include <stdlib.h>

int *twin_sum_solutions(int *input, size_t input_len, size_t *output_len) {
if (input_len < 2) {
*output_len = 0;
return NULL; // Return NULL if there are no pairs
}

int *res = malloc(input_len * sizeof(int));  // Allocate memory for the result
if (!res) {
    *output_len = 0;
    return NULL;  // Return NULL if memory allocation fails
}

*output_len = 0;  // Initialize the result counter

for (size_t i = 0; i < input_len - 1; i++) {
    if (input[i] == input[i + 1]) {  // Check if it's a matching pair
        res[*output_len] = input[i] * 2;  // Store the "twin sum"
        (*output_len)++;
        i++;  // Skip the next element as it's already processed
    }
}

// Resize the memory to the correct size of results
int *final_result = realloc(res, *output_len * sizeof(int));

// If realloc fails and there are results, free the memory
if (!final_result && *output_len > 0) {
    free(res);
    *output_len = 0;
    return NULL;
}

return final_result;  // Return the result (or NULL if no results)

}