我在为面试做编码挑战。获取此错误：

Reading symbols from Solution...done.
[New LWP 183007]
Core was generated by `./Solution'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007fb1eb4fdcf8 in ?? ()

字符串
守则如下：

#include <assert.h>
#include <ctype.h>
#include <limits.h>
#include <math.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char* readline();
/*
 * Complete the 'newPassword' function below.
 *
 * The function is expected to return a STRING.
 * The function accepts following parameters:
 *  1. STRING a
 *  2. STRING b
 */
/*
 * To return the string from the function, you should either do static allocation or dynamic allocation
 *
 * For example,
 * char* return_string_using_static_allocation() {
 *     static char s[] = "static allocation of string";
 *
 *     return s;
 * }
 *
 * char* return_string_using_dynamic_allocation() {
 *     char* s = malloc(100 * sizeof(char));
 *
 *     s = "dynamic allocation of string";
 *
 *     return s;
 * }
 *
 */
char* newPassword(char* a, char* b) {
     int len = strlen(a);
     char* c = "";
     int i = 0;
     for(; i < len; i++){
         strcat(c, &a[i]);
         strcat(c, &b[i]);
     }
     for(int z = i; z < strlen(b); z++){
         strcat(c, &b[z]);
     }
     return c;
}
int main()
{
    FILE* fptr = fopen(getenv("OUTPUT_PATH"), "w");
    char* a = readline();
    char* b = readline();
    char* result = newPassword(a, b);
    fprintf(fptr, "%s\n", result);
    fclose(fptr);
    return 0;
}
char* readline() {
    size_t alloc_length = 1024;
    size_t data_length = 0;
    char* data = malloc(alloc_length);
    while (true) {
        char* cursor = data + data_length;
        char* line = fgets(cursor, alloc_length - data_length, stdin);
        if (!line) {
            break;
        }
        data_length += strlen(cursor);
        if (data_length < alloc_length - 1 || data[data_length - 1] == '\n') {
            break;
        }
        alloc_length <<= 1;
        data = realloc(data, alloc_length);
        if (!data) {
            data = '\0';
            break;
        }
    }
    if (data[data_length - 1] == '\n') {
        data[data_length - 1] = '\0';
        data = realloc(data, data_length);
        if (!data) {
            data = '\0';
        }
    } else {
        data = realloc(data, data_length + 1);
        if (!data) {
            data = '\0';
        } else {
            data[data_length] = '\0';
        }
    }
    return data;
}

型
我不是一个真正的C/C++的人，所以需要一些见解。