malloc causes <Access violation writing location>

Question

I am trying to read a file line by line, and get each line as a char * to a dynamic string, the code I am using used to work and without changing it (or noticing it), it has ceased to work, accsesing the reed information results in an error. Here is a MRE of my code for getting one line:

#include <stdio.h>
#include <string.h>
 
#define MAX_STR_SIZE 10000

int main(void)
{
char* filePath; // is set by other working part of program to a real readable file address.
    while (fgetc(filePath) != EOF) // an extra chracter is in the data files to account for this cheack.
    {
        char tempStr[MAX_STR_SIZE] = { 0 };
        char* str = NULL;
        fgets(tempStr, MAX_STR_SIZE, filePath);
        tempStr[strcspn(tempStr, "\n")] = 0;
        str = malloc(sizeof(char) * (strlen(tempStr) + 1)); // does not work
        strcpy(str, tempStr); 
    }
}

The error: Exception thrown at 0x00007ff95448d215 in GifProject.exe: Access violation writing location 0xFFFFFFFFEA1854F0.

Answer 1

It is difficult to diagnose your problem without a complete compilable program that exhibits the problem, but from the code fragment and the debugging information in the image, it seems you do not include <stdlib.h> and the prototype inferred by the compiler for malloc() from the actual argument is int malloc(size_t), leading to undefined behavior when you store the return value into the pointer str: because of the missing prototype, the compiler generates code that converts the return value from int to char *, sign extending from 32-bit to 64-bits, producing a meaningless pointer.

Note that you should also test the return value of fgets to properly handle end of file, and you should test for potential malloc failure before calling strcpy or better: use strdup that allocates and copies a string in a single call.

Here is a modified version:

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

#define MAX_STR_SIZE  4096

char *readline(FILE *file) {
    char tempStr[MAX_STR_SIZE];
    if (!fgets(tempStr, sizeof tempStr, file)) {
        /* end of file: return a null pointer */
        return NULL;
    }
    /* strip the trailing newline if any */
    tempStr[strcspn(tempStr, "\n")] = '\0';
    /* allocate a copy of the string and return it */
    return strdup(tempStr);
}