Program breaks after a couple hundred lines have been read

Question

I made a scanning function which essentially just scans in the lines of a file into a single char * called buffer. However, after a couple hundred lines are read, the program just stops working. I just get a program has stopped working pop up window. Assuming I did something wrong with memory allocation, but I am unsure what.

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

char *scan_file(FILE *fp);

#define MAX_LINE                200


int main(void) {
    FILE *fp = fopen("test.txt", "r");
    char *contents = scan_file(fp);
    printf("%s\n", contents);
    return 0;
}
// Scan in file into a buffer. Returns a malloc-ed string
char *scan_file(FILE *fp) {

    int buf_len = 1;
    int contents_len = buf_len;

    char *buffer = malloc(sizeof(char) * (MAX_LINE + 1));
    char *contents = malloc(sizeof(char) * (buf_len + 1));
    strcpy(contents, "\0");

    while (fgets(buffer, MAX_LINE, fp) != NULL) {
        buf_len = strlen(buffer);
        contents_len += buf_len;
        realloc(contents ,contents_len);
        strcat(contents, buffer);
        strcpy(buffer, "\0");
    }

    free(buffer);
    return contents;
}

Answer 1

Code fails to use the return value form realloc()

Allocation sizes off by 1.

Repeated strcat() make for a slow (n*n) solution.

Consider using size_t for array sizes vs. int.

Rather than call the variable ...len, consider ...size to acknowledge the presence of the final null character in a string.

char *scan_file(FILE *fp) {
    // int buf_len = 1;
    size_t buf_size = 1;

    // int contents_len = buf_len;
    size_t contents_size = buf_size;

    // char *buffer = malloc(sizeof(char) * (MAX_LINE + 1));
    // fgets(..., MAX_LINE, ...) will only read up to MAX_LINE - 1 characters.
    char *buffer = malloc(MAX_LINE);

    char *contents = malloc(buf_size + 1u);
    if (buffer == NULL || contents == NULL) {
      fprintf(stderr, "Out of memory\n");
      return EXIT_FAILURE;
    }

    // strcpy(contents, "\0");
    contents[0] = '\0';

    while (fgets(buffer, MAX_LINE, fp) != NULL) {
        // buf_len = strlen(buffer);
        buf_size = strlen(buffer) + 1u;

        // contents_len += buf_len;

        // realloc(contents ,contents_len);
        void *p = realloc(contents ,contents_size + buf_size);
        if (p == NULL) {
          fprintf(stderr, "Out of memory\n");
          return EXIT_FAILURE;
        }
        contents = p;

        // strcat(contents, buffer);
        strcpy(contents + contents_size, buffer);

        // now add
        contents_size += buf_size;

        // Code here not truly needed, yet helps in debugging.
        // strcpy(buffer, "\0");
        buffer[0] = '\0';
    }

    free(buffer);
    return contents;
}