Why does this loop iterating over a file buffer throw a BAD ACCESS exception?

Question

I've been picking at this for hours and can't figure out why the loop would try to access out of bounds memory.... Any help would be super appreciated!

int CountCharacters(FILE *fp, const char* filename){
    // Get file size
    fseek(fp, 0L, SEEK_END);
    long size = ftell(fp);
    if (size == -1){ perror("Failed: "); return 0;} 
    rewind(fp); //seek back to file's beginning

    // Allocate approrpiately sized buffer ( size + 1 for null-termination)
    char *buf = malloc(sizeof (char) * (size + 1)); 

    // Read the entire file into memory
    size_t newLen = fread(buf, sizeof(char), size, fp);
    if ( ferror( fp ) != 0 ) {
        fputs("Error reading file", stderr);
    } else {
        buf[newLen++] = '\0'; /* Just to be safe. */
    }

    //Try to get byte count from buffer
    int byte_count[256] = {0}; //initialize character counts to 0
    for (int i = 0; i < size; ++i){
        byte_count[(int) buf[i]]++; //BAD ACCESS ERROR HERE
    }

    /* Do something with byte_count here */

    return (1);
}

Did you check the value of `buf[i]` when the access error occurs? — kaylum, Feb 08 '20 at 10:36
@Renat Wow it looks like that did the trick! Does that mean the bad access was coming from the byte_count array, and not the buf? — Adi Shah, Feb 08 '20 at 10:42
Does this answer your question? [What is an unsigned char?](https://stackoverflow.com/questions/75191/what-is-an-unsigned-char) Essentially, buf contained negative numbers, which you were then using as an index into byte_count — Korosia, Feb 08 '20 at 10:49
For better debugging, you could split the line into separate instructions. This way you will see exactly, where something goes wrong. And you could insert `printf()` or `assert()` or anthing you see fit to debug. — the busybee, Feb 08 '20 at 14:31

Why does this loop iterating over a file buffer throw a BAD ACCESS exception?

0 Answers0