Thread: The mystical art of debugging

Hi all. I am learning how to use the debugger.

Here is a program that I wrote in the early days of learning C for displaying the contents of a text file to the screen.

I had always thought that it would grab 100 characters of a line(at a time) and then output to the screen, looping repeatedly, until fgets() == NULL. To my surprise, it seems to be storing several lines somewhere, and then only outputting them when fgets() == NULL. So it seems to output only once. Why is it doing this, and how can it be doing this when the buffer size is only 100?

Thanks.

Code:

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

#define MAX_FILE_LENGTH 40
#define MAX_STRING_LENGTH 100

int main(void)
{
    int count, file_length, string_length;
    char filename[MAX_FILE_LENGTH], line[MAX_STRING_LENGTH];
    FILE *openfile;

    puts("Enter file to open");
    fgets(filename, MAX_FILE_LENGTH, stdin);

    string_length = strlen(filename);

    filename[string_length-1] = '\0';

    puts(filename);

    if ((openfile = fopen(filename, "r")) != NULL)
        puts("File opened successfully\n");

    else
        {
            puts("Error opening file");
            exit(1);
        }

    while(fgets(line, MAX_STRING_LENGTH, openfile ) != NULL)
        {
            printf("%s", line);
        }

    fclose(openfile);

    return 0;
}