Files and Streams

beginner13 min readLesson 58 of 148

A FILE* is a handle to an open stream; modes decide read, write, or append.

Opening and closing

#include <stdio.h>

FILE *f = fopen("notes.txt", "r");   // mode: how you intend to use it
if (f == NULL) {
    perror("notes.txt");             // WHY it failed, on stderr
    return 1;
}
// ... use f ...
fclose(f);                           // every open gets a close

fopen returns NULL on failure — missing file, no permission, bad path. Unconditionally assuming success is a beginner signature move; check it.

The modes that matter now

| mode | meaning | if file exists | if missing | |------|---------|----------------|------------| | "r" | read | reads from start | NULL | | "w" | write | TRUNCATED to empty | created | | "a" | append | writes at end | created |

"w" destroys existing content the moment you open it. "a" preserves it.

Streams you already use

stdout and stderr are FILE* too. printf(...) is fprintf(stdout, ...). That is why the same fprintf/fgets family works everywhere.