Reading & Writing Files

Read a text file line by line

fgets returns NULL at end-of-file, which doubles as the loop condition.

FILE *fp = fopen("notes.txt", "r");
if (fp == NULL) { perror("open"); return 1; }

char line[100];
while (fgets(line, sizeof(line), fp) != NULL) {
    printf("%s", line);
}
fclose(fp);

fwrite / fread for raw data

Binary mode reads/writes exact byte layouts — useful for structs, not just text.

struct Point { int x, y; } p = {3, 4};

FILE *fp = fopen("point.bin", "wb");
fwrite(&p, sizeof(p), 1, fp);
fclose(fp);

struct Point q;
fp = fopen("point.bin", "rb");
fread(&q, sizeof(q), 1, fp);
fclose(fp);
printf("%d,%d\n", q.x, q.y);

Output:

3,4

Quick check: What does fopen return if the file cannot be opened?

• NULL
• An empty string
• It crashes the program