File I/O (`stdio.h`)

Files are accessed through FILE* streams.

FILE *fp = fopen("data.txt", "r");
if (!fp) { perror("open"); exit(EXIT_FAILURE); }

char line[128];
while (fgets(line, sizeof line, fp)) {
    // process line
}

fclose(fp);

Modes:

• r – read (file must exist).
• w – write (creates/overwrites).
• a – append.
• r+, w+, a+ – read/write variants.

Always check the return of fread, fwrite, fprintf, and close the file with fclose.