c 3/5 30 XP

File I/O

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Quick Reference

File modes:

Mode	Meaning	Creates?	Truncates?
`"r"`	Read only	No (must exist)	No
`"w"`	Write (create/overwrite)	Yes	Yes
`"a"`	Append	Yes	No

Read/write functions:

Function	Purpose	Check return
`fprintf(fp, fmt, ...)`	Write formatted to file	—
`fscanf(fp, fmt, ...)`	Read formatted from file	`== expected_count`
`fgets(buf, size, fp)`	Read one line safely	`!= NULL`
`fputs(str, fp)`	Write string to file	—

Standard pattern:

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

char line[256];
while (fgets(line, sizeof(line), fp) != NULL) {
    line[strcspn(line, "\n")] = '\0';
    // process line
}

fclose(fp);

Shell ↔ C mapping:

Shell	C equivalent
`printf(...)`	`fprintf(stdout, ...)`
`./prog > file`	`fprintf(fp, ...)`
`scanf(...)`	`fscanf(stdin, ...)`
`./prog 2> err`	`fprintf(stderr, ...)`

Common Pitfalls

while (!feof(fp)) — Processes garbage on last iteration. Use while (fscanf(...) == n) or while (fgets(...) != NULL).
"w" truncates — Opens and erases. Use "a" for append.
Not checking fopen — Returns NULL on failure. Dereferencing NULL FILE * crashes.
Forgetting fclose — Buffered data may not be written. Always close.
fgets newline — Includes \n. Strip with strcspn.
Mixing fgets and fscanf — fscanf leaves \n in buffer. Clear with while(fgetc(fp)!='\n'){} or use fgets consistently.

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