Module 37 · Interactive lesson

File Handling Basics

Data that outlives the program

Variables disappear when a program ends — files do not. In the lab below you see the file on the disk next to your program: fopen it in different modes, write with fprintf, read it back with fscanf, fgetc and fgets, watch the file position move, and find out what happens when the file is missing.

fopenopens a file — or returns NULL
'w' / 'a'empty and write / add at the end
EOFmarks the end of the file

Mode Ann Bob

main.c

Ready

Statements run 0 Lines printed 0

Ready

Keyboard: ← → step · Space play / pause · R reset · Esc exit classroom mode

How it works

The ideas behind the lab

Exactly what you watched happen, in plain steps.

Open, use, close

Working with a file always follows the same three steps. fopen(name, mode) opens it and returns a FILE * — a pointer to a structure that remembers the file and the current position — or NULL when it fails. Then you read or write with functions that look like the console ones with an f in front (fprintf, fscanf, fgets, fputs, fgetc, fputc). Finally fclose writes out anything still waiting and releases the file.

FILE *fp = fopen("data.txt", "w");   // 1. open
if (fp == NULL) {                      //    always check!
    perror("data.txt");
    return 1;
}
fprintf(fp, "%d\n", 42);               // 2. use
fclose(fp);                            // 3. close

fopen modes for text files
Mode	Meaning	File must exist?	Old content
"r"	read	yes — else NULL	kept
"w"	write	no — it is created	ERASED
"a"	append (write at the end)	no — it is created	kept
"r+"	read and write	yes	kept
"w+"	read and write	no	erased
"a+"	read, and write at the end	no	kept

Opening and closing

FILE *fp = fopen("name.txt", "r"); — FILE is declared in stdio.h.
fopen returns NULL if the file is missing ("r"), the folder is wrong or access is denied.
perror("name.txt") prints why, e.g. "name.txt: No such file or directory".
fclose(fp) flushes the data to disk and frees the FILE.
A program can only have a limited number of files open — close them when done.

Writing text

fprintf(fp, format, …) — printf into a file.
fputs("text", fp) writes a string (no newline added).
fputc('A', fp) writes one character.
"w" starts with an empty file; "a" adds to the end.
Writes are buffered: they reach the disk at fclose (or fflush).

Reading text and EOF

fscanf(fp, "%d", &n) returns how many values it read — or EOF at the end.
fgets(line, sizeof line, fp) reads one line (with '\n') or returns NULL at the end.
fgetc(fp) returns the next character as an int, or EOF (−1).
Loop on the read function's result: while (fgets(…) != NULL).
Each read moves the file position forward; it starts at 0 for "r".

Walkthrough

A small diary: append a line, then read them all

Every line, explained.

1 #include <stdio.h>

3 int main(void)

4 {

5 FILE *fp = fopen("diary.txt", "a"); Creates the file the first time, adds to it afterwards.

6 if (fp == NULL) {

7 perror("diary.txt"); Prints the reason the file could not be opened.

8 return 1;

9 }

10 fprintf(fp, "Learned files in C\n");

11 fclose(fp);

13 fp = fopen("diary.txt", "r"); Open again, now for reading, at position 0.

14 if (fp == NULL) return 1;

15 char line[100];

16 int n = 0;

17 while (fgets(line, sizeof line, fp) != NULL) One line per round, until NULL.

18 printf("%d: %s", ++n, line); line already ends with '\n'.

19 fclose(fp);

20 printf("%d entries\n", n); Grows by one every time you run the program.

21 return 0;

22 }

Common mistakes

Spot the bug

Mistakes almost every beginner makes — and how to fix them.

Not checking fopen

✗ Wrong

FILE *fp = fopen("data.txt", "r");
fscanf(fp, "%d", &n);   // crash if fp is NULL

✓ Right

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

Missing files are normal. Using a NULL FILE * crashes the program.

Using "w" when you meant "a"

✗ Wrong

fopen("log.txt", "w");   // old log erased

✓ Right

fopen("log.txt", "a");   // add to the end

"w" empties the file the moment it is opened.

while (!feof(fp))

✗ Wrong

while (!feof(fp)) {
    fscanf(fp, "%d", &n);   // last value counted twice
    sum += n;
}

✓ Right

while (fscanf(fp, "%d", &n) == 1)
    sum += n;

feof only becomes true AFTER a read has failed. Test the read itself.

Storing fgetc in a char

✗ Wrong

char c;
while ((c = fgetc(fp)) != EOF)

✓ Right

int c;
while ((c = fgetc(fp)) != EOF)

EOF is −1, an int. In a char it can be confused with a real character or never match.

Forgetting fclose

✗ Wrong

fprintf(fp, "%d", total);
return 0;   // data may never reach the disk

✓ Right

fprintf(fp, "%d", total);
fclose(fp);
return 0;

Writes are buffered. Close (or fflush) to be sure they are saved.

Practice

Check your understanding

Answer each question — you will see the explanation straight away.

0 / 5 correct

Question 1

What does fopen return when the file cannot be opened?

Question 2

A file holds "old". It is opened with "w" and "new" is written. What does it hold?

Question 3

The same file holding "old" is opened with "a" and "new" is written. What does it hold?

Question 4

What does fgets return at the end of the file?

Question 5

Why should the variable that receives fgetc be an int?

Assessment

File Handling Basics quiz

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