FILE and fopen

Learn the FILE struct and opening files with fopen.

FILE and fopen

To work with a file in C, you use a handle of type FILE*. The fopen function opens a file in a specific mode and returns this handle.

Example: FILE *f = fopen("test.txt", "w");. Here "w" is write mode (truncates existing content). Modes: "r" read, "w" write, "a" append. If fopen fails it returns NULL; that's why checking before use is required. File operations are declared in stdio.h; so #include <stdio.h> is needed.

NULL check

fopen isn't always successful. If the file doesn't exist, permission is denied, or the disk is full, it returns NULL. Operating on a NULL handle is undefined behavior; that's why checking is required.

Example pattern: FILE *f = fopen("x.txt", "r"); if (f == NULL) return 1;. This pattern safely exits the program when the file can't be opened. Production code does more detailed error handling; but the basic pattern is this. Putting a NULL check for every fopen is a sign of robust C code.

Exercise Write to a file and confirm

Open a file in write mode with FILE *f = fopen("test.txt", "w");. Check with if (f == NULL) return 1;. Write with fprintf(f, "Merhaba\n");. Close with fclose(f);. Print confirmation with printf("Dosya yazıldı\n");.

Show example solution

#include <stdio.h>

int main(void) {
    FILE *f = fopen("test.txt", "w");
    if (f == NULL) return 1;
    fprintf(f, "Merhaba\n");
    fclose(f);
    printf("Dosya yazıldı\n");
    return 0;
}

Test yourself

Which mode opens a file for reading?
- r
- a
- rw
- w
Show answer

r — The "r" mode opens a file for reading.

Complete the fopen call

FILE *f = fopen("notlar.txt", "____");

Show answer

FILE *f = fopen("notlar.txt", "w");

Match modes to purpose

"r"

Read mode
"w"

Write mode
"a"

Append mode
NULL

Open failed

Next lesson: fprintf and fscanf

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