File Handling in C

What is file handling?

Normally, everything a C program stores in variables vanishes the moment the program ends. File handling lets a program save data to a file on disk so it survives — you can write it now and read it back tomorrow. This is how programs remember scores, settings, records and reports.

C does all of this through a special FILE pointer and a set of functions in <stdio.h>. The pattern is always the same: open a file, use it, close it.

The FILE pointer and fopen

To work with a file, you first open it with fopen, which gives you back a FILE pointer. Every later operation uses that pointer.

FILE *fp;                       // a file pointer
fp = fopen("data.txt", "w");    // open for writing

if (fp == NULL) {               // opening can fail!
    printf("Could not open file.\n");
    return 1;
}

The fopen call takes two things: the file name and a mode that says what you want to do with it.

File opening modes

Writing to a file

Once open in a write mode, fprintf works just like printf but sends text into the file.

#include <stdio.h>
int main() {
    FILE *fp = fopen("data.txt", "w");
    if (fp == NULL) return 1;

    fprintf(fp, "Hello, file!\n");
    fprintf(fp, "Marks: %d\n", 95);

    fclose(fp);                 // save and close
    printf("Data written.\n");
    return 0;
}

Reading from a file

To read, open the file in "r" mode and use fgets (for lines) or fscanf (for formatted values).

#include <stdio.h>
int main() {
    FILE *fp = fopen("data.txt", "r");
    if (fp == NULL) {
        printf("File not found.\n");
        return 1;
    }
    char line[100];
    while (fgets(line, sizeof(line), fp) != NULL)
        printf("%s", line);      // print each line

    fclose(fp);
    return 0;
}

The fgets loop keeps reading until it returns NULL, which marks the end of the file.

Always check and close

Two habits make file handling safe and reliable:

• Check for NULL right after fopen — the file may be missing or locked.
• Call fclose when done — it flushes buffered data to disk and frees the file.

Common mistakes

• Using the FILE pointer without checking it for NULL first.
• Opening an existing file in "w" and accidentally erasing it.
• Forgetting fclose, which can lose buffered data.
• Trying to read a file opened in write mode, or write to one opened as read-only.

Summary

• File handling saves data permanently using a FILE pointer and stdio.h functions.
• Open with fopen(name, mode); always check the result for NULL.
• Modes: "r" read, "w" overwrite, "a" append, and the + variants.
• Write with fprintf; read with fgets or fscanf.
• Always fclose to flush data and release the file.