Reading from files in C

Introduction

Reading from files lets your program get information saved on your computer. This helps your program use data that was saved before or from other sources.

You want to load a list of names saved in a file to use in your program.

You need to read settings or preferences stored in a file to customize your program.

You want to process a text file with data like scores or messages.

You want to read a log file to check what happened earlier.

You want to read a file that a user selects to show its content.

Syntax

C

FILE *file_pointer;
file_pointer = fopen("filename.txt", "r");
if (file_pointer == NULL) {
    // handle error
}
// read from file
fclose(file_pointer);

fopen opens the file. The "r" means open for reading.

Always check if fopen returns NULL. That means the file could not be opened.

Examples

Open a file named data.txt for reading and check if it opened successfully.

C

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) {
    printf("Cannot open file\n");
}

Read the file line by line and print each line.

C

char line[100];
while (fgets(line, sizeof(line), fp)) {
    printf("%s", line);
}

Read the file character by character and print each character.

C

int c;
while ((c = fgetc(fp)) != EOF) {
    putchar(c);
}

Sample Program

This program opens a file named example.txt for reading. It reads the file line by line and prints each line to the screen. If the file cannot be opened, it prints an error message.

C

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        printf("Failed to open file.\n");
        return 1;
    }

    char line[256];
    while (fgets(line, sizeof(line), file)) {
        printf("%s", line);
    }

    fclose(file);
    return 0;
}

Important Notes

Remember to close the file with fclose() to free resources.

fgets() reads one line at a time including the newline character.

Use fgetc() to read one character at a time if needed.

Summary

Use fopen() with "r" to open a file for reading.

Check if the file opened successfully by testing if the pointer is NULL.

Read file content using fgets() for lines or fgetc() for characters.