How to Read File Line by Line in C: Simple Guide

To read a file line by line in C, open the file with fopen, then use fgets in a loop to read each line into a buffer until the end of the file. Always check if the file opened successfully and close it with fclose after reading.

Syntax

Here is the basic syntax to read a file line by line in C:

• FILE *fp = fopen("filename", "r"); opens the file for reading.
• char buffer[SIZE]; declares a buffer to store each line.
• while (fgets(buffer, SIZE, fp) != NULL) reads each line until the end.
• fclose(fp); closes the file when done.

c

FILE *fp = fopen("filename.txt", "r");
char buffer[256];
if (fp == NULL) {
    // handle error
}
while (fgets(buffer, sizeof(buffer), fp) != NULL) {
    // process the line stored in buffer
}
fclose(fp);

Example

This example opens a file named example.txt, reads it line by line, and prints each line to the screen.

c

#include <stdio.h>

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

    char line[256];
    while (fgets(line, sizeof(line), fp) != NULL) {
        printf("%s", line);
    }

    fclose(fp);
    return 0;
}

Output

Hello, this is line one.
This is line two.
And this is line three.

Common Pitfalls

Common mistakes when reading files line by line include:

• Not checking if fopen returns NULL, which means the file failed to open.
• Using fgets without a large enough buffer, causing lines to be cut off.
• Not handling the newline character that fgets keeps at the end of each line.
• Forgetting to close the file with fclose, which can cause resource leaks.

c

// Wrong way: Not checking file open
FILE *fp = fopen("file.txt", "r");
char line[10];
while (fgets(line, sizeof(line), fp) != NULL) {
    printf("%s", line);
}
// Missing fclose(fp);

// Right way:
FILE *fp2 = fopen("file.txt", "r");
if (fp2 == NULL) {
    // handle error
}
char line2[256];
while (fgets(line2, sizeof(line2), fp2) != NULL) {
    printf("%s", line2);
}
fclose(fp2);

Quick Reference

Function	Purpose
fopen(filename, "r")	Open file for reading
fgets(buffer, size, file)	Read one line into buffer
fclose(file)	Close the opened file
NULL check	Verify file opened successfully

Key Takeaways

Always check if fopen returns NULL before reading the file.

Use fgets in a loop to read each line safely into a buffer.

Choose a buffer size large enough to hold the longest expected line.

Remember to close the file with fclose to free resources.

fgets keeps the newline character; handle it if needed.