C fgets() Function | Read a String from a File

Introduction

The fgets() function is used for reading a line of text from a stream and storing it into a buffer. It is commonly used for reading strings from files or user input.

fgets() Function Syntax

The syntax for the fgets() function is as follows:

char *fgets(char *str, int n, FILE *stream);

Parameters:

• str: A pointer to an array of characters where the string read is stored.
• n: The maximum number of characters to be read (including the null terminator).
• stream: A pointer to a FILE object that specifies an input stream.

Returns:

• The function returns str on success, and NULL if an error occurs or end-of-file is reached while no characters have been read.

Examples

Reading a Line from a File

To demonstrate how to use fgets() to read a line from a file, we will write a simple program.

Example

#include <stdio.h>

int main() {
    FILE *file;
    char buffer[100];

    // Open the file for reading
    file = fopen("example.txt", "r");
    if (file == NULL) {
        printf("Error: Could not open file for reading.\n");
        return 1;
    }

    // Read a line from the file
    if (fgets(buffer, sizeof(buffer), file) != NULL) {
        // Print the read line
        printf("Read line: %s", buffer);
    } else {
        printf("Error: Could not read from file.\n");
    }

    // Close the file
    fclose(file);

    return 0;
}

Output (assuming example.txt contains some text):

Read line: This is an example line from the file.

Reading a Line from Standard Input

This example shows how to use fgets() to read a line from standard input.

Example

#include <stdio.h>

int main() {
    char buffer[100];

    // Prompt the user for input
    printf("Enter a line of text: ");

    // Read a line from standard input
    if (fgets(buffer, sizeof(buffer), stdin) != NULL) {
        // Print the read line
        printf("You entered: %s", buffer);
    } else {
        printf("Error: Could not read from standard input.\n");
    }

    return 0;
}

Output (example user input "Hello, World!"):

Enter a line of text: Hello, World!
You entered: Hello, World!

Real-World Use Case

Reading Configuration Lines from a File

In real-world applications, the fgets() function can be used to read lines from a configuration file.

Example

#include <stdio.h>

int main() {
    FILE *file;
    char buffer[100];

    // Open the configuration file for reading
    file = fopen("config.txt", "r");
    if (file == NULL) {
        printf("Error: Could not open configuration file for reading.\n");
        return 1;
    }

    // Read lines from the file and process them
    while (fgets(buffer, sizeof(buffer), file) != NULL) {
        // Process the configuration line (here we simply print it)
        printf("Config line: %s", buffer);
    }

    // Close the file
    fclose(file);

    return 0;
}

Output (assuming config.txt contains configuration lines):

Config line: url=http://example.com
Config line: timeout=5000
Config line: maxConnections=100

Conclusion

The fgets() function in C is a standard library function that reads a line from the specified stream and stores it into the string pointed to by str. It is part of the C standard library (stdio.h) and is commonly used for reading input from a file or from the standard input (stdin).