Reading and writing files in C are fundamental operations that allow you to manipulate data stored in files on disk. The C Standard Library provides several functions for these operations, mainly through the <stdio.h> header. Below is an overview of how to read from and write to files in C.

Reading from Files

You can read data from files using various functions provided by the C Standard Library:

1. fgetc()

This function reads a single character from the file.

Syntax:

int fgetc(FILE *stream);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r"); // Open the file in read mode
    if (file == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    int ch;
    while ((ch = fgetc(file)) != EOF) { // Read characters until end of file
        putchar(ch); // Print each character to standard output
    }

    fclose(file); // Close the file
    return 0;
}

2. fgets()

This function reads a line from the file and stores it in a string.

Syntax:

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

• str: The string where the read line will be stored.
• n: The maximum number of characters to read.
• stream: The file pointer.

Example:

#include <stdio.h>

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

    char line[100];
    while (fgets(line, sizeof(line), file)) { // Read a line from the file
        printf("%s", line); // Print the line
    }

    fclose(file);
    return 0;
}

3. fscanf()

This function reads formatted input from the file, similar to scanf().

Syntax:

int fscanf(FILE *stream, const char *format, ...);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "r"); // Open the file in read mode
    if (file == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    int age;
    char name[50];
    while (fscanf(file, "%s %d", name, &age) == 2) { // Read formatted data
        printf("Name: %s, Age: %d\n", name, age);
    }

    fclose(file);
    return 0;
}

Writing to Files

You can write data to files using various functions as well:

1. fputc()

This function writes a single character to the file.

Syntax:

int fputc(int char, FILE *stream);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("output.txt", "w"); // Open the file in write mode
    if (file == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    for (char ch = 'A'; ch <= 'Z'; ch++) {
        fputc(ch, file); // Write characters to the file
    }

    fclose(file);
    return 0;
}

2. fputs()

This function writes a string to the file.

Syntax:

int fputs(const char *str, FILE *stream);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("output.txt", "w");
    if (file == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    fputs("Hello, World!\n", file); // Write a string to the file

    fclose(file);
    return 0;
}

3. fprintf()

This function writes formatted output to the file, similar to printf().

Syntax:

int fprintf(FILE *stream, const char *format, ...);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("output.txt", "w");
    if (file == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    int age = 30;
    fprintf(file, "Name: Alice, Age: %d\n", age); // Write formatted data

    fclose(file);
    return 0;
}

Important Notes

• Error Checking: Always check if the file opened successfully by verifying that the file pointer is not NULL.

• File Modes: When writing to a file, be aware of the file modes ("w", "a", etc.) to avoid losing existing data unintentionally.

• Buffering: C uses buffering for file I/O, so data may not be written immediately. Use fflush(FILE *stream) to flush the output buffer if necessary.

Summary

• Reading Files: Use functions like fgetc(), fgets(), and fscanf() to read data from files.
• Writing Files: Use fputc(), fputs(), and fprintf() to write data to files.
• Error Handling: Always check for errors when opening files and handle them appropriately.

Understanding file I/O in C enables you to manage persistent data effectively, allowing your programs to store and retrieve information efficiently.