A Complete Guide - C Programming File Handling Basics FILE fp

Step-by-Step Guide to File Handling in C

Step 1: Including Necessary Header File

To perform file handling operations in C, you need to include the standard I/O library header file:

#include <stdio.h>

Step 2: Opening a File

Use the fopen function to open a file. The fopen function returns a FILE pointer which points to the file:

FILE *fp;
fp = fopen("filename.txt", "mode");

• "filename.txt" is the name of the file.
• "mode" specifies the purpose of opening the file (e.g., reading, writing).

Common modes are:

• "r": Open for reading (the file must exist).
• "w": Open for writing (create a new file; if it exists, it will be overwritten).
• "a": Open for appending (create a new file if it does not exist; if it exists, the file pointer will be placed at the end of the file, and data will be written to the end).
• "rb", "wb", "ab": Binary versions of the above options.

Example:

#include <stdio.h>

int main() {
    FILE *fp;
    fp = fopen("example.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    fclose(fp);
    return 0;
}

Step 3: Writing to a File

Use fprintf, fputc, fwrite, etc., to write to a file.

Using fprintf:

#include <stdio.h>

int main() {
    FILE *fp;
    fp = fopen("example.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    fprintf(fp, "Hello, World!\n");
    fclose(fp);
    return 0;
}

Using fputc:

#include <stdio.h>

int main() {
    FILE *fp;
    fp = fopen("example.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    fputc('H', fp);
    fputc('e', fp);
    fputc('l', fp);
    fputc('l', fp);
    fputc('o', fp);
    fclose(fp);
    return 0;
}

Using fwrite:

#include <stdio.h>

int main() {
    FILE *fp;
    char data[100] = "Hello, World!\n";
    fp = fopen("example.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    fwrite(data, 1, sizeof(data), fp);
    fclose(fp);
    return 0;
}

Step 4: Reading from a File

Use fscanf, fgetc, fgets, fread, etc., to read from a file.

Using fscanf:

#include <stdio.h>

int main() {
    FILE *fp;
    char buffer[100];
    fp = fopen("example.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    while (fscanf(fp, "%s", buffer) == 1) {
        printf("%s ", buffer);
    }
    fclose(fp);
    return 0;
}

Using fgetc:

#include <stdio.h>

int main() {
    FILE *fp;
    char ch;
    fp = fopen("example.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);
    }
    fclose(fp);
    return 0;
}

Using fgets:

#include <stdio.h>

int main() {
    FILE *fp;
    char buffer[100];
    fp = fopen("example.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    while (fgets(buffer, sizeof(buffer), fp) != NULL) {
        printf("%s", buffer);
    }
    fclose(fp);
    return 0;
}

Using fread:

#include <stdio.h>

int main() {
    FILE *fp;
    char buffer[100];
    fp = fopen("example.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return(-1);
    }
    if (fread(buffer, 1, sizeof(buffer), fp) > 0) {
        printf("%s", buffer);
    }
    fclose(fp);
    return 0;
}

Step 5: Closing a File

Always close the file after completing file operations using fclose:

fclose(fp);

Complete Example

Here is a complete example that writes to a file and then reads from it: