C File Pointers

C File Pointers – ftell(), fseek(), and rewind()

In C, a file pointer of type FILE * is used to manage file operations. It keeps track of the current position in the file and is used in almost all file functions like fopen(), fclose(), fgetc(), and more.

🔍 FILE * Pointer

When you open a file using fopen(), it returns a pointer of type FILE * which acts like a connection between your program and the file. All operations like read/write happen using this pointer.

🧪 Example: Basic FILE Pointer Usage

#include <stdio.h>

int main() {
    FILE *fp = fopen("info.txt", "w");

    if (fp == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    fprintf(fp, "Learning C File Pointers!\n");
    fclose(fp);
    printf("File written using FILE pointer.\n");

    return 0;
}

📌 ftell() – Get Current Position

The ftell() function returns the current position of the file pointer in bytes from the beginning.

🧪 Example: Using ftell()

#include <stdio.h>

int main() {
    FILE *fp = fopen("info.txt", "r");

    if (fp == NULL) {
        printf("File not found.\n");
        return 1;
    }

    fgetc(fp); // Read a character
    long pos = ftell(fp);
    printf("Current file position: %ld\n", pos);

    fclose(fp);
    return 0;
}

📍 fseek() – Move File Pointer

fseek() allows you to move the file pointer to a specific location in the file.

• SEEK_SET – Beginning of the file
• SEEK_CUR – Current position
• SEEK_END – End of file

🧪 Example: Using fseek()

#include <stdio.h>

int main() {
    FILE *fp = fopen("info.txt", "r");

    if (fp == NULL) {
        printf("Cannot open file.\n");
        return 1;
    }

    fseek(fp, 0, SEEK_END); // Move to end
    long size = ftell(fp);  // Get size
    printf("Size of file: %ld bytes\n", size);

    fclose(fp);
    return 0;
}

🔁 rewind() – Reset File Pointer

rewind() resets the file pointer to the beginning of the file.

🧪 Example: Using rewind()

#include <stdio.h>

int main() {
    FILE *fp = fopen("info.txt", "r");

    if (fp == NULL) {
        printf("Unable to read file.\n");
        return 1;
    }

    fgetc(fp); // read a char
    rewind(fp); // go back to start
    char ch = fgetc(fp);
    printf("First character after rewind: %c\n", ch);

    fclose(fp);
    return 0;
}

🧠 Quick Recap

• FILE * is your gateway to any file in C.
• ftell() tells you where you are in the file.
• fseek() lets you jump to any position.
• rewind() resets everything back to the start!

💡 Practice Tip

Create your own file, write some text, then try using fseek() to jump to different positions and read from there!