File Pointers

Key Functions

• fopen – Open a file and obtain a file pointer.
• fclose – Close a file pointer and release resources.
• fseek – Move the file pointer to a specific location.
• ftell – Retrieve the current offset of the file pointer.
• rewind – Reset the file pointer to the beginning of the file.
• feof – Test for end‑of‑file condition.
• ferror – Detect I/O errors on the stream.

Using File Pointers – Example

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    FILE *fp = fopen("example.txt", "w+");
    if (!fp) {
        perror("fopen");
        return EXIT_FAILURE;
    }

    // Write data
    fputs("Hello, world!\n", fp);
    fputs("File pointers are useful.\n", fp);

    // Move to the beginning
    rewind(fp);

    // Read and display
    char buffer[64];
    while (fgets(buffer, sizeof(buffer), fp)) {
        puts(buffer);
    }

    // Seek to the second line (offset after first line)
    fseek(fp, 14, SEEK_SET); // 14 bytes offset
    fputs("INSERTED LINE\n", fp);

    // Clean up
    fclose(fp);
    return EXIT_SUCCESS;
}

Best Practices

• Always check the return value of fopen before using the pointer.
• Use fclose to prevent resource leaks.
• Validate the result of fseek and ftell for error handling.
• Prefer binary mode ("rb", "wb") when dealing with non‑text data.
• When performing random access, calculate offsets based on sizeof your data structures.

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