C fseek()

The fseek() function in C is used to set the file position indicator (cursor) within a file. It allows you to move the cursor to a specific position in the file, which is useful for reading or writing data at a particular location within the file. fseek() is typically used with files opened in binary mode ("rb" for reading or "wb" for writing).

Here’s the prototype of the fseek() function:

int fseek(FILE *stream, long int offset, int whence);

• stream: A pointer to the FILE structure representing the open file.
• offset: The number of bytes by which the file position indicator is to be moved. A positive value moves the indicator forward, and a negative value moves it backward.
• whence: An integer that specifies the reference point for the offset. It can take one of the following values:
• SEEK_SET (0): Set the position relative to the beginning of the file.
• SEEK_CUR (1): Set the position relative to the current position of the file pointer.
• SEEK_END (2): Set the position relative to the end of the file.

The fseek() function returns 0 on success and a non-zero value on failure.

Here are some common uses of fseek():

1. Move to the Beginning of a File:

 fseek(file, 0, SEEK_SET);

2. Move to the End of a File:

 fseek(file, 0, SEEK_END);

3. Move Forward or Backward a Specific Number of Bytes:

 fseek(file, 100, SEEK_CUR); // Move 100 bytes forward from the current position
 fseek(file, -50, SEEK_CUR); // Move 50 bytes backward from the current position

4. Move to a Specific Position in the File:

 fseek(file, 200, SEEK_SET); 
 // Move to the 200th byte from the beginning of the file

Here’s a simple example that demonstrates the use of fseek() to move the cursor within a file:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "rb"); // Open a file for reading in binary mode

    if (file == NULL) {
        perror("Error opening the file");
        return 1;
    }

    fseek(file, 10, SEEK_SET); // Move to the 10th byte from the beginning of the file

    // Read and print the content from this position
    char buffer[100];
    if (fread(buffer, 1, sizeof(buffer), file) > 0) {
        printf("Data read from the file: %s\n", buffer);
    } else {
        perror("Error reading the file");
    }

    fclose(file); // Close the file

    return 0;
}

In this example, fseek() is used to move to the 10th byte from the beginning of the file, and then data is read and printed from that position.