C Library - fseek() function

Quiz

The C library fseek(FILE *stream, long int offset, int whence) function sets the file position of the stream to the given offset.This function is a part of the C Standard Library and is used for file handling.

Syntax

Following is the C library syntax of the fseek() function −

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

Parameters

This function accept three parameters −

FILE *stream: A pointer to a FILE object that identifies the stream.

long int offset: The number of bytes to move the file pointer.

int whence: The position from which the offset is added. It can have one of the following values:

SEEK_SET: Beginning of the file.
SEEK_CUR: Current position of the file pointer.
SEEK_END: End of the file.

Return Value

The function returns 0 on success and Non-zero value on failure. The function also sets the errno to indicate the error.

Example 1: Moving to the Beginning of the File

This program opens a file and moves the file pointer to the beginning of the file before reading the first character.

Below is the illustration of the C library fseek() function.

#include <stdio.h>

int main() {
   FILE *file = fopen("example1.txt", "r");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   fseek(file, 0, SEEK_SET);

   char ch = fgetc(file);
   if (ch != EOF) {
       printf("First character in the file: %c\n", ch);
   }

   fclose(file);
   return 0;
}

Output

The above code produces following result−

First character in the file: [First character of the file]

Example 2: Moving Backward from the Current Position

In this example, we move the file pointer to the 10th byte from the beginning and then moves it 3 bytes backward, resulting in the file pointer being at the 7th byte, from which it reads the character.

#include <stdio.h>

int main() {
   FILE *file = fopen("example3.txt", "r");
   if (file == NULL) {
       perror("Error opening file");
       return 1;
   }

   fseek(file, 10, SEEK_SET);  // Move to the 10th byte from the beginning
   fseek(file, -3, SEEK_CUR);  // Move 3 bytes backward from the current position

   char ch = fgetc(file);
   if (ch != EOF) {
       printf("Character after moving backward: %c\n", ch);
   }

   fclose(file);
   return 0;
}

Output

After execution of above code, we get the following result

Character after moving backward: [Character at position 7]

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