C library function - fseek()

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Description

The C library function int fseek(FILE *stream, long int offset, int whence) sets the file position of the stream to the given offset.

Declaration

Following is the declaration for fseek() function.

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

Parameters

• stream − This is the pointer to a FILE object that identifies the stream.

• offset − This is the number of bytes to offset from whence.

• whence − This is the position from where offset is added. It is specified by one of the following constants −

Sr.No.	Constant & Description
1	SEEK_SET Beginning of file
2	SEEK_CUR Current position of the file pointer
3	SEEK_END End of file

Return Value

This function returns zero if successful, or else it returns a non-zero value.

Example

The following example shows the usage of fseek() function.

#include <stdio.h>

int main () {
   FILE *fp;

   fp = fopen("file.txt","w+");
   fputs("This is tutorialspoint.com", fp);
  
   fseek( fp, 7, SEEK_SET );
   fputs(" C Programming Language", fp);
   fclose(fp);
   
   return(0);
}

Let us compile and run the above program that will create a file file.txt with the following content. Initially program creates the file and writes This is tutorialspoint.com but later we had reset the write pointer at 7th position from the beginning and used puts() statement which over-write the file with the following content −

This is C Programming Language

Now let's see the content of the above file using the following program −

#include <stdio.h>

int main () {
   FILE *fp;
   int c;

   fp = fopen("file.txt","r");
   while(1) {
      c = fgetc(fp);
      if( feof(fp) ) {
         break;
      }
      printf("%c", c); 
   }
   fclose(fp);
   return(0);
}

Let us compile and run the above program to produce the following result −

This is C Programming Language