Fseek

Description

• Reposition stream position indicator
• Sets the position indicator associated with the stream to a new position.
• For streams open in binary mode, the new position is defined by adding offset to a reference position specified by origin.
• For streams open in text mode, offset shall either be zero or a value returned by a previous call to ftell, and origin shall necessarily be SEEK_SET.
• If the function is called with other values for these arguments, support depends on the particular system and library implementation (non-portable).
• The end-of-file internal indicator of the stream is cleared after a successful call to this function, and all effects from previous calls to ungetc on this stream are dropped.
• On streams open for update (read+write), a call to fseek allows to switch between reading and writing.

Syntax

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

Parameters

stream

Pointer to a FILE object that identifies the stream.

offset

Binary files: Number of bytes to offset from origin.

Text files: Either zero, or a value returned by ftell.

origin

Position used as reference for the offset. It is specified by one of the following constants defined in <cstdio> exclusively to be used as arguments for this function:

Constant	Reference position
SEEK_SET	Beginning of file
SEEK_CUR	Current position of the file pointer
SEEK_END	End of file. Note: Library implementations are allowed to not meaningfully support SEEK_END (therefore, code using it has no real standard portability).

Return Value

• If successful, the function returns zero.
• Otherwise, it returns non-zero value.
• If a read or write error occurs, the error indicator (ferror) is set.

Example

#include <stdio.h>

main()
{
	char fname[21] ;
	int n ;
	long num ;
	FILE * fstr ;

	printf ("File to open? ") ;
	scanf ("%20s", fname) ;
	fstr = fopen (fname, "r") ;
	while ( printf ("character to read (number)? "),
    	    scanf ("%ld", &num), num )
	    { fseek (fstr, sizeof(int)*(num-1), SEEK_SET) ;
	      fread (&n, sizeof(int), 1, fstr) ;
	      printf (" value: %d \n", n) ;
        }
    fclose (fstr) ;
}