Fseek

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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) ;
}