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Showing content from https://cplusplus.com/reference/cstdio/fseek/ below:

function

<cstdio>

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

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.



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 * * 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 
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/* fseek example */
#include <stdio.h>

int main ()
{
  FILE * pFile;
  pFile = fopen ( "example.txt" , "wb" );
  fputs ( "This is an apple." , pFile );
  fseek ( pFile , 9 , SEEK_SET );
  fputs ( " sam" , pFile );
  fclose ( pFile );
  return 0;
}

After this code is successfully executed, the file example.txt contains:


See also
ftell
Get current position in stream (function)
fsetpos
Set position indicator of stream (function)
rewind
Set position of stream to the beginning (function)

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