How to seek to a position of a file? How to know current position? How to rewind position back to beginning?

Overview

file is like byte array located in secondary memory device. Programmer can seek to any offset of an array by changing offset. However seeking to a file offset is not the same as the size of a file often too large to store in primary memory. File functions works on current file position. This position can be changed by fseek. Programmer need to provide a desired offset and origin.

FSEEK

Fseek allows programmer to change current file position to a particulate offset with reference to origin.

Syntax

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

Parameters

  • fp - Pointer to a FILE object that identifies the stream.
  • offset - Number of bytes to offset from origin.
  • origin - Position used as reference for the offset. It is specified by one of the following constants.
    • SEEK_SET - Beginning of file
    • SEEK_CUR - Current position of the file pointer
    • SEEK_END - End of file

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.

FTELL

Returns the current value of the position indicator of the file.

SYNTAX

long int ftell ( FILE * fp );

Parameters

fp - Pointer to a FILE object that identifies the stream.

Return Value

On success, the current value of the position indicator is returned. On failure, -1L is returned, and errno is set to a system-specific positive value.

REWIND

Sets the position indicator associated with stream to the beginning of the file.

Syntax

void rewind ( FILE * fp );

Parameters

fp - Pointer to a FILE object that identifies the stream.

Return Value

none

fseek, ftell, rewind example


/* fseek, ftell, rewind example : getting size of a file */
#include <stdio.h>

int main ()
{
  FILE * fp;
  long size;
  long chunk;
  char buffer[100];

  fp = fopen ("myfile.txt","rb");
  if (fp == NULL) {
    perror ("Error opening file");
  }
  else
  {
    fseek (fp, 0, SEEK_END);
    size= ftell (fp);
    
    printf ("Size of myfile.txt: %ld bytes.\n",size);
  rewind(fp); /* Same as fseek (fp, 0, SEEK_SET); */
  
  while((chunk = fread(buffer, 100, 1, fp)) > 0 && size) {
  size -= chunk;
  fwrite(buffer, chunk, 1, fp);
  
  }
  fclose (pFile);
  }
  return 0;
}

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