Fseek

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fseek is a C function belonging to the ANSI C standard library, and included in the file stdio.h. It's purpose is to change the file position indicator for the specified stream.

[edit] Function prototype

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

Argument meaning:

• stream_pointer is a pointer to the stream FILE structure of which the position indicator should be change;
• offset is a long integer which specified the number of bytes from origin where the position indicator should be place ;
• origin is an integer which specified the origin position. It can be:
  • SEEK_SET : set the position indicator to the start of the stream ;
  • SEEK_CUR : keep the current position indicator ;
  • SEEK_END : set the position indicator the end of the stream ;

[edit] Return value

The return value is an integer which mean:

• 0 (zero) : function performed successfully in the stream
• nonzero : an error occurred
• On devices incapable of seeking, the return value is undefined.

Notes that each error number has a distinct meaning. The meaning can be revealed by checking errno.h.

[edit] Example

#include <stdio.h>

int main(int argc, char **argv) {
  FILE *file_pointer;
  file_pointer = fopen("text.txt","r");
  if(fseek(file_pointer, 0, SEEK_SET)) {
    puts("An error occurred");
  }
  else {
    char buffer[100];
    fgets(buffer, 100, file_pointer);
    puts("The first line of the file is:");
    puts(buffer);
  }
  fclose(file_pointer);
  return 0;
}

This program code opens a file called text.txt in read-only mode, tries to force the file pointer to the beginning of the file, and outputs the first line of the file.