fseek()

Move the file position indicator.

  #include <stdio.h>
  
  int fseek(FILE *stream, long offset, int whence);    

stream

A pointer to a file stream.

  offset   

The number of characters to move.

whence

The starting position to move from.

The fseek() function moves the file position indicator to allow random access to a file.

The function moves the file position indicator either absolutely or relatively. The whence argument can be one of three values defined in stdio.h:

• SEEK_SET sets the file position indicator to offset bytes from the beginning of the file. In this case offset must be equal or greater than zero.
• SEEK_CUR sets the file position indicator to offset bytes from its current position. The offset argument can be a negative or positive value.
• SEEK_END moves the file position indicator to offset bytes from the end of the file. The offset argument must be equal or less than zero.

The fseek() function undoes the last ungetc() call and clears the end-of-file status of stream.

The function has limited use when used with MS-DOS text files opened in text mode because of carriage return/line feed translations. The seek operations may be incorrect near the end of the file due to end-of-file translations.

The only operations guaranteed to work in MS-DOS text files opened in text mode are:

• Using the offset returned from ftell() and seeking from the beginning of the file.
• Seeking with an offset of zero from SEEK_SET, SEEK_CUR and SEEK_END.

This facility may not be available on configurations of the EWL that run on platforms without file systems.

fseek() returns zero if it is successful and returns a nonzero value if it fails.

#include <stdio.h>

#include <stdlib.h>

int main(void)

{

FILE *f;

long int pos1, pos2, newpos;

char filename[80], buf[80];

// get a filename from the user

printf("Enter a filename to read.\n");

gets(filename);

// open a file for input

if (( f = fopen(filename, "r")) == NULL) {

printf("Can't open %s.\n", filename);

exit(1);

}

printf("Reading last half of first line.\n");

// get the file position indicator before and after

// reading the first line

pos1 = ftell(f);

fgets(buf, 80, f);

pos2 = ftell(f);

printf("Whole line: %s\n", buf);

// calculate the middle of the line

newpos = (pos2 - pos1) / 2;

fseek(f, newpos, SEEK_SET);

fgets(buf, 80, f);

printf("Last half: %s\n", buf);

fclose(f);

return 0;

}

Output:

Enter a filename to read.

itwerks

Reading last half of first line.

Whole line: The quick brown fox

Last half: brown fox