fsetpos

    int fsetpos(FILE *stream, const fpos_t *pos); 

reset the position indicator to using fsetpos() stores the current file position indicator to the location indicated by the information in pos Returns non-zero on error.

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
 
int main( void )
{
    FILE *stream;
    fpos_t file_pos;
    int c;
    int i;
 
    if((stream=fopen("test.txt","r"))==NULL) {
        printf("Cannot open file.\n");
        return 1;
    }
 
   fgetpos(stream, &file_pos); 
 
   while((c = fgetc(stream) ) != EOF) {
     if ( c == '#' ) { 
             fgetpos(stream, &file_pos); 
           }
        printf("%c", c);
     }
 
   printf("1st Position of # is the %d Byte\n", file_pos);
 
   printf("\nset to given position\nand print it again\n");
   fsetpos(stream, &file_pos);
 
   while((c = fgetc(stream) ) != EOF) {
        printf("%c", c);
     }
 
      fclose(stream);
}

  output:  ./fsetpos 
  test 1234,
   ... # ...
  this is a test for code-reference.com 
  1st Position of # is the 17 Byte
  
  set to given position
  and print it again
   ...
  this is a test for code-reference.com