manipulating files in place

[caroundw5h]

I'm trying to understand file streams and how they work and I'm still a little frustrated as to why this code wouldn't work - without calling fseek,

/*manipulating files in place */
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main(int argc, char *argv[]){
    int ch,lower;
    FILE *file;

   
    
    if ( (file = fopen(argv[1], "r+") ) == NULL){
    	puts("Unable to open file");
    	getchar();
    	return 1;
     }   
  
    	
   	while( ( ch = fgetc(file) ) != EOF){
   	    if( (isupper(ch) ) )
   	      tolower(ch); 
   	      
    }    
       
    if ( fclose(file) !=0)
       puts("Unable to close file");
       
     getchar();
     return 0;
 }

I got this

If the file is opened in the update mode (+), then output cannot be directly followed by input and input cannot be directly followed by output without an intervening fseek, fsetpos, rewind, or fflush.

but why?

[Sebastiani]

read this post.