c - Is there any difference without fflush in that code? -

in cpp reference, claims fflush is:

causes output file stream synchronized actual contents of file.

indeed, don't understand means. wonder, in code, if take out fflush, there difference?

i tested it, seems there little bit difference, can't find pattern. 1 explain in details me? in advance.

#include<stdio.h> int i;  int main() {     fork();     for(i=0;i<1000;i++)     {         printf("%d\n",i);         fflush(stdout);// without fflush, there difference?     } } 

the standard output flushed when write newline. if want test properly, open file , write it. tests useful, have write lot more data few integers. should find omitting fflush result in faster code. try timing these 2 loops...

with flushing:

file * fp = fopen("scratch", "w"); for( int = 0; < 1000000; i++ ) {     fprintf( fp, "hello world" );     fflush(fp); } fclose(fp); 

without flushing:

file * fp = fopen("scratch", "w"); for( int = 0; < 1000000; i++ ) {     fprintf( fp, "hello world" ); } fclose(fp); 

on machine, results are:

with fflush:    4.57 seconds without fflush: 0.24 seconds