C/File/Stream Buffer
Change stream buffering: how to setbuf
#include <stdio.h>
int main ()
{
char buffer[256];
FILE *file1, *file2;
file1=fopen ("f1.txt","w");
file2=fopen ("f2.txt","a");
setbuf ( file1 , buffer );
fputs ("This is sent to the buffered stream: file1",file1);
fflush (file1);
setbuf ( file2 , NULL );
fputs ("This is sent to the unbuffered stream: file2",file2);
fclose (file1);
fclose (file2);
return 0;
}