Fixed moving window.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

int main(){

setbuf(stdout,NULL); //disable output buffering

char *str=malloc(9);
strcpy(str,"abcdefgh");

str=realloc(str,strlen(str)+1+8);
strcat(str,"efghefgh");     //sprintf(str,"%s%s",str,"efghefgh");

unsigned int imax=1024/strlen(str)*1024*4;

printf("%s","exec.tm.sec\tstr.length\n"); //fflush(stdout);

time_t starttime=time(NULL);
char *gstr=malloc(0);
unsigned int i=0;
size_t lngth;

char *pos_c=gstr;
size_t str_len=strlen(str);

    while(i++ < imax+1000){
        lngth=strlen(str)*i;
        gstr=realloc(gstr,lngth+str_len);
        strcat(gstr,str);    //sprintf(gstr,"%s%s",gstr,str);
        pos_c=(gstr + (str_len * (i - 1)));

        while((pos_c=strstr(pos_c,"efgh"))){
            memcpy(pos_c,"____",4);
        }

        if(lngth % (1024*256)==0){
            printf("%lusec\t\t%lukb\n",time(NULL)-starttime,lngth/1024); //fflush(stdout);
        }
    }
//printf("%s\n",gstr);

}