modify popen output and print it in c language

Question

The following code is producing the complete strace output. Why wouldn't it produce only the first 10 characters? How can I control the output of strace in my code?

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

#define ARRL 250

void execute_ps(){
    int count=0;
    char p;
    FILE *fp;
    char cmd[ARRL];

    //insert system command
    sprintf(cmd, "strace echo hello");
    //open channel to see output of command executed
    fp = popen(cmd,"r");
    if(fp==NULL){printf("popen err:%s\n",strerror(errno));exit(1);}
    while((p=fgetc(fp))!=EOF){
        printf("%c",p);
        if (count==10){break;}
        count++;
    }

pclose(fp);
}


int main(int argc, char **argv)
{
    execute_ps();
    exit(0);
}

Answer 1

strace prints to stderr, while popen only looks at stdout. Removing the print statement completely will still cause output to be produced. Changing strace echo hello to strace echo hello 2>&1 (see this) seems to fix the problem and cause only ten characters to be produced.