A short code example of using popen in c to start a process then write to it using fprintf.

#include <stdio.h>


int main(int argc, char **argv) {
	FILE *fp = popen("cat", "w");
	int i = 0;

	for(i=0;i<10;i++)
		fprintf(fp, "Count = %d\n", i);

	pclose(fp);

	return 0;
}