/* Orion Lawlor's Short UNIX Examples, olawlor@acm.org 2003/8/18

Connect our own stdout to a pipe, for a weird way 
to read back our own printf's.

Note that because ordinary pipes block when they reach 
4K of unconsumed input, this program deadlocks if you 
try to do too much output.  To fix this, use a network 
socket (for a bigger buffer), fork off a reader, or 
redirect stdout to a file.
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>

/* Redirect stdout to this file pointer.
   This is the central hack for this program.
 */
void redirectStdout(int to_fd) {
	int stdout_fd=1;
	if (-1==dup2(to_fd,stdout_fd)) {perror("dup2 stdout"); exit(1);}
}

/* Return true if this socket is readable right now.
  (Poor man's non-blocking I/O). */
int readable(int fd) {
	fd_set read;
	struct timeval to;
	to.tv_sec=0; to.tv_usec=0;
	FD_ZERO(&read);
	FD_SET(fd,&read);
	if (-1==select(fd+1,&read,NULL,NULL,&to))
		return 0; /* timeout */
	return FD_ISSET(fd,&read)?1:0;
}

/* Can read stuff written to stdout from this fd */
int readStdout;

/* Clean out the stdout pipe */
int readPipes(void) {
	char buf[1001];
	int ret;
	if (!readable(readStdout)) return 0;
	ret=read(readStdout,buf,1000);
	if (ret>0) {
	  buf[ret]=0;
	  fprintf(stderr,"--- Read from stdout (%d bytes) ---\n"
	  		"%s"
			"---\n",ret,buf);
	}
	return 1;
}

int main() {
	/* Set up our stdout socket: */
	int pair[2];
	if (-1==pipe(pair)) {perror("pipe stdout"); exit(1);}
	readStdout=pair[0];
	redirectStdout(pair[1]);
	
	/* Test it out */
	fprintf(stderr,"Doing some printfs...\n");
	printf("Hello, world!\n"); /*Shows nothing*/	
	printf("Yes, it works!\n");
	fflush(stdout);
	fprintf(stderr,"Printfs all returned: reading pipes.\n");
	
	/* Write the output */
	while (readPipes()) {}
	
	return 0;
}
/*<@>
<@> ******** Program output: ********
<@> Doing some printfs...
<@> Printfs all returned: reading pipes.
<@> --- Read from stdout (29 bytes) ---
<@> Hello, world!
<@> Yes, it works!
<@> ---
<@> */