A patch from Ted Phelps to fix popen(). Ted writes:

    There is a minor bug in the implementation of popen().  In the case
    where the file descriptor returned from its call to pipe() is the same
    as the file descriptor that it is meant to be (where the dup2() branch
    is not taken), the end of the pipe is still closed by close(reading)
    even though it shouldn't be.
Thanks Ted!

libc/stdio/popen.c

@@ -34,8 +34,8 @@ FILE *popen (const char *command, const char *mode)
 		if ((fp = fdopen(pnr, mode)) != NULL) {
 			if ((pid = vfork()) == 0) {	/* vfork -- child */
 				close(pnr);
-				close(reading);
 				if (pr != reading) {
+					close(reading);
 					dup2(pr, reading);
 					close(pr);
 				}