C: popen reading from stdout blocks - c

Question

The manpage for popen says "reading from a "popened" stream reads the command's standard output".
However, I can't seem to get the subprocess output in the trivial program below. The "reader" parent process blocks on the read (whether using fgets or fread)
What am I missing?
Attaching to the pinger program with gdb shows it is looping and calling printf to output text. Just nothing detected by fgets on the parent's side...
PINGER.C
#include <string.h>
#include <stdio.h>
#include <unistd.h>
int
main(int argc, char **argv)
{
int i = 0;
while (1)
{
printf("stdout %d\n", i++);
sleep(1);
}
}
POPENTEST.C
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
int
main(int argc, char **argv)
{
char *cmd = "./pinger";
printf("Running '%s'\n", cmd);
FILE *fp = popen(cmd, "r");
if (!fp)
{
perror("popen failed:");
exit(1);
}
printf("fp open\n");
char inLine[1024];
while (fgets(inLine, sizeof(inLine), fp) != NULL)
{
printf("Received: '%s'\n", inLine);
}
printf("feof=%d ferror=%d: %s\n", feof(fp), ferror(fp), strerror(errno));
pclose(fp);
}
OUTPUT
$ ./popenTest
fp open

Answer 1

By default, C buffers writes to the stdout when stdout is not connected to a tty. This means that from the OS' perspective, the program has not written anything to stdout until either the buffer is full or you manually flushed the output:
#include <string.h>
#include <stdio.h>
#include <unistd.h>
int
main(int argc, char **argv)
{
int i = 0;
while (1)
{
printf("stdout %d\n", i++);
fflush(stdout);
sleep(1);
}
}
When connected to a tty, the stdout is automatically flushed on every newline. But this automatic flushing does not happen when the stdout is connected to a pipe.