Calling fgets() on popen() of 'ssh' is flushing the beginning of stdin of the calling process (ptty issue) - ssh

Question

I have now whittled this down to a minimal test case. Thus far I have been able to determine that this is an issue related to pseudo-terminals which come about with the pipe of ssh. Adding the '-t -t' to the ssh call improved things, in that now, it takes a second call to fgets() to cause the issue. I suspect that the stderr output of the ssh command somehow works into the issue, for now I have redirected stderr to stdout in the ssh code to execute. I do wonder if the "tcgetattr: Invalid argument" error is part of the problem, but am not sure how to get rid of that. It seems to come from the -t -t being present. I believe the -t -t is moving in the right direction, but I have to set up the pseudo terminal for stderr somehow and perhaps the test will work properly?
The Makefile:
test:
gcc -g -DBUILD_MACHINE='"$(shell hostname)"' -c -o test.o test.c
gcc -g -o test test.o
.PHONY: clean
clean:
rm -rf test.o test
The test.c source file:
#include <unistd.h>
#include <string.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
const unsigned int bufSize = 32;
char buf1[bufSize];
char buf2[bufSize];
int ssh = argv[1][0] == 'y';
const char *cmd = ssh ? "ssh -t -t " BUILD_MACHINE " \"ls\" 2>&1" : "ls";
FILE *fPtr = popen(cmd, "r");
if (fPtr == NULL) {
fprintf(stderr,"Unable to spawn command.\n");
perror("popen(3)");
exit(1);
}
printf("Command: %s\n", cmd);
if (feof(fPtr) == 0 && fgets(buf2, bufSize, fPtr) != NULL) {
printf("First result: %s\n", buf2);
if (feof(fPtr) == 0 && fgets(buf2, bufSize, fPtr) != NULL) {
printf("Second result: %s\n", buf2);
int nRead = read(fileno(stdin), buf1, bufSize);
if (nRead == 0) {
printf("???? popen() of ssh consumed the beginning of stdin ????\n");
} else if (nRead > 0) {
if (strncmp("The quick brown fox jumped", buf1, 26) != 0) {
printf("??? Failed ???\n");
} else {
printf("!!!!!!! Without ssh popen() did not consume stdin !!!!!!!\n");
}
}
}
}
}
This shows it running the passing way:
> echo "The quick brown fox jumped" | ./test n
Command: ls
First result: ARCH.linux_26_i86
Second result: Makefile
!!!!!!! Without ssh popen() did not consume stdin !!!!!!!
This shows it running the failing way:
> echo "The quick brown fox jumped" | ./test y
Command: ssh -t -t hostname "ls" 2>&1
First result: tcgetattr: Invalid argument
Second result: %backup%~ gmon.out
???? popen() of ssh consumed the beginning of stdin ????

Answer 1

Okay, I have got this working finally. The secret was to supply /dev/null as the input to my ssh command as follows from the test case above:
const char *cmd
= ssh ? "ssh -t -t " BUILD_MACHINE " \"ls\" 2>&1 < /dev/null" : "ls";
However, while the code works correctly, I get a nasty message which apparently I can ignore for my purposes (although I'd like to make the message go away):
tcgetattr: Inappropriate ioctl for device