piping stockfish misbehaves in fedora - c++

Question

Somewhere in my project I use fork and pipe to execute another process and pipe its I/O to communicate with it (I'm writing it in C++). There is no problem when I compile it in Ubuntu 14.04, it will work just fine, but I compiled it in fedora on a WMWare virtual machine and strange things began to happen. If I run the binary in terminal, there is no error but nothing will be written in the pipe (but getting streams of characters will work). I tried to debug my code in fedora, I put a break point in my code, but then a broken pipe signal was given when process tried to read from pipe (there were no signals when executing in terminal).
So, have any of you encountered such problems before? Is there any difference in piping between debian and red hat linux? Or is it because I'm running fedora on a virtual machine?
CODE:
int mFD_p2c [2];
int mFD_c2p [2];
int mEnginePID;
if (pipe(mFD_p2c) != 0 || pipe(mFD_c2p) != 0)
{
cout << "Failed to pipe";
exit(1);
}
mEnginePID = fork();
if (mEnginePID < 0)
{
cout << "Fork failed";
exit(-1);
}
else if (mEnginePID == 0)
{
if (dup2(mFD_p2c[0], 0) != 0 ||
close(mFD_p2c[0]) != 0 ||
close(mFD_p2c[1]) != 0)
{
cout << "Child: failed to set up standard input";
exit(1);
}
if (dup2(mFD_c2p[1], 1) != 1 ||
close(mFD_c2p[1]) != 0 ||
close(mFD_c2p[0]) != 0)
{
cout << "Child: failed to set up standard output";
exit(1);
}
string engine = "stockfish";
execlp(engine.c_str(), (char *) 0);
cout << "Failed to execute " << engine;
exit(1);
}
else
{
close(mFD_p2c[0]);
close(mFD_c2p[1]);
string str = "uci";
int nbytes = str.length();
if (write(mFD_p2c[1], str.c_str(), nbytes) != nbytes)
{
cout << "Parent: short write to child";
exit(1);
}
cout << "The following string has been written to engine:\n"
<< string(1, '\t') << str;
char readBuffer[2];
string output = "";
while (1)
{
int bytes_read = read(mFD_c2p[0], readBuffer, sizeof(char));
if (readBuffer[0] == '\n')
break;
readBuffer[bytes_read] = '\0';
output += readBuffer;
}
cout << "Got: " << output;
}

Answer 1

I see you're using Stockfish. I too have exactly experienced this behavior from Stockfish. The problem lies within how it handles output. Defined in misc.h:
#define sync_cout std::cout << IO_LOCK
And looking at the code again we'll see that IO_LOCK is an enum which is used in an overloaded friend operator for cout:
std::ostream& operator<<(std::ostream& os, SyncCout sc) {
static Mutex m;
if (sc == IO_LOCK)
m.lock();
if (sc == IO_UNLOCK)
m.unlock();
return os;
}
What I see here is that during using cout, a mutex is locked. I don't know how exactly this affects cout's output in a pipe instead of stdout, but I'm positive that this is the cause for the problem. You can check it by removing the lock functionality.
Edit: I forgot to mention that the pipe behavior is not different in linux based systems as mentioned before, but there might be slight differences between distributions handling mutexes used with pipes.

Answer 2

There are no differences in piping between debian and red hat, but the following list of questions may help you:
-Are the Ubuntu and the Fedora using the same architecture (64 bit vs 32) ?
-Are you using the same version of gcc (or any other compiler) ?
(Suggestion: use cerr for your error outputs, and maybe your debug output too -> you dup the standard outputs and inputs, so if something fails you may not see it)

Answer 3

Anyhow, here's how you turn it into a self-contained, compilable example:
stockfish
#cat stockfish
tr a-z A-Z #just so we do something
echo #need to end with a "\n" or else the parent won't break out of the while loop
Run command:
make pipes && PATH=.:$PATH pipes
pipes.cc
//pipes.cc
#include <iostream>
#include <fstream>
#include <string>
#include <unistd.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
using namespace std;
int mFD_p2c [2];
int mFD_c2p [2];
int mEnginePID;
if (pipe(mFD_p2c) != 0 || pipe(mFD_c2p) != 0)
{
cout << "Failed to pipe";
exit(1);
}
mEnginePID = fork();
if (mEnginePID < 0)
{
cout << "Fork failed";
exit(-1);
}
else if (mEnginePID == 0)
{
if (dup2(mFD_p2c[0], 0) != 0 ||
close(mFD_p2c[0]) != 0 ||
close(mFD_p2c[1]) != 0)
{
cout << "Child: failed to set up standard input";
exit(1);
}
if (dup2(mFD_c2p[1], 1) != 1 ||
close(mFD_c2p[1]) != 0 ||
close(mFD_c2p[0]) != 0)
{
cout << "Child: failed to set up standard output";
exit(1);
}
string engine = "stockfish";
char *const args[]={};
int ret;
execvp(engine.c_str(), args);
//I need the endl here or else it doesn't show for me when the execvp fails; I wasn't able to compile the original exec command so I used a different one from the exec* family
cout << "Failed to execute " << engine << endl;
exit(1);
}
else
{
close(mFD_p2c[0]);
close(mFD_c2p[1]);
string str = "uci";
int nbytes = str.length();
if (write(mFD_p2c[1], str.c_str(), nbytes) != nbytes)
{
cout << "Parent: short write to child";
exit(1);
}
//My particular child process tries to read to the end, so give it the EOF
close(mFD_p2c[1]);
cout << "The following string has been written to engine:\n"
<< string(1, '\t') << str;
char readBuffer[2];
string output = "";
while (1)
{
int bytes_read = read(mFD_c2p[0], readBuffer, sizeof(char));
if (readBuffer[0] == '\n')
break;
readBuffer[bytes_read] = '\0';
output += readBuffer;
}
cout << "Got: " << output;
}
return 0;
}
output:
The following string has been written to engine:
uciGot: UCI