Is C++'s system() synchronized? - c++

Question

I'm writing a small utility which is supposed to launch several commands in parallel using system() and wait for their results for logging purposes. However, even though I'm calling system() on different threads, by looking at my Activity Monitor I only see one instance of each command at a time. It looks like system is internally synchronized on a mutex, and only one execution is allowed at each time, however this looks like a huge limitation, can someone confirm this behavior? Do you have any ideas on how to solve it?
Update by looking at the threads execution flow, it looks like they're effectively synchronizing on a mutex. Is there an alternative system() which doesn't do that?
I should mention I'm using C++11 (w/ clang and libc++) on Mac OS 10.7.5.
Update code is:
void Batch::run()
{
done.clear();
generator->resetGeneration();
while(generator->hasMoreParameters())
{
// Lock for accessing active
unique_lock<mutex> lock(q_mutex, adopt_lock);
// If we've less experiments than threads
if (active.size() < threads)
{
Configuration conf = generator->generateParameters();
Experiment e(executable, conf);
thread t(&Experiment::run, e, reference_wrapper<Batch>(*this));
thread::id id = t.get_id();
active.insert(id);
t.detach();
}
// Condition variable
q_control.wait(lock, [this] { return active.size() < threads; } );
}
}
void Batch::experimentFinished(std::thread::id pos)
{
unique_lock<mutex> lock(q_mutex, adopt_lock);
active.erase(pos);
lock.unlock();
q_control.notify_all();
}
void Experiment::run(Batch& caller)
{
// Generate run command
stringstream run_command;
run_command << executable + " ";
ParameterExpression::printCommandLine(run_command, config);
if (system(run_command.str().c_str()))
stats["success"] = "true";
else
stats["success"] = "false";
caller.experimentFinished(this_thread::get_id());
}
Just to be clear: the spawning and handling of threads works fine and does what it needs to do, but it looks like you can have just one instance of system() running at a time.
Thanks

Answer 1

POSIX has this to say about system(3):
Using the system() function in more than one thread in a process or when the SIGCHLD signal is being manipulated by more than one thread in a process may produce unexpected results.
Due to the way that SIGCHLD must be blocked during the execution, running system calls concurrently doesn't really work. If you want multiple threads to run external tasks, you'll need to write a bit more code (handling fork/exec/wait yourself).

Answer 2

To whoever comes later, popen did the trick, as it doesn't internally keep a mutex. The code to make it work is
FILE* proc;
char buff[1024];
// Keep track of the success or insuccess of execution
if (!(proc = popen(run_command.str().c_str(), "r")))
stats["success"] = "false";
else
stats["success"] = "true";
// Exhaust output
while(fgets(buff, sizeof(buff), proc) != nullptr);
pclose(proc);

Answer 3

In case this helps, I wrote some fork/exec/wait code in C++ a while back. It captures output into a std::string.
As #Mat points out, fork, exec, and wait aren't really designed to be uses in a multi-threaded process.
So this is more useful if multi-process can be a substitute for multi-threaded in your application.
bool Utility::execAndRedirect(std::string command, std::vector<std::string> args, std::string& output, int& status)
{
int error;
int pipefd[2];
int localStatus;
if (pipe(pipefd) == -1)
{
error = errno;
cerr << "Executing command '" << command << "' failed: " << strerror(error) << endl;
return false;
}
pid_t pid = fork();
if (pid == 0)
{
char** argsC;
argsC = new char*[args.size() + 2];
argsC[0] = new char[command.size() + 1];
strncpy(argsC[0], command.c_str(), command.size());
argsC[0][command.size()] = '\0';
for (size_t count = 0; count < args.size(); count++)
{
argsC[count + 1] = new char[args[count].size() + 1];
strncpy(argsC[count + 1], args[count].c_str(), args[count].size());
argsC[count + 1][args[count].size()] = '\0';
}
argsC[args.size() + 1] = NULL;
close(pipefd[0]);
if (dup2(pipefd[1], STDOUT_FILENO) == -1)
{
error = errno;
cerr << "Executing command '" << command << "' failed: " << strerror(error) << endl;
exit(1);
}
if (dup2(pipefd[1], STDERR_FILENO) == -1)
{
error = errno;
cerr << "Executing command '" << command << "' failed: " << strerror(error) << endl;
exit(1);
}
close(pipefd[1]);
if (execvp(command.c_str(), argsC) == -1)
{
error = errno;
cerr << "Executing command '" << command << "' failed: " << strerror(error) << endl;
exit(1);
}
}
else if (pid > 0)
{
size_t BUFFER_SIZE = 1024;
char buffer[BUFFER_SIZE + 1];
close(pipefd[1]);
ostringstream oss;
ssize_t num_b;
while ((num_b = read(pipefd[0], buffer, BUFFER_SIZE)) != 0)
{
buffer[num_b] = '\0';
oss << buffer;
}
output = oss.str();
waitpid(pid, &localStatus, 0);
close(pipefd[0]);
}
else
{
error = errno;
cerr << "Executing command '" << command << "' failed: " << strerror(error) << endl;
return false;
}
if(WIFEXITED(localStatus))
{
status = WEXITSTATUS(localStatus);
//DateTime current = DateTime::now(); //this is a custom class
if(status == 0)
{
return true;
}
else
{
return false;
}
}
else
{
error = errno;
cerr << "Executing command '" << command << "' failed: child didn't terminate normally" << endl;
return false;
}
}