The following program, crashes with
libc++abi.dylib: terminating with uncaught exception of type stxxl::io_error: Error in virtual void stxxl::ufs_file_base::lock() : fcntl(,F_SETLK,) path=/var/tmp/stxxl fd=5 : Resource temporarily unavailable: unspecified iostream_category error
Abort trap: 6
This looks a lot like my two threads are trying to use the same file handler/file for doing updates to the stxxl file til /var/tmp.
Is there a trick for having multiple threads using multiple files in stxxl?
#include <stxxl/queue>
#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
void test() {
typedef stxxl::queue<unsigned int> queue;
queue my_queue;
for(unsigned long long i = 0; i != 1024L * 1024 * 1024; i++)
my_queue.push(10);
std::cout << "queue_size " << my_queue.size() << std::endl;
while(my_queue.size() != 0)
my_queue.pop();
std::cout << "queue_size " << my_queue.size() << std::endl;
}
int main()
{
pid_t pid;
pid_t cpid;
int status;
pid = fork();
if (pid == 0)
{
test();
exit(0);
} else
{
test();
if ((cpid=wait(&status)) == pid){
std::cout << "Child " << pid << " returned" << std::endl;
}
}
return 0;
}
With STXXL 1.4.0 you can also use "###" in a .stxxl config file. The "###" is replaced with the current pid when files are opened.
Note that the disk files are automatically opened when the first STXXL functions are called. So one must delay such calls until after the fork(), which like you did in your example.
I found the answer my self, a solution is to give every thread its own virtual disk, this I have done like this:
stxxl::config * cfg = stxxl::config::get_instance();
std::string result;
std::ostringstream convert;
convert << id; // the id of the stream, to make th filenames different
result = convert.str();
std::string base_file = "/var/tmp/stxxl" + result;
stxxl::disk_config disk1(base_file, 100 * 1024 * 1024, "syscall autogrow delete_on_exit");
cfg->add_disk(disk1);
Related
Use the answer in the question: simultaneous read and write to child's stdio using boost.process,
I refactored the code and hybridized the new method using the Boost library. I've been successful in making a pipes connection with Stockfish, but this is also where I get errors I've never seen before, not even Google helps.
Here is what I have tried:
#include <stdio.h>
#include <time.h>
#include <string>
#include <memory.h>
#include <unistd.h>
#include <iostream>
#include <stddef.h>
#include <execinfo.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <fstream>
#include </usr/local/include/backtrace.h>
#include </usr/local/include/backtrace-supported.h>
#include <boost/process.hpp>
#include <boost/asio.hpp>
#include <boost/process/async.hpp>
#include <vector>
#include <iomanip>
#include <stdlib.h>
#include <string.h>
using namespace std;
namespace bp = boost::process;
using boost::system::error_code;
using namespace std::chrono_literals;
string errDetails = "Error Details: ";
void delay(int number_of_seconds) {
int ms = 1000 * number_of_seconds;
clock_t start_time = clock();
while (clock() < start_time + ms)
;
}
static void full_write(int fd, const char* buf, size_t len) {
while (len > 0) {
ssize_t ret = write(fd, buf, len);
if ((ret == -1) && (errno != EINTR)) {
break;
}
buf += (size_t) ret;
len -= (size_t) ret;
}
}
void print_backtrace() {
static const char start[] = "--------BACKTRACE--------\n\n";
static const char end[] = "-------------------------\n\n";
void *bt[1024];
int bt_size;
char **bt_syms;
int i;
bt_size = backtrace(bt, 1024);
bt_syms = backtrace_symbols(bt, bt_size);
full_write(STDERR_FILENO, start, strlen(start));
full_write(STDERR_FILENO, errDetails.c_str(), strlen(errDetails.c_str()));
for (i = 1; i < bt_size; i++) {
size_t len = strlen(bt_syms[i]);
full_write(STDERR_FILENO, bt_syms[i], len);
full_write(STDERR_FILENO, "\n", 1);
}
full_write(STDERR_FILENO, end, strlen(end));
free(bt_syms);
}
void abort_application() {
size_t memLeakCount, staticMemLeakCount;
uint64_t memLeakSize, staticMemLeakSize;
for (int i = 0; i < 3; i++) {
/**
* Delay
*/
delay(1);
}
print_backtrace();
abort();
}
inline bool stockfish_check_exists(const std::string& name) {
struct stat buffer;
return (stat(name.c_str(), &buffer) == 0);
}
int main() {
std::future<std::string> data;
boost::asio::io_service svc;
bp::async_pipe in{svc}, out{svc};
string proc = "";
char command[64];
string output = "";
if (stockfish_check_exists("stockfish")) {
proc = "stockfish"; } else {
errDetails = "Stockfish not found!\n\n";
abort_application();
}
std::string const program_dir = proc;
auto on_exit = [](int code, std::error_code ec) {
std::cout << "Exited " << code << "(" << ec.message() << ")\n";
};
bp::child process(proc, bp::std_in < in, svc);
boost::asio::streambuf recv_buffer;
std::cout << "uci send" << std::endl;
boost::asio::async_write(in, boost::asio::buffer("uci\n"),
[&](boost::system::error_code ec, size_t transferred) {
std::cout << "Write: " << transferred << "\n" << std::endl;
in.close();
}
);
std::cout << "isready send" << std::endl;
boost::asio::async_write(in, boost::asio::buffer("isready\n"),
[&](boost::system::error_code ec, size_t transferred) {
std::cout << "Write: " << transferred << "\n" << std::endl;
in.close();
}
);
cout << "Enter your command: ";
cin >> command;
cout << "Your command is: " << command << endl;
if (strcmp(command, "quit") == 0) {
cout << "Quiting......." << endl;
boost::asio::async_write(in, boost::asio::buffer("quit"),
[&](boost::system::error_code ec, size_t transferred) {
std::cout << "Write: " << transferred << std::endl;
in.close();
cout << "Engine quit!" << endl;
}
);
}
svc.run();
return 0;
}
To make it easier to follow, I left out std::std_out > out at the line:
bp::child process(proc, bp::std_in < in, svc);
so that the engine results are immediately displayed in the Terminal window, so I'll know if I've gone astray. And this is when I discovered the strange thing
When I launch the application, it outputs on Terminal as follows:
[2022-01-14 20:25:55]
duythanh#DuyThanhs-MacBook-Pro:/Volumes/Data/ChessGUI$ ./ChessGUI
uci send
isready send
Enter your command: Stockfish 120122 by the Stockfish developers (see AUTHORS file)
id name Stockfish 120122
id author the Stockfish developers (see AUTHORS file)
option name Debug Log File type string default
option name Threads type spin default 1 min 1 max 512
option name Hash type spin default 16 min 1 max 33554432
option name Clear Hash type button
option name Ponder type check default false
option name MultiPV type spin default 1 min 1 max 500
option name Skill Level type spin default 20 min 0 max 20
option name Move Overhead type spin default 10 min 0 max 5000
option name Slow Mover type spin default 100 min 10 max 1000
option name nodestime type spin default 0 min 0 max 10000
option name UCI_Chess960 type check default false
option name UCI_AnalyseMode type check default false
option name UCI_LimitStrength type check default false
option name UCI_Elo type spin default 1350 min 1350 max 2850
option name UCI_ShowWDL type check default false
option name SyzygyPath type string default <empty>
option name SyzygyProbeDepth type spin default 1 min 1 max 100
option name Syzygy50MoveRule type check default true
option name SyzygyProbeLimit type spin default 7 min 0 max 7
option name Use NNUE type check default true
option name EvalFile type string default nn-ac07bd334b62.nnue
uciok
Unknown command: isready
Contrasting with the code above, the two commands were sent through pipes. is uci and isready, this is fine. The first uci command runs successfully, but the isready command, instead of returning readyok, it returns:
Unknown command: isready
I keep trying to type quit, which sends a quit command to the pipe as the exit engine, and it also fails:
Your command is: quit
Quiting.......
Write: 5
Write: 9
Unknown command: quit
Write: 5
Engine quit!
The program will then exit with the engine. I'm still wondering what was going on at the time, but the clues are really hazy as to what was going on behind the scenes.
Please help me. Any help is highly appreciated. Thank you so much everyone
UPDATE: The error continued when Unknown Command: Quit appeared. I typed these commands in Terminal while running Stockfish directly through Terminal, they work as a result, but my program still can't
You are printing to cout as if the async operations happen immediately. That's not the case. The async operations only happen when the io service runs.
svc.run();
Is at the very end of your code. So no async_ operation ever completes (or even starts) before that.
Other problems:
Your out async pipe is never used (not even connected). It's unclear to me how you intend to communicate with the child process that way.
In fairness, you only every write to the child process, so maybe you're not at all interested in the output. (But then perhaps recv_buffer can be deleted just as well).
Your buffers include the terminating NUL characters. (asio::buffer("uci\n") sends {'u','c','i','\n','\0'}). That's going to mess up the child processes's parsing.
You do in.close() in response to every single async_write completion. This guarantees that subsequent writes never can happen, as you closed the pipe.
Then when you send quit you fail to include the '\n' as well
You are reading into a char[64] with operator>> which makes no sense at all. Maybe you are using c++20 (so width of 64 might be assumed) but you never set a width. Most likely you would want to read into a string instead.
However, doing so cannot accept commands with whitespace (because std::ios::skipws is set by default). So, likely you wanted std::getline instead...
The fact that you include a boatload of C headers makes me think you're porting some C code (badly). That's also exemplified by the strcmp use and others, e.g. no need to use ::stat
Don't use using namespace std; (Why is "using namespace std;" considered bad practice?)
Don't use global variables (errDetails)
Don't use loops to wait for a time delay
No need to manually print backtraces. Instead, use Boost:
void abort_application(std::string const& errDetails) {
std::cerr << errDetails << "\n";
std::cerr << boost::stacktrace::stacktrace{} << std::endl;
std::this_thread::sleep_for(3s);
abort();
}
Existing Stockfish Client: Playing Games
You're in luck: I have a written full demo using stockfish on this site: Interfacing with executable using boost in c++.
This example shows how to correctly await and parse expected replies from the child process(es).
You will note that I chose coroutines for the async version:
Just for completeness, I thought I'd try an asynchronous implementation. Using the default Asio callback style this could become unwieldy, so I thought to use Boost Coroutine for the stackful coroutines. That makes it so the implementation can be 99% similar to the synchronous version
Just for comparison, here's what your code should look like if you didn't use coroutines:
Fixing Up Your Code
Live On Coliru
#include <boost/asio.hpp>
#include <boost/process.hpp>
#include <boost/process/async.hpp>
#include <boost/stacktrace/stacktrace.hpp>
#include <chrono>
#include <iomanip>
#include <iostream>
namespace bp = boost::process;
using boost::system::error_code;
using namespace std::literals;
static void abort_application(std::string const& errDetails) {
std::cerr << errDetails << "\n";
std::cerr << boost::stacktrace::stacktrace{} << std::endl;
std::this_thread::sleep_for(3s);
abort();
}
inline static bool stockfish_check_exists(std::string& name) {
return boost::filesystem::exists(name);
}
int main() {
boost::asio::io_service svc;
bp::async_pipe in{svc};
std::string proc = "/usr/games/stockfish";
if (!stockfish_check_exists(proc)) {
abort_application("Stockfish not found!");
}
auto on_exit = [](int code, std::error_code ec) {
std::cout << "Exited " << code << "(" << ec.message() << ")\n";
};
bp::child process(proc, bp::std_in < in, svc, bp::on_exit = on_exit);
std::function<void()> command_loop;
std::string command_buffer;
command_loop = [&] {
std::cout << "Enter your command: " << std::flush;
// boost::asio::streambuf recv_buffer;
if (getline(std::cin, command_buffer)) {
std::cout << "Your command is: " << command_buffer << std::endl;
command_buffer += '\n';
async_write( //
in, boost::asio::buffer(command_buffer),
[&](error_code ec, size_t transferred) {
std::cout << "Write: " << transferred << " (" << ec.message() << ")" << std::endl;
if (command_buffer == "quit\n") {
std::cout << "Quiting......." << std::endl;
// in.close();
std::cout << "Engine quit!" << std::endl;
} else {
command_loop(); // loop
}
});
}
};
std::cout << "uci send" << std::endl;
async_write(
in, boost::asio::buffer("uci\n"sv),
[&](error_code ec, size_t transferred) {
std::cout << "Write: " << transferred << "\n" << std::endl;
std::cout << "isready send" << std::endl;
async_write(in, boost::asio::buffer("isready\n"sv),
[&](error_code ec, size_t n) {
std::cout << "Write: " << n << std::endl;
command_loop(); // start command loop
});
});
svc.run(); // only here any of the operations start
}
Prints, e.g.
Or if Stockfish is in fact installed:
I have the following reduced program that spins up a bunch of child processes and then uses a boost::interprocess::message_queue to send a message to each one. This works when number of processes is small (about 4 on my machine) but as that number rises I get the following message:
head (81473): "./a.out"
Assertion failed: (res == 0), function do_wait, file /usr/local/include/boost/interprocess/sync/posix/condition.hpp, line 175.
I'm guessing it's a problem with my synchronisation.. have I done something wrong or is the boost::interprocess::scoped_lock not enough?
My program is here:
#include <boost/interprocess/ipc/message_queue.hpp>
#include <boost/interprocess/sync/interprocess_mutex.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/process.hpp>
#include <iostream>
auto main(int argc, char **argv) -> int
{
namespace ip = boost::interprocess;
boost::filesystem::path self{argv[0]};
if (argc == 1) {
std::cout << "head (" << ::getpid() << "): " << self << std::endl;
// create a message queue.
ip::message_queue::remove("work_queue");
ip::message_queue tasks{ip::create_only, "work_queue", 100, sizeof(int)};
// mutex for writing to the queue.
ip::interprocess_mutex mutex{};
// spawn off a bunch of processes.
const auto cores{5 * std::thread::hardware_concurrency()};
std::vector<boost::process::child> workers{};
for (auto i = 0; i < cores; ++i) {
workers.emplace_back(self, "child");
}
// send message to each core.
for (auto i = 0; i < cores; ++i) {
ip::scoped_lock<decltype(mutex)> lock{mutex};
tasks.send(&i, sizeof(i), 0);
}
// wait for each process to finish.
for (auto &worker : workers) {
worker.wait();
}
} else if (argc == 2 && std::strcmp(argv[1], "child") == 0) {
// connect to message queue.
ip::message_queue tasks{ip::open_only, "work_queue"};
// mutex for reading from the queue.
ip::interprocess_mutex mutex{};
unsigned int priority;
ip::message_queue::size_type recvd_size;
{
ip::scoped_lock<decltype(mutex)> lock{mutex};
int number;
tasks.receive(&number, sizeof(number), recvd_size, priority);
std::cout << "child (" << ::getpid() << "): " << self << ", received: " << number << std::endl;
}
}
return 0;
}
You create an interprocess_mutex instance on the stack. So each process has it's own mutex and locking it does not synchronize anything. You need to create a shared memory region, place mutex there and then open the same shared memory region in child process to access the mutex created by parent process.
I am learning C++ [Java background fwiw] and trying to write a UNIX shell as a project. I am running into a funny little problem with tokenizing the input for execution. The tok function is getting called twice and I'm not sure why. My current test code is the following:
#include <iostream>
#include <vector>
#include <sstream>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <sys/wait.h>
#include <sys/types.h>
using namespace std;
void tok(string, char**);
int main(){
const char* EXIT = "exit";
string input;
cout << "shell>> ";
getline(cin, input);
pid_t pid = fork();
char* args[64]; //arbitrary size, 64 possible whitespace-delimited tokens in command
tok(input, args);
return 0;
}
//copied from http://stackoverflow.com/questions/14265581/parse-split-a-string-in-c-using-string-delimiter-standard-c
void tok(string inStr, char** args){
int last = 0, next = 0, i = 0;
while( (next = inStr.find(' ', last)) != -1){
cout << i++ << ": " << inStr.substr(last, next-last) << endl;
*args++ = strdup(inStr.substr(last, next-last).c_str());
last = next + 1;
}
cout << i++ << ": " << inStr.substr(last) << endl;
*args++ = strdup(inStr.substr(last).c_str());
*args = '\0';
cout << "done tokenizing..." << endl;
}
My output when I actually run the program is:
$ ./a.out
shell>> ls -l
0: ls
1: -l
done tokenizing...
0: ls
1: -l
done tokenizing...
I'm not sure why it would do that. Can anyone guide me in the right direction please? Thank you
The fork function returns twice, once in the original process and once in the newly-created, forked process. Both of those processes then call tok.
There doesn't seem to be any clear reason why you called fork. So the fix may be as simple as eliminating the call to fork.
When you call fork, you create two processes. Each process has nearly the exact same state except for the respective pid_t you receive. If that value is greater than 0, then you are in the parent process (main), and otherwise you are in the child (or fork failed).
Without performing a check on the returned pid_t, both processes will call tok, resulting in the double call behavior you witnessed.
Hide the call behind a check on pid like so:
pid_t pid = fork();
if (pid > 0) // have parent process call tok
{
char* args[64]; //arbitrary size, 64 possible whitespace-delimited tokens in command
tok(input, args);
}
To see what else parent and child processes have in common (or not): check the docs
following code may work fine
#include <iostream>
#include <vector>
#include <sstream>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <sys/wait.h>
#include <sys/types.h>
using namespace std;
void tok(string, char**);
int main(){
const char* EXIT = "exit";
string input;
cout << "shell>> ";
getline(cin, input);
// pid_t pid = fork();
char* args[64];
tok(input, args);
return 0;
}
void tok(string inStr, char** args){
int last = 0, next = 0, i = 0;
while( (next = inStr.find(' ', last)) != -1){
cout << i++ << ": " << inStr.substr(last, next-last) << endl;
*args++ = strdup(inStr.substr(last, next-last).c_str());
last = next + 1;
}
cout << i++ << ": " << inStr.substr(last) << endl;
*args++ = strdup(inStr.substr(last).c_str());
*args = '\0';
cout << "done tokenizing..." << endl;
}
I'm trying to port a program from Windows to Linux.
I encountered a problem when I found out that there isn't a "real" ReadProcessMemory counterpart on Linux; I searched for an alternative and I found ptrace, a powerful process debugger.
I quickly coded two small console applications in C++ to test ptrace, before using it in the program.
TestApp
This is the tracee; it keeps printing two integers every 50 milliseconds while increasing their value by 1 every time.
#include <QCoreApplication>
#include <QThread>
#include <iostream>
using namespace std;
class Sleeper : public QThread
{
public:
static void usleep(unsigned long usecs){QThread::usleep(usecs);}
static void msleep(unsigned long msecs){QThread::msleep(msecs);}
static void sleep(unsigned long secs){QThread::sleep(secs);}
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
int value = 145;
int i = 0;
do {
cout << "i: " << i << " " << "Value: " << value << endl;
value++;
i++;
Sleeper::msleep(50);
} while (true);
return a.exec();
}
MemoryTest
This is the tracer; it asks for the process name and retrieves the PID using the command pidof -s, then ptrace attaches to the process and retrieves the memory address' value every 500 milliseconds, for 10 times.
#include <QCoreApplication>
#include <QThread>
#include <iostream>
#include <string>
#include <sys/ptrace.h>
#include <errno.h>
using namespace std;
class Sleeper : public QThread
{
public:
static void usleep(unsigned long usecs){QThread::usleep(usecs);}
static void msleep(unsigned long msecs){QThread::msleep(msecs);}
static void sleep(unsigned long secs){QThread::sleep(secs);}
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
char process_name[50];
cout << "Process name: ";
cin >> process_name;
char command[sizeof(process_name) + sizeof("pidof -s ")];
snprintf(command, sizeof(command), "pidof -s %s", process_name);
FILE* shell = popen(command, "r");
char pidI[sizeof(shell)];
fgets(pidI, sizeof(pidI), shell);
pclose(shell);
pid_t pid = atoi(pidI);
cout << "The PID is " << pid << endl;
long status = ptrace(PTRACE_ATTACH, pid, NULL, NULL);
cout << "Status: " << status << endl;
cout << "Error: " << errno << endl;
unsigned long addr = 0x012345; // Example address, not the true one
int i = 0;
do {
status = ptrace(PTRACE_PEEKDATA, pid, addr, NULL);
cout << "Status: " << status << endl;
cout << "Error: " << errno << endl;
i++;
Sleeper::msleep(500);
} while (i < 10);
status = ptrace(PTRACE_DETACH, pid, NULL, NULL);
cout << "Status: " << status << endl;
cout << "Error: " << errno << endl;
return a.exec();
}
Everything works fine, but TestApp is paused (SIGSTOP) until ptrace detaches from it.
Also, when it attaches to the process, the status is 0 and the error is 2; the first time it tries to retrieve the memory address value it fails with status -1 and error 3. Is it normal?
Is there a way to prevent ptrace from sending the SIGSTOP signal to the process?
I already tried using PTRACE_SEIZE instead of PTRACE_ATTACH, but it doesn't work: status -1 and error 3.
Update: Using Sleeper in MemoryTest before the "do-while" loop fixes the problem of the first memory address value retrieval, even if the value of seconds, milliseconds or microseconds is 0. Why?
After a lot of research I'm pretty sure that there isn't a way to use ptrace without stopping the process.
I found a real ReadProcessMemory counterpart, called process_vm_readv, which is much more simple.
I'm posting the code in the hope of helping someone who is in my (previous) situation.
Many thanks to mkrautz for his help coding MemoryTest with this beautiful function.
#include <QCoreApplication>
#include <QThread>
#include <sys/uio.h>
#include <stdint.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <iostream>
using namespace std;
class Sleeper : public QThread
{
public:
static void usleep(unsigned long usecs){QThread::usleep(usecs);}
static void msleep(unsigned long msecs){QThread::msleep(msecs);}
static void sleep(unsigned long secs){QThread::sleep(secs);}
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
char process_name[50];
cout << "Process name: ";
cin >> process_name;
char command[sizeof(process_name) + sizeof("pidof -s ")];
snprintf(command, sizeof(command), "pidof -s %s", process_name);
FILE* shell = popen(command, "r");
char pidI[sizeof(shell)];
fgets(pidI, sizeof(pidI), shell);
pclose(shell);
pid_t pid = atoi(pidI);
cout << "The PID is " << pid << endl;
if (pid == 0)
return false;
struct iovec in;
in.iov_base = (void *) 0x012345; // Example address, not the true one
in.iov_len = 4;
uint32_t foo;
struct iovec out;
out.iov_base = &foo;
out.iov_len = sizeof(foo);
do {
ssize_t nread = process_vm_readv(pid, &out, 1, &in, 1, 0);
if (nread == -1) {
fprintf(stderr, "error: %s", strerror(errno));
} else if (nread != in.iov_len) {
fprintf(stderr, "error: short read of %li bytes", (ssize_t)nread);
}
cout << foo << endl;
Sleeper::msleep(500);
} while (true);
return a.exec();
}
Davide,
Have you had a look at the /proc filesystem? It contains memory map files that can be used to peek at the full process space. You can also write in the space to set a breakpoint. There is a wealth of other information in /proc as well.
The PTRACE_CONT command can be used to continue a process. Generally, the target will be paused with a PTRACE_ATTACH when the debugger attaches.
The man page says PTRACE_SIEZE should not pause the process. What flavor and version of Linux are you using? PTRACE_SIEZE has been around for quite awhile so I'm not sure why you are having trouble there.
I note the addr value is set to 0x12345. Is this a valid address in the target space? Or was that just an example? How is the stack address of interest (&value) communicated between the two processes?
I'm not too sure about the return codes. Generally a 0 means all is well, the errno may just be a hangover value from the last error.
--Matt
I am struggling with process creation and piping the child process' output into a string of the parent process. I got it working on Windows (using CreatePipe and CreateProcess and ReadFile), but can't seem to get the exact analog on Unix to work. This is my code:
#include <spawn.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
int exit_code;
int cout_pipe[2];
int cerr_pipe[2];
posix_spawn_file_actions_t action;
if(pipe(cout_pipe) || pipe(cerr_pipe))
cout << "pipe returned an error.\n";
posix_spawn_file_actions_init(&action);
posix_spawn_file_actions_addclose(&action, cout_pipe[0]);
posix_spawn_file_actions_addclose(&action, cerr_pipe[0]);
posix_spawn_file_actions_adddup2(&action, cout_pipe[1], 1);
posix_spawn_file_actions_adddup2(&action, cerr_pipe[1], 2);
posix_spawn_file_actions_addclose(&action, cout_pipe[1]);
posix_spawn_file_actions_addclose(&action, cerr_pipe[1]);
vector<string> argmem = {"bla"};
vector<char*> args = {&argmem[0][0], nullptr}; // I don't want to call new.
pid_t pid;
if(posix_spawnp(&pid, "echo", &action, NULL, &args[0], NULL) != 0)
cout << "posix_spawnp failed with error: " << strerror(errno) << "\n";
//close(cout_pipe[0]);
//close(cerr_pipe[0]);
close(cout_pipe[1]);
close(cerr_pipe[1]);
waitpid(pid,&exit_code,0);
cout << "exit code: " << exit_code << "\n";
// Read from pipes
const size_t buffer_size = 1024;
string buffer;
buffer.resize(buffer_size);
ssize_t bytes_read = read(cout_pipe[0], &buffer[0], buffer_size);
while ((bytes_read = read(cout_pipe[0], &buffer[0], buffer_size)) > 0)
{
cout << "read " << bytes_read << " bytes from stdout.\n";
cout << buffer.substr(0, static_cast<size_t>(bytes_read)+1) << "\n";
bytes_read = read(cout_pipe[0], &buffer[0], buffer_size);
}
if(bytes_read == -1)
cout << "Failure reading from stdout pipe.\n";
while ((bytes_read = read(cerr_pipe[0], &buffer[0], buffer_size)) > 0)
{
cout << "read " << bytes_read << " bytes from stderr.\n";
cout << buffer.substr(0, static_cast<size_t>(bytes_read)+1) << "\n";
bytes_read = read(cout_pipe[0], &buffer[0], buffer_size);
}
if(bytes_read == -1)
cout << "Failure reading from stderr pipe.\n";
posix_spawn_file_actions_destroy(&action);
}
The output is:
exit code: 0
So I suppose everything is working except the actual piping. What is wrong here? I also wonder if there is a way to read the piped bytes in a waitpid loop, but when I try that, the parent process hangs infinitely.
posix_spawn is interesting and useful, which makes this question worth necromancing -- even if it is no longer relevant to the OP.
There are some significant bugs in the code as posted. I suspect that some of these were the result of hacking in desperation, but I don't know which was the original bug:
The args array does not include the argv[0] that would represent the executable name. This results in the echo program never seeing the intended argv[1] ("bla").
The read() function is called from different places in a way that just doesn't make sense. A correct way to do this would be to only call read as part of the control expression for the while loops.
waitpid() is called before reading from the pipes. This prevents the I/O from completing (in non-trivial cases at least).
A more subtle issue with this code is that attempts to read all of the child's stdout before reading anything from stderr. In principle, this could cause the child to block while attempting to write to stderr, thus preventing the program from completing. Creating an efficient solution to this is more complicated as it requires that you can read from whichever pipe has available data. I used poll() for this. Another approach would be to use multiple threads.
Additionally, I have used sh (the command shell, i.e. bash) as the child process. This provides a great deal of additional flexibility, such as running a pipeline instead of a single executable. In particular, though, using sh provides the simple convenience of not having to manage the parsing of the command-line.
/*BINFMTCXX: -std=c++11 -Wall -Werror
*/
#include <spawn.h> // see manpages-posix-dev
#include <poll.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <iostream>
#include <string>
#include <vector>
using namespace std;
int main()
{
int exit_code;
int cout_pipe[2];
int cerr_pipe[2];
posix_spawn_file_actions_t action;
if(pipe(cout_pipe) || pipe(cerr_pipe))
cout << "pipe returned an error.\n";
posix_spawn_file_actions_init(&action);
posix_spawn_file_actions_addclose(&action, cout_pipe[0]);
posix_spawn_file_actions_addclose(&action, cerr_pipe[0]);
posix_spawn_file_actions_adddup2(&action, cout_pipe[1], 1);
posix_spawn_file_actions_adddup2(&action, cerr_pipe[1], 2);
posix_spawn_file_actions_addclose(&action, cout_pipe[1]);
posix_spawn_file_actions_addclose(&action, cerr_pipe[1]);
//string command = "echo bla"; // example #1
string command = "pgmcrater -width 64 -height 9 |pgmtopbm |pnmtoplainpnm";
string argsmem[] = {"sh","-c"}; // allows non-const access to literals
char * args[] = {&argsmem[0][0],&argsmem[1][0],&command[0],nullptr};
pid_t pid;
if(posix_spawnp(&pid, args[0], &action, NULL, &args[0], NULL) != 0)
cout << "posix_spawnp failed with error: " << strerror(errno) << "\n";
close(cout_pipe[1]), close(cerr_pipe[1]); // close child-side of pipes
// Read from pipes
string buffer(1024,' ');
std::vector<pollfd> plist = { {cout_pipe[0],POLLIN}, {cerr_pipe[0],POLLIN} };
for ( int rval; (rval=poll(&plist[0],plist.size(),/*timeout*/-1))>0; ) {
if ( plist[0].revents&POLLIN) {
int bytes_read = read(cout_pipe[0], &buffer[0], buffer.length());
cout << "read " << bytes_read << " bytes from stdout.\n";
cout << buffer.substr(0, static_cast<size_t>(bytes_read)) << "\n";
}
else if ( plist[1].revents&POLLIN ) {
int bytes_read = read(cerr_pipe[0], &buffer[0], buffer.length());
cout << "read " << bytes_read << " bytes from stderr.\n";
cout << buffer.substr(0, static_cast<size_t>(bytes_read)) << "\n";
}
else break; // nothing left to read
}
waitpid(pid,&exit_code,0);
cout << "exit code: " << exit_code << "\n";
posix_spawn_file_actions_destroy(&action);
}