Compiling pthreads using g++

stulli · 05-15-2009, 07:19 PM

Hi.
I´m pretty new to programming in C++ and I´m using pthreads. I´m cross compiling my code for OpenWRT but for some reason I get segmentation fault when I run the program on my board but it runs fine on my PC. I suspect that the error occurs in the linking stage of the compilation because I tried a small C program and that worked fine. Also if I change the name of the file to .cpp and compile it with g++ it also works.

The main:

Code:

#define INPUT_PORT "20"
#define POWER_PORT "21"
#define OUTPUTPORT 3
#define HIGHVALUE 1
#define LOWVALUE 0
#define EMERGENCY_FOLDER "emergency"
#define NORMAL_FOLDER "normal"

#include <iostream>
#include <string>
#include <time.h>
#include <unistd.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#include "sender.h"
#include "Camera.h"
#include "Ports.h"

#include "UDPServer.h"

using namespace std;

//initializes the ports
//Ports pow(POWER_PORT, "out", 0);
//Ports button(INPUT_PORT, "in", 0);

//Tread Functions
Camera* camera;

void *CPUSimFunction(void *);
void *securityThread(void *);
int main() {

	camera = new Camera("192.168.1.67", "admin", "admin", "image.jpg", "/cameratest");

	cout << "at least I´m running here" << endl;
	pthread_t doorThread;
	pthread_create(&doorThread, NULL, CPUSimFunction, NULL);

	while (1) {
		camera->savePicture(NORMAL_FOLDER);
		sleep(20);
	}

}

void *CPUSimFunction(void *dummyPtr) {
	int x;
	while (1) {
		cout << "Press 1 to open and 0 to close: ";
		cin >> x;
		if (x == 1) {
			pthread_t emergencyCamThread;
			pthread_create(&emergencyCamThread, NULL, securityThread, NULL);
			cout << "door is open" << endl;
		} else if (x == 0) {
			cout << "door is closed" << endl;
		} else {
			cout << "Choose 1 or 0";
		}
	}

}

void *securityThread(void *dummyPtr) {
	for (int i = 0; i < 10; i++) {
		cout << "saving emergency picture" << endl;
		camera->savePicture(EMERGENCY_FOLDER);
		usleep(1000000);
	}
}

The output of the eclipse console when compiled:

Code:

**** Build of configuration Release for project ButtonProgram ****

make all 
Building file: ../src/ButtonProgram.cpp
Invoking: GCC C++ Compiler
mipsel-linux-g++ -O3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/ButtonProgram.d" -MT"src/ButtonProgram.d" -o"src/ButtonProgram.o" "../src/ButtonProgram.cpp"  -ldl -pthread
../src/ButtonProgram.cpp: In function 'void* CPUSimFunction(void*)':
../src/ButtonProgram.cpp:73: warning: no return statement in function returning non-void
../src/ButtonProgram.cpp: In function 'void* securityThread(void*)':
../src/ButtonProgram.cpp:81: warning: no return statement in function returning non-void
../src/ButtonProgram.cpp:81: warning: control reaches end of non-void function
mipsel-linux-g++: -ldl: linker input file unused because linking not done
Finished building: ../src/ButtonProgram.cpp
 
Building file: ../src/Camera.cpp
Invoking: GCC C++ Compiler
mipsel-linux-g++ -O3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/Camera.d" -MT"src/Camera.d" -o"src/Camera.o" "../src/Camera.cpp"  -ldl -pthread
../src/Camera.cpp: In member function 'void Camera::savePicture(std::string) const':
../src/Camera.cpp:53: warning: unused variable 'mkdirOut'
mipsel-linux-g++: -ldl: linker input file unused because linking not done
Finished building: ../src/Camera.cpp
 
Building file: ../src/Ports.cpp
Invoking: GCC C++ Compiler
mipsel-linux-g++ -O3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/Ports.d" -MT"src/Ports.d" -o"src/Ports.o" "../src/Ports.cpp"  -ldl -pthread
mipsel-linux-g++: -ldl: linker input file unused because linking not done
Finished building: ../src/Ports.cpp
 
Building file: ../src/UDPServer.cpp
Invoking: GCC C++ Compiler
mipsel-linux-g++ -O3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/UDPServer.d" -MT"src/UDPServer.d" -o"src/UDPServer.o" "../src/UDPServer.cpp"  -ldl -pthread
mipsel-linux-g++: -ldl: linker input file unused because linking not done
Finished building: ../src/UDPServer.cpp
 
Building file: ../src/sender.cpp
Invoking: GCC C++ Compiler
mipsel-linux-g++ -O3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/sender.d" -MT"src/sender.d" -o"src/sender.o" "../src/sender.cpp"  -ldl -pthread
mipsel-linux-g++: -ldl: linker input file unused because linking not done
Finished building: ../src/sender.cpp
 
Building target: ButtonProgram
Invoking: GCC C++ Linker
mipsel-linux-g++  -o"ButtonProgram"  ./src/ButtonProgram.o ./src/Camera.o ./src/Ports.o ./src/UDPServer.o ./src/sender.o    -static -pthread
Finished building target: ButtonProgram

rriggs · 05-15-2009, 09:35 PM

Try returning NULL from void *securityThread(void *dummyPtr) and see if that helps. Always pay attention to the warnings from your compiler.

Code:

../src/ButtonProgram.cpp: In function 'void* securityThread(void*)':
../src/ButtonProgram.cpp:81: warning: no return statement in function returning non-void
../src/ButtonProgram.cpp:81: warning: control reaches end of non-void function

stulli · 05-16-2009, 07:44 AM

Hi
That didn´t work. I tried removing all thread code and compiling with the -lpthread and I still get segfault. Also I never get to the main since the "At least I´m running here" string doesn´t print. If I remove the -lpthread flag from the compiler the code (without the treads) runs on the board.

dwhitney67 · 05-16-2009, 11:27 AM

Please post the code for Camera.cpp so that we can take a look at the constructor.

Edit: Btw, cout is not thread-safe. Also, do you not require the pthread_join()?

jiml8 · 05-16-2009, 12:01 PM

Looking at the Camera constructor seems to be a good plan. Since the first cout isn't printing, that is the place to start.

You also should look at your variable scope rules. You are asking for trouble with this statement: pthread_t emergencyCamThread;
being placed inside an if statement which is itself inside a while statement.

stulli · 05-16-2009, 01:51 PM

The above code might seem strange but the fact is I have been testing alot of different things so there might be things there that shouldn´t be used the way I´m using them. Also I have tried moving the camera into the main but with no results.
About the pthread_join()... Well I don´t know yet if I will need it. I´m using threads for the first time so im just experimenting and I don´t have a final picture of this in my head.

Here is another sample of a code that will run on my PC as expected but will only run on the board when compiled as a C program.

Code:

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

void *run(void *dummyPtr) {
	printf("I am a thread...\n");
	return NULL;
}

int main(int argc, char **argv) {
	printf("Main start...\n");
	pthread_t connector;
	pthread_create(&connector, NULL, run, NULL);
	pthread_join(connector, NULL);
	printf("Main end...\n");
	return 0;
}

Sergei Steshenko · 05-16-2009, 02:52 PM

I think in/with C++ one should use 'boost' threads - it is also cross-platform. 'boost' developers have already taken care of system-specific things, so it should just compile. I also think that on Linux 'boost' threads in the end are implemented through 'pthreads'.

I am not a C++ guy, though.

dwhitney67 · 05-16-2009, 03:43 PM

Quote:

Originally Posted by stulli

The above code might seem strange but the fact is I have been testing alot of different things so there might be things there that shouldn´t be used the way I´m using them. Also I have tried moving the camera into the main but with no results.
About the pthread_join()... Well I don´t know yet if I will need it. I´m using threads for the first time so im just experimenting and I don´t have a final picture of this in my head.

Here is another sample of a code that will run on my PC as expected but will only run on the board when compiled as a C program.

So you can get a C program to work, but not a C++ program? Maybe certain runtime libraries are missing from the board?

Here's a basic C++ thread implementation:

Code:

#include <pthread.h>
#include <iostream>

class Thread
{
public:
  Thread() {}

  void start() {
    pthread_start(&m_tid, 0, helper, this);
  }

  void waitTillFinished() {
    pthread_join(m_tid, 0);
  }

private:
  void run() {
    std::cout << "thread is running." << std::endl;  // not thread-safe!
  }

  static void* helper(void* arg) {
    Thread* t = static_cast<Thread*>(arg);
    t->run();
    pthread_exit(0);
    return 0;
  }

  pthread_t m_tid;
};

int main()
{
  Thread thread;
  thread.start();
  thread.waitTillFinished();  // this is not typically done this way
}

Can you please build this app to test on your board and report the results? Here's the basics on how to build it with the standard g++:

Code:

g++ ThreadTest.cpp -lpthread -o ThreadTest

Note your board probably requires that you use a special cross compiler, perhaps not the same for building apps to run on your PC.

stulli · 05-16-2009, 06:16 PM

Quote:

Can you please build this app to test on your board and report the results? Here's the basics on how to build it with the standard g++:
Code:

g++ ThreadTest.cpp -lpthread -o ThreadTest

I get the same segmentation fault. I´m using a crosscompiler and the command I use is:

Code:

mipsel-linux-g++ ThreadTest.cpp -lpthread -o ThreadTest -static

About the libraries... If I understand correctly the -static flag compiles the libraries needed. The output file is at least alot bigger.

dwhitney67 · 05-16-2009, 09:13 PM

Try without the -static. This linker option is used to include within the executable the library instructions; without it, the shared-object (.so) library is accessed during runtime.

Unless you have a libpthread.a that has been specifically built for your target (mipsel-linux?), then you should not be using the -static option.

stulli · 05-17-2009, 05:32 AM

Removing the -static works

Thanks a million for your help, basicly saved my semester.