What happened to my threads?

I used threads in a C++ program before and now I have done a new part to it. When I put the parts together the thread did not work. The program died with a segmentation fault.

So I tried to do a simpe thread-program:

#include <iostream>
using namespace std;

void *newfunc(void*);

int main() {
        pthread_t th;
        int iret;
        int arg = 1;

        cout << "main: create thread" << endl;
        iret = pthread_create(&th, NULL, newfunc, (void*)arg);
        cout << "main: thread dead" << endl;
}

void *newfunc(void*) {
        cout << "thread: i am alive" << endl;
}

This can be compiled but cannot run properly:

$ g++ temp.cc -o blah
$ ./blah
main: create thread
Segmentation fault

Why is this? I am kinda confused...

Let's see ... arg is 1 and you're passing it as a void pointer. So basically you're passing the memory address 1 as the argument. Memory address 1 is probably not anywhere valid for your program, so this is likely what's causing the problem. You never do dereference the argumentt, though, so I'm slightly surprised it's segfaulting, still, you probably meant:

iret = pthread_create(&th, NULL, newfunc, (void*)&arg);

FYI, I tested this out, and you don't see the thread's output unless it happens to run before the main process exits. You should put a sleep call before the thread dead output in main, because there is no guarantee that the thread you create will run before the main thread. The correct way to wait on a thread is with pthread_join. HTH.

Yeah, I mean (void*)&.

But hell. The problem was not the code. It all worked when I compiled with "-lpthread". I didn't know that not using -lpthread would cause segmentation fault but when I used it everything worked.

Pretty simple. I hate things like this. :P

When you use -lpthread it links your program with the pthread library, that library is responsable for many thread operations. It must have to compile it using -lpthread. Segmentation Fault means that your program is accessing some memory that is not allocated to your program. maybe some of your memory allocation have to be allocated by pthread library, so if you don't use it you get the error.
None of your thread program will run properly without compiling using -lpthread aurgument. You must read about shared libraries.

I have code that compiles on both Linux and Solaris 2.6, but it segfaults on Linux, while it runs on Solaris. This is my very first pthreads app, so I'm not sure what might be wrong.

I have a linked list (not typedef'd) which you can use dynamically {struct list *blah=(struct list *) malloc(sizeof(struct list));} or statically {struct list blah;}. In this case, I'm creating it "statically" because I need 64 lists, and I don't want a list of lists, which would have been required for "dynamic" lists. If I've confused you, just look at the code:


struct list commands[MAX_THREADS];

/* some initialization and population takes place here */

thread_status=pthread_create(thread_id, NULL, download, (void *) &commands[offset]);


and the function is declared as:

void *download(void *login)
{
struct list *commands=(struct list *) login;


There is way too much code to include it all. Let me just state that I read the previous post and was able to compile and run the short example (with the sleep included, and a printf in the newfunc thread to confirm).

Even with an empty "download" function, I still segfault in Linux. It works fine in Solaris. I would hate to have to admit this to my co-workers. I can't seem to figure out why.

Never mind; I figured it out. It was so stoooopid.

I was doing:

pthread_t *thread_id;
pthread_create(thread_id...

Syntactically, it's correct; i.e., I am passing a pointer. I feel so stupid for this, because I did it correctly everywhere else in my code. I didn't malloc! I could have done:

pthread_t *thread_id=(pthread_t *) malloc(sizeof(pthread_t));

but that's silly, because:

pthread_t thread_id;
pthread_create(&thread_id ...

is much more consice. I don't need memory for a thread id dynamically allocated. Let me just remember to KISS!