threadDeath2.cc File Reference

slightly complex pthread example More...

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

Include dependency graph for threadDeath2.cc:

Go to the source code of this file.

Classes
struct	func_params
	wrapper function parameters/arguments More...

Functions
void *	myfunc (void *arg)
	user defined function More...
void *	func (void *arg)
	A wrapper function for the user defined function. More...
int	main (int argc, char *argv[])

Detailed Description

slightly complex pthread example

Purpose:

Additional Example program to threadDeath1.cc that creates a thread that uses a function that calls a user defined function. Also demonstrates casing examples for the pthread_join(void **) return type. Often the return type from pthread_join() is confusing so we demonstrate only a string * usage here.

This program is slightly more complicated than threadDeath1.cc because we are passing around additional parameters.

The intention here is that we want to demonstrate passing pointers around when creating threads and then accessing those pointers as we go allong.

Definition in file threadDeath2.cc.

Function Documentation

void* func

(

void *

arg

)

A wrapper function for the user defined function.

Purpose:

This function wrapps the user defined function so that additional opperations may be performed on a thread once it is created. Unified signal handling or thread ID tracking can be performed from this function as well as pthread_cleanup_push() and pthread_cleanup_pop() operations.

Returns

NOTHING calls pthread_exit() to be joined later.

Parameters

arg	is a void pointer that is expected to point to a func_params object.

Note

This function should never return -it calls pthread_exit() instead

Definition at line 54 of file threadDeath2.cc.

References func_params::argument, func_params::other_info, and func_params::user_func.

Referenced by main().

{
  //grab the parameters for redability (demonstration purposes)
  struct func_params *params = (struct func_params *)arg;

  //print out other info from the paramater structure
  cout << (char *)params->other_info << endl;

  //call the user function with the intended argument (void *)
  void *return_value = params->user_func(params->argument);

  //exit thread
  pthread_exit(return_value);

  return NULL;
}

int main	(	int	argc,
		char *	argv[]
	)

Purpose:

Demonstrate the use of a wrapper function to call a user defined function from our wrapper function. A pointer to the user defined function is passed to the wrapper function through a func_params object. Casting is used to pass the func_params object through pthread_create to the wrapper function.

The func_params object also contains a pointer to an argument that will be passed on to the user defined function.

Definition at line 86 of file threadDeath2.cc.

References func_params::argument, func(), myfunc(), func_params::other_info, and func_params::user_func.

{
  //track the thread ID
  pthread_t tid;

  //capture the return value of our user defined function (ultimately
  //from a call from pthread_exit in our wrapper function).
  void **ret;

  //create a string to pass to our user defined function
  string arg("string from main");
  
  //create a func_params object to pass to our wrapper function
  struct func_params params;

  //set the members of the func_params object
  params.user_func = myfunc;    // pointer to user defined function
  params.argument = &arg;       // object passed to user function

  //more data -for demonstration 
  params.other_info = (void *)"literal string in main params"; 

  //create the thread (passing our wrapper function as the function
  //argument)
  pthread_create(&tid, (pthread_attr_t *)NULL, func, (void *)&params);

  //join the thread -simple enough
  pthread_join(tid, ret);

  //check the return value 
  if(*ret != NULL)
    {
      //if the contents of the pointer to a pointer (ret) is not NULL...

      //print the contents -casting it as what we expect it to be
      cout << *((string *)(string *)*ret) << endl;

      //delete the pointer because we don't want memory leaks
      delete ((string *)(string *)*ret);
    }

  //arg falls out of scope and is handled by the string destructor
  //after return.

  //exit normally
  return(0);
}

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void* myfunc

(

void *

arg

)

user defined function

Purpose:

A user defined function. This function, for demonstration purposes, dynamically creates a string object that will be returned through a call to pthread_join() later on.

Note

This function SHOULD NOT call pthread_exit() as it is intended, in this case, to return to the wrapper function func(). Calling pthread_exit() from here will terminate the thread and will result in "undefined behavior".

Parameters

arg	is a string object pointer passed in as a void *

Returns

a string object that is dynamically allocated

Warning

DO NOT call pthread_exit

Definition at line 149 of file threadDeath2.cc.

Referenced by main().

{
  //print out the argument passed in
  cout << *(string *)arg << endl;
  
  //allocate for a new string object
  string *s = new string("string from myfunc");

  //display our string: for demonstration
  cout << "myfunc returning:" << *s <<endl;

  //pthread_exit((void *)s);    // BAD, BAD, BAD

  //return the object as a void pointer
  return( (void *)s);
}