Posix线程参数问题（C）

Question

I'm running into a bit of trouble with C. I'm a relatively new programmer and I'm trying to create a structure and pass it into two thread by reference. I want one thread to put information into the structure and the other thread to add the information and print it out. Pseudo-code of what I'm talking about is below:

typedef struct{ int x, y }addme;
main{
  addme argstopass;
  create_thread(method_store, (void*)&argstopass);
  create_thread(method_calc, (void*)&argstopass);
  //Code to tell store thread 'only' to run
  //Code to tell calc thread to run when store is finished.
  join_both_threads;
}

void method_store(void* args){
  addme info = *((addme*)args);
  info.a = 7;
  info.b = 3;
}  

void method_calc(void* args){
  addme info = *((addme*)args);
  print(info.a+info.b);
}

The issue is that when I try to add the information it's like the store method had never updated it. The reference passed into the threads is the same, so I can't see why they wouldn't be able to access the same information as long as they both have a pointer to it.

Hopefully someone here can enlighten me as to what I'm doing wrong. If anything isn't clear, comment and I'll help to clarify.

Answer 1

addme info = *((addme*)args);

creates a locale variable on stack and copies content of argstopass into it. Modifications happen on this local variable only and won't be seen by the second thread hence.

Use

addme *info = args;
info->a = 7;

and ditto for the second thread. You will have to ensure that second thread waits with its printf() until first thread modified the values.

Answer 2

void method_store(void* args){
  addme info = *((addme*)args);
  info.a = 7;
  info.b = 3;
}  

This method creates a local copy of your structure field, updates that local copy, and then returns, destroying the copy and not doing anything to your main structure.