Accessing variables in the parent via pointer after clone()

问题

I'm sorry for possibly asking an idiot question, but i'm a bloody beginner in C. Now my problem is, i need to access two variables, declared in main, in a child process and modify them. Sounds very simple, but i have to use clone and after backcasting the variables to my array, they are completely messed up concerining the value.

int main (){
uint64_t N = 10000000;
uint64_t tally = 0;
int status;

void** child_stack = (void**)malloc(65536);
uint64_t** package = (uint64_t**)malloc(sizeof(uint64_t*)*2);
package[0] = &tally;
package[1] = &N;

pid_t cpid;
cpid = clone(child_starter, &child_stack, CLONE_VM , (void*)package);
waitpid(cpid, &status, __WCLONE);
return 0;
}

the child_starter functions looks as follows:

int child_starter(void* package){
printf( "Tally is in the child %" PRIu64 "\n", *((int64_t**)package)[0] );
return 0;
}

since tally is 0, i thought my printf should actually print out 0, but it's more like 140541190785485 changing from run to run..

Hope you can help me :)

回答1:

child_stack already is a pointer to valid memory, so do not pass its address, but its value.
Stack grows top to bottom, at least on nearly all Linux implementations, so the stack pointer passed needs to be the end of the memory allocated for the stack.

cpid = clone(child_starter, ((char *) child_stack) + 65536, CLONE_VM , package);

Also in C there is not need to cast to (void*).

Also^2 printf() isn't reentrant so it's use here is error prone.

来源：https://stackoverflow.com/questions/20442072/accessing-variables-in-the-parent-via-pointer-after-clone

标签

clone