test runner - Page 106 - Developer IT

Question about unions and heap allocated memory

- by Dennis Miller

I was trying to use a union to so I could update the fields in one thread and then read allfields in another thread. In the actual system, I have mutexes to make sure everything is safe. The problem is with fieldB, before I had to change it fieldB was declared like field A and C. However, due to a third party driver, fieldB must be alligned with page boundary. When I changed field B to be allocated with valloc, I run into problems. Questions: 1) Is there a way to statically declare fieldB alligned on page boundary. Basically do the same thing as valloc, but on the stack? 2) Is it possible to do a union when field B, or any field is being allocated on the heap?. Not sure if that is even legal. Here's a simple Test program I was experimenting with. This doesn't work unless you declare fieldB like field A and C, and make the obvious changes in the public methods. #include <iostream> #include <stdlib.h> #include <string.h> #include <stdio.h> class Test { public: Test(void) { // field B must be alligned to page boundary // Is there a way to do this on the stack??? this->field.fieldB = (unsigned char*) valloc(10); }; //I know this is bad, this class is being treated like //a global structure. Its self contained in another class. unsigned char* PointerToFieldA(void) { return &this->field.fieldA[0]; } unsigned char* PointerToFieldB(void) { return this->field.fieldB; } unsigned char* PointerToFieldC(void) { return &this->field.fieldC[0]; } unsigned char* PointerToAllFields(void) { return &this->allFields[0]; } private: // Is this union possible with field B being // allocated on the heap? union { struct { unsigned char fieldA[10]; //This field has to be alligned to page boundary //Is there way to be declared on the stack unsigned char* fieldB; unsigned char fieldC[10]; } field; unsigned char allFields[30]; }; }; int main() { Test test; strncpy((char*) test.PointerToFieldA(), "0123456789", 10); strncpy((char*) test.PointerToFieldB(), "1234567890", 10); strncpy((char*) test.PointerToFieldC(), "2345678901", 10); char dummy[11]; dummy[10] = '\0'; strncpy(dummy, (char*) test.PointerToFieldA(), 10); printf("%s\n", dummy); strncpy(dummy, (char*) test.PointerToFieldB(), 10); printf("%s\n", dummy); strncpy(dummy, (char*) test.PointerToFieldC(), 10); printf("%s\n", dummy); char allFields[31]; allFields[30] = '\0'; strncpy(allFields, (char*) test.PointerToAllFields(), 30); printf("%s\n", allFields); return 0; }

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