Programmatically get all frame variables in objective-c - objective-c

Question

I'm trying to create my own custom assert. However, I would like my assertion to automatically include all of the relevant variables. This seems really basic to me, and I've searched around for about an hour but I can't seem to find a way get access to all the relevant stack frame variables. Does anyone know how to get these variables?
FYI - I don't need to access the variables in the debugger, I need to access them programmatically. I would like to upload them along with the crash report to give me more information about the crash. I also know that I can print them out manually...that is exactly what I'm looking to avoid.

Answer 1

You are basically asking to re-invent a good sized chunk of the debugger.
Without symbols, there isn't anything you can interrogate to figure out the layout of the local frame. Even with symbols, it is quite likely that the optimizer will have stomped on any local variables as the optimizer will re-use stack slots at whim once it determines the variable is no longer needed within the frame.
Note that many crashes won't be able to be caught at all or, if caught, the frame within which they occurred will have long since been destroyed.
Since you mention that you are creating a custom assertion, it sounds like you really aren't looking to introspect crashes as much as dump a snap of the local frame when you programatically detect that things have gone off the rails. While there really isn't a means of automatically reporting on local stack state, you could do something like:
{ ... some function ....
... local variables ...
#define reportblock ^{ ... code that summarizes locals ... ; return summary; }
YourAssert( cond, "cond gone bad. summary: %#", reportblock());
}
Note that the #define ensures that each YourAssert() captures the state at the time of the assertion. Note also that the above might have a potentially significant impact on performance.
Note also that I just made that code up. It seems like it is worthy of investigation, but may prove non-viable for a number of reasons.

Answer 2

If you're willing to use Objective-C++, then this is definitely a possibility, as long as you are also willing to declare your variables differently, and understand that you will only be able to grab your own variables with this method.
Also note that it will increase your stack frame size with extra __stack_ variables, which could cause memory issues (although I doubt it, personally).
It won't work with certain constructs such as for-loops, but for 95% of cases, this should work for what you need:
#include <vector>
struct stack_variable;
static std::vector<const stack_variable *> stack_variables;
struct stack_variable {
void **_value;
const char *_name;
const char *_type;
const char *_file;
const char *_line;
private:
template<typename T>
stack_variable(const T& value, const char *type, const char *name, const char *file, const char *line) : _value((void **) &value), _type(type), _name(name), _file(file), _line(line) {
add(*this);
}
static inline void add(const stack_variable &var) {
stack_variables.push_back(static_cast<const stack_variable *>(&var));
}
static inline void remove(const stack_variable &var) {
for (auto it = stack_variables.begin(); it != stack_variables.end(); it++) {
if ((*it) == &var) {
stack_variables.erase(it);
return;
}
}
}
public:
template<typename T>
static inline stack_variable create(const T& value, const char *type, const char *name, const char *file, const char *line) {
return stack_variable(value, type, name, file, line);
}
~stack_variable() {
remove(*this);
}
void print() const {
// treat the value as a pointer
printf("%s:%s - %s %s = %p\n", _file, _line, _type, _name, *_value);
}
static void dump_vars() {
for (auto var : stack_variables) {
var->print();
}
}
};
#define __LINE_STR(LINE) #LINE
#define _LINE_STR(LINE) __LINE_STR(LINE)
#define LINE_STR _LINE_STR(__LINE__)
#define LOCAL_VAR(type, name, value)\
type name = value;\
stack_variable __stack_ ## name = stack_variable::create<type>(name, #type, #name, __FILE__, LINE_STR);\
(void) __stack_ ## name;
Example:
int temp() {
LOCAL_VAR(int, i_wont_show, 0);
return i_wont_show;
}
int main(){
LOCAL_VAR(long, l, 15);
LOCAL_VAR(int, x, 192);
LOCAL_VAR(short, y, 256);
temp();
l += 10;
stack_variable::dump_vars();
}
Output (note the junk extra bytes for the values smaller than sizeof(void *), there isn't much I can do about that):
/Users/rross/Documents/TestProj/TestProj/main.mm:672 - long l = 0x19
/Users/rross/Documents/TestProj/TestProj/main.mm:673 - int x = 0x5fbff8b8000000c0
/Users/rross/Documents/TestProj/TestProj/main.mm:674 - short y = 0xd000000010100
Threads will royally screw this up, however, so in a multithreaded environment this is (almost) worthless.

Answer 3

I decided to add this as a separate answer, as it uses the same approach as my other one, but this time with an all ObjC code. Unfortunately, you still have to re-declare all of your stack variables, just like before, but hopefully now it will work better with your existing code-base.
StackVariable.h:
#import <Foundation/Foundation.h>
#define LOCAL_VAR(p_type, p_name, p_value)\
p_type p_name = p_value;\
StackVariable *__stack_ ## p_name = [[StackVariable alloc] initWithPointer:&p_name\
size:sizeof(p_type)\
name:#p_name\
type:#p_type\
file:__FILE__\
line:__LINE__];\
(void) __stack_ ## p_name;
#interface StackVariable : NSObject
-(id) initWithPointer:(void *) ptr
size:(size_t) size
name:(const char *) name
type:(const char *) type
file:(const char *) file
line:(const int) line;
+(NSString *) dump;
#end
StackVariable.m:
#import "StackVariable.h"
static NSMutableArray *stackVariables;
#implementation StackVariable {
void *_ptr;
size_t _size;
const char *_name;
const char *_type;
const char *_file;
int _line;
}
-(id) initWithPointer:(void *)ptr size:(size_t)size name:(const char *)name type:(const char *)type file:(const char *)file line:(int)line
{
if (self = [super init]) {
if (stackVariables == nil) {
stackVariables = [NSMutableArray new];
}
_ptr = ptr;
_size = size;
_name = name;
_type = type;
_file = file;
_line = line;
[stackVariables addObject:[NSValue valueWithNonretainedObject:self]];
}
return self;
}
-(NSString *) description {
NSMutableString *result = [NSMutableString stringWithFormat:#"%s:%d - %s %s = { ", _file, _line, _type, _name];
const uint8_t *bytes = (const uint8 *) _ptr;
for (size_t i = 0; i < _size; i++) {
[result appendFormat:#"%02x ", bytes[i]];
}
[result appendString:#"}"];
return result;
}
+(NSString *) dump {
NSMutableString *result = [NSMutableString new];
for (NSValue *value in stackVariables) {
__weak StackVariable *var = [value nonretainedObjectValue];
[result appendString:[var description]];
[result appendString:#"\n"];
}
return result;
}
-(void) dealloc {
[stackVariables removeObject:[NSValue valueWithNonretainedObject:self]];
}
#end
Example:
#include "StackVariable.h"
int temp() {
LOCAL_VAR(int, i_wont_show, 0);
return i_wont_show;
}
int main(){
LOCAL_VAR(long, l, 15);
LOCAL_VAR(int, x, 192);
LOCAL_VAR(short, y, 256);
temp();
l += 10;
puts([[StackVariable dump] UTF8String]);
}
Output:
/Users/rross/Documents/TestProj/TestProj/main.m:676 - long l = { 19 00 00 00 00 00 00 00 }
/Users/rross/Documents/TestProj/TestProj/main.m:677 - int x = { c0 00 00 00 }
/Users/rross/Documents/TestProj/TestProj/main.m:678 - short y = { 00 01 }
This requires ARC (and all of it's magic) enabled for any file you want to test this in, or you will manually have to release the __stack_ variables, which won't be pretty.
However, it now gives you a hex dump of the variable (rather than the weird pointer one), and if you really tried hard enough (using __builtin_types_compatible), it could detect whether the result was an object, and print that.
Once again, threads will mess this up, but a simple way to fix that would be to create a NSDictionary of NSArrays, with a NSThread as the key. Makes it a bit slower, but let's be honest, if you're using this over the C++ version, you aren't going for performance.