I've got code (where commit_id is already set) like:
git_note* note;
git_note_read(¬e, repo, "refs/notes/label", &commit_oid);
printf("%s\n", note->message);
git_note_free(note);
It doesn't compile, complaining:
.../importer_test.cc:103:22: error: member access into incomplete type 'git_note'
printf("%s\n", note->message);
^
.../include/git2/types.h:160:16: note: forward declaration of 'git_note'
typedef struct git_note git_note;
If I just copy/paste from src/notes.h into this file:
struct git_note {
git_oid id;
git_signature *author;
git_signature *committer;
char *message;
};
It compiles and runs correctly. But surely that's not the right solution?
git_note is an opaque type. You're not meant to access members of the data directly. You should be using the accessor functions to read from it. In your case, you would want to use git_note_message() to get the message.
Related
I have to provide a C-style callback for a specific C library in an iOS app. The callback has no void *userData or something similar. So I am not able to loop in a context. I'd like to avoid introducing a global context to solve this. An ideal solution would be an Objective-C block.
My question: Is there a way to 'cast' a block into a function pointer or to wrap/cloak it somehow?
Technically, you could get access to a function pointer for the block. But it's totally unsafe to do so, so I certainly don't recommend it. To see how, consider the following example:
#import <Foundation/Foundation.h>
struct Block_layout {
void *isa;
int flags;
int reserved;
void (*invoke)(void *, ...);
struct Block_descriptor *descriptor;
};
int main(int argc, char *argv[]) {
#autoreleasepool {
// Block that doesn't take or return anything
void(^block)() = ^{
NSLog(#"Howdy %i", argc);
};
// Cast to a struct with the same memory layout
struct Block_layout *blockStr = (struct Block_layout *)(__bridge void *)block;
// Now do same as `block()':
blockStr->invoke(blockStr);
// Block that takes an int and returns an int
int(^returnBlock)(int) = ^int(int a){
return a;
};
// Cast to a struct with the same memory layout
struct Block_layout *blockStr2 = (struct Block_layout *)(__bridge void *)returnBlock;
// Now do same as `returnBlock(argc)':
int ret = ((int(*)(void*, int a, ...))(blockStr2->invoke))(blockStr2, argc);
NSLog(#"ret = %i", ret);
}
}
Running that yields:
Howdy 1
ret = 1
Which is what we'd expect from purely executing those blocks directly with block(). So, you could use invoke as your function pointer.
But as I say, this is totally unsafe. Don't actually use this!
If you want to see a write-up of a way to do what you're asking, then check this out:
http://www.mikeash.com/pyblog/friday-qa-2010-02-12-trampolining-blocks-with-mutable-code.html
It's just a great write-up of what you would need to do to get this to work. Sadly, it's never going to work on iOS though (since you need to mark a page as executable which you're not allowed to do within your app's sandbox). But nevertheless, a great article.
If your block needs context information, and the callback does not offer any context, I'm afraid the answer is a clear no. Blocks have to store context information somewhere, so you will never be able to cast such a block into a no-arguments function pointer.
A carefully designed global variable approach is probably the best solution in this case.
MABlockClosure can do exactly this. But it may be overkill for whatever you need.
I know this has been solved but, for interested parties, I have another solution.
Remap the entire function to a new address space. The new resulting address can be used as a key to the required data.
#import <mach/mach_init.h>
#import <mach/vm_map.h>
void *remap_address(void* address, int page_count)
{
vm_address_t source_address = (vm_address_t) address;
vm_address_t source_page = source_address & ~PAGE_MASK;
vm_address_t destination_page = 0;
vm_prot_t cur_prot;
vm_prot_t max_prot;
kern_return_t status = vm_remap(mach_task_self(),
&destination_page,
PAGE_SIZE*(page_count ? page_count : 4),
0,
VM_FLAGS_ANYWHERE,
mach_task_self(),
source_page,
FALSE,
&cur_prot,
&max_prot,
VM_INHERIT_NONE);
if (status != KERN_SUCCESS)
{
return NULL;
}
vm_address_t destination_address = destination_page | (source_address & PAGE_MASK);
return (void*) destination_address;
}
Remember to handle pages that aren't required anymore and note that it takes a lot more memory per invocation than MABlockClosure.
(Tested on iOS)
I am trying to write some code that interacts with an USB device in Objective C, and I got stuck on setting the callback function for incoming reports. In my case it's an IOKIT function but I think the problem is more general as I (apparently) don't know how to correctly set a C callback function in Objective-C. I've got a Class "USBController" that handles the io functions
USBController.m:
#include <CoreFoundation/CoreFoundation.h>
#include <Carbon/Carbon.h>
#include <IOKit/hid/IOHIDLib.h>
#import "USBController.h"
static void Handle_IOHIDDeviceIOHIDReportCallback(
void * inContext, // context from IOHIDDeviceRegisterInputReportCallback
IOReturn inResult, // completion result for the input report operation
void * inSender, // IOHIDDeviceRef of the device this report is from
IOHIDReportType inType, // the report type
uint32_t inReportID, // the report ID
uint8_t * inReport, // pointer to the report data
CFIndex InReportLength) // the actual size of the input report
{
printf("hello"); //just to see if the function is called
}
#implementation USBController
- (void)ConnectToDevice {
...
IOHIDDeviceRegisterInputReportCallback(tIOHIDDeviceRefs[0], report, reportSize,
Handle_IOHIDDeviceIOHIDReportCallback,(void*)self);
...
}
...
#end
All the functions are also declared in the header file.
I think I did pretty much the same as what I've found here, but it doesn't work. The project compiles nicely and everything works up till the moment there is input and the callback function is to be called. Then I get an "EXC_BAD_ACCESS" error. The first three arguments of the function are correct. I'm not so sure about the context..
What did I do wrong?
I am not sure at all that your EXEC_BAD_ACCESS depends on your callback. Indeed, if you say that it is called (I suppose you see the log) and since it only logs a message, there should be no problem with this.
EXEC_BAD_ACCESS is caused by an attempt to access an already deallocated object. You can get more information in two ways:
execute the program in debug mode, so when it crashes you will be able to see the stack content;
activate NSZombies or run the program using the performance tool Zombies; this will tell you exactly which object was accessed after its deallocation.
I know how to fix this. When calling this:
IOHIDDeviceRegisterInputReportCallback(tIOHIDDeviceRefs[0], report, reportSize,
Handle_IOHIDDeviceIOHIDReportCallback,(void*)self);
You don't include the code for the creation/type of the value called report. However the method name "Handle_IOHIDDeviceIOHIDReportCallback" comes from an Apple document where there is an error in the creation of the report value. https://developer.apple.com/library/archive/technotes/tn2187/_index.html
CFIndex reportSize = 64;
uint8_t report = malloc( reportSize ); // <---- WRONG
IOHIDDeviceRegisterInputReportCallback( deviceRef,
report,
reportSize,
Handle_IOHIDDeviceIOHIDReportCallback,
context );
Instead do this:
uint8_t *report = (uint8_t *)malloc(reportSize);
I am declaring an array of NSString* in a header file of a class.
PolygonShape.h
NSString* POLYGON_NAMES[] = {#"Invalid Polygon", #"Monogon", ...};
Now I am using this in PolyginShape.m as follows:
- (NSString*) name {
return (POLYGON_NAMES [self.numberOfSides]);
}
numberOfSides is an iVar which will indicate the index at which the polygon name is stored
So far so good ... it was compiling without any errors
Then I added PolygonShape.h in my file that implements main method (note: these does not have any class definition and call functions C-Style rather than obj-c Style)
#import "PolygonShape.h"
Now when I compile, I am getting a build (linking) error
ld: duplicate symbol _POLYGON_NAMES in /Users/../Projects/CS193P/1B/What_A_Tool/build/What_A_Tool.build/Debug/What_A_Tool.build/Objects-normal/i386/PolygonShape.o and /Users/../Projects/CS193P/1B/What_A_Tool/build/What_A_Tool.build/Debug/What_A_Tool.build/Objects-normal/i386/What_A_Tool.o
collect2: ld returned 1 exit status
So I went thru stack overflow and other forums and mostly the advice was to make the global variable extern and so I did ...
extern NSString* POLYGON_NAMES[] = {#"Invalid Polygon", #"Monogon" .. };
However I am still getting the linking error and also getting 2 warnings now that says
warning: 'POLYGON_NAMES' initialized and declared 'extern'
at both the places where i am importing PolygonShape.h
What am I missing here?
Thanks.
In your header file declare the array as:
extern const NSString* POLYGON_NAMES[];
In your source file, define the array and initialize the contents:
const NSString* POLYGON_NAMES[] = {#"Invalid Polygon", #"Monogon" };
I use this code to set my constants
// Constants.h
extern NSInteger const KNameIndex;
// Constants.m
NSInteger const KNameIndex = 0;
And in a switch statement within a file that imports the Constant.h file I have this:
switch (self.sectionFromParentTable) {
case KNameIndex:
self.types = self.facilityTypes;
break;
...
I get error at compile that read this: "error:case label does not reduce to an integer constant"
Any ideas what might be messed up?
For C/C++ and Objective-C must the case statement have fixed values - "reduced to an integer (read value)" at compile time
Your constants is not a real "constant" because it is a variable and I imagine it can be changed through a pointer - ie &KNameIndex
Usually one defines constants as enum
enum {
KNameIndex = 0,
kAnotherConstant = 42
};
If you would use C++, or Objective-C++ (with .mm as file extension) you could use a const statement as
const int KNameIndex = 0;
You can use
#define KNameIndex 0
...
switch (self.sectionFromParentTable) {
case KNameIndex:
self.types = self.facilityTypes;
break;
...
and it should work.
Just had the same problem and I decided to go with #define rather than enum. Works for me™ ;-)
This is a stab in the dark because I haven't used Cocoa / ObjC in a long time now, but is the member variable sectionFromParentTable not of int type?
I have not worked with Objective C, but I'd try chucking the 'extern'. At least if this were C++, the Constants.m file would not be part of the compilation unit of Other.m, so the value of KNameIndex would be unknown to the compiler. Which would explain the error; an unknowable value can't be a constant.
Does putting the definition, not just the declaration, in the Constants.h file help?
I think you are stuck with using a const int instead of a const NSInteger as switch only works with built in integral types. (not sure about your syntax with const flipped around after the type).
Take a look at the related question: Objective-C switch using objects?
The offsetof macro seems not to work under C++/CLI.
This works fine in unmanaged C++, but throws "error C2275: 'Entity' :illegal use of this type as an expression" error in CLI.
struct Property{
char* label;
PropertyTypes type;
unsigned int member_offset;
unsigned int position;
unsigned char bit_offset;
};
struct Entity{
...
bool transparent;
...
};
Property property = {"Transparent",
TYPE_BOOL,
offsetof(Entity, transparent),
0,
0}; // C2275 HERE
Does CLI have some replacement?
My guess would be that the compiler message boils down to: "offsetof" is not a known macro and if it was a function its parameters must not contain a typename.
Edit: As somebody pointed out in the comments, offsetof is actually part of the std lib. So what's missing is probably just
#include <cstddef>
Alternatively, you can use this macro implementation (taken from Win32/MFC headers):
#ifdef _WIN64
#define OFFSET_OF( s, m )\
(size_t)((ptrdiff_t)&reinterpret_cast<const volatile char&>((((s*)0)->m)) )
#else
#define OFFSET_OF( s, m )\
(size_t)&reinterpret_cast<const volatile char&>((((s*)0)->m))
#endif
Standard C++ already has an alternative; &Entity::transparent. You'll probably want to use templates when redesigning the Propery class. The type of a pointer-to-member is non-trivial.
You will need to provide the type of the object you are assigning to. Looks like there is some type-mismatch for the member in question.
See this for sample usage.
Just a shot in the dark and without a chance to double-check this - should
offsetof(Entity, transparent),
perhaps rather read
offsetof( struct Entity, transparent ),
???