How we pass a function into objc2?

[b01o]

I am currently using objc2_core_media to construct a CMSampleBuffer. However, I cannot figure out how to pass a "c-unwind" ABI style function into objc2 function.

For example, the following code compiles but it segfaults when the newly constructed CMBlockBuffer goes out of scope.

use objc2::rc::Retained;
use objc2_core_foundation::*;
use objc2_core_media::*;

use std::{
    ffi::c_void,
    ptr::{NonNull, null_mut},
};

fn main() {
    unsafe extern "C-unwind" fn free_block(
        ref_con: *mut c_void,
        block: NonNull<c_void>,
        block_size: usize,
    ) {
        println!("free_block called");
    }

    let data = vec![65u8; 1024];
    let data = data.leak();
    let custom_block_source = CMBlockBufferCustomBlockSource {
        version: 0,
        AllocateBlock: None,
        FreeBlock: Some(free_block),
        refCon: null_mut(),
    };
    let block_buffer: *mut CMBlockBuffer = null_mut();
    let status = unsafe {
        CMBlockBuffer::create_with_memory_block(
            None,
            data.as_ptr() as *mut std::ffi::c_void,
            data.len(),
            kCFAllocatorNull,
            &custom_block_source,
            0,
            data.len(),
            0,
            NonNull::new(&block_buffer as *const _ as *mut _).unwrap(),
        )
    };
    assert_eq!(status, 0);

    let block_buffer = unsafe { Retained::from_raw(block_buffer).unwrap() };
    assert_eq!(block_buffer.retain_count(), 1);
}

Note that the free_block does not being called as well. Am I missing something on this?

[madsmtm]

A few things:

• Use mut block_buffer + NonNull::from(&mut block_buffer).
• Similarly, you probably want data.as_mut_ptr().

But even with those fixes, I see the same crash you're seeing. Tried to read the docs myself, can't figure out what's wrong here?

Maybe you could rewrite it to Objective-C, and see if the problem is present there with -fobjc-arc?

[b01o]

A few things:

• Use mut block_buffer + NonNull::from(&mut block_buffer).
• Similarly, you probably want data.as_mut_ptr().

Thanks for pointing out!

Maybe you could rewrite it to Objective-C, and see if the problem is present there with -fobjc-arc?

Tried it in Objective-C, it did segfaults. I hardly found any examples online about CMBlockBufferCreateWithMemoryBlock other than the official documents. Is there any way we can find out the problem at?

Here is the code written in Objective-C:

main.m

#import <CoreFoundation/CoreFoundation.h>
#import <CoreMedia/CoreMedia.h>

static void freeBlock(void *refCon, void *block, size_t size) {
  // This function is called to free the memory block.
  NSLog(@"Freeing block of size %zu", size);
  if (block != NULL) {
    free(block);
    block = NULL;
  }
}

// Define the struct with explicit 8-byte alignment
typedef struct __attribute__((aligned(8))) {
  uint32_t version;
  void *(*AllocateBlock)(void *refCon, size_t sizeInBytes);
  void (*FreeBlock)(void *refCon, void *doomedMemoryBlock, size_t sizeInBytes);
  void *refCon;
} AlignedCMBlockBufferCustomBlockSource;

int main(int argc, const char *argv[]) {
  size_t dataSize = 1024;
  uint8_t *data = (uint8_t *)malloc(dataSize);
  if (data == NULL) {
    NSLog(@"Memory allocation failed");
    return 1;
  }
  memset(data, 65, dataSize);

  AlignedCMBlockBufferCustomBlockSource customBlockSource = {
      .version = kCMBlockBufferCustomBlockSourceVersion,
      .AllocateBlock = NULL,
      .FreeBlock = &freeBlock,
      .refCon = NULL,
  };

  CMBlockBufferRef blockBuffer = NULL;
  OSStatus status = CMBlockBufferCreateWithMemoryBlock(
      kCFAllocatorDefault, data, dataSize, kCFAllocatorNull,
      (CMBlockBufferCustomBlockSource *)&customBlockSource, 0, dataSize, 0,
      &blockBuffer);

  if (status != noErr) {
    NSLog(@"Error creating block buffer: %d", (int)status);
    free(data);
    return 1;
  }

  CFIndex retainCount = CFGetRetainCount(blockBuffer);
  if (retainCount != 1) {
    NSLog(@"Unexpected retain count: %ld", (long)retainCount);
  }

  CFRelease(blockBuffer);
  return 0;
}

compile and run:

clang -framework CoreMedia -framework CoreFoundation -fobjc-arc main.m -o prog1 && ./prog1

output:

Job 1, './prog1' terminated by signal SIGSEGV (Address boundary error)

[madsmtm]

Ah, thanks. That's kinda what I suspected though, I don't think there's really anything I / the objc2 project can do further about this.

I'd recommend that you submit it upstream to Apple via. their Feedback Assisant app (if you choose SDKs + CoreMedia, I think you're more likely to get a response).

Alternatively, try to see if you can use a different API, either with a custom allocator, or with CFData and kCFAllocatorDefault? Might be that it's just the kCFAllocatorNull case that's broken? Or maybe pass null_mut() for the data ptr, and provide an allocation callback to create the data in there?

[b01o]

Yes, I do plan to use api the another way. Appreciate your help, I will close this for now. Update when I get responses from upstream.

[b01o]

Finally I was able to find the way to create a CMBlockBuffer with the custom block in Objective-C, but I try to do as same in the rust side, it segfaults. Here is the code to reproduce.

Objective-C:

// main.m
#import <CoreMedia/CoreMedia.h>

static void freeBlock(void *refCon, void *block, size_t size) {
  assert(block != NULL);
  char *data = (char *)block;
  NSLog(@"Freeing block of size %zu, [%c, %c].", size, data[0], data[1]);
  NSLog(@"Getting RefCon: %d", *(int *)refCon);
  free(block);
}

int main(int argc, const char *argv[]) {
  size_t dataSize = 2;
  char *data = (char *)malloc(dataSize);
  assert(data != NULL); // Memory allocation fail, exit early
  memset(data, 'A', dataSize);

  int refData = 42;
  CMBlockBufferCustomBlockSource source = {0};
  source.version = kCMBlockBufferCustomBlockSourceVersion;
  source.FreeBlock = &freeBlock;
  source.refCon = &refData; // Reference to an integer, can be any data type

  CMBlockBufferRef blockBuffer = NULL;
  OSStatus status = CMBlockBufferCreateWithMemoryBlock(
      kCFAllocatorDefault, data, dataSize, kCFAllocatorNull, &source, 0,
      dataSize, 0, &blockBuffer);

  assert(status == noErr); // block buffer creation failed, exit early
  assert(CFGetRetainCount(blockBuffer) == 1); // retain count must be 1
  CFRelease(blockBuffer);
  return 0;
}

compile and run:

clang -framework CoreMedia -framework CoreFoundation -fobjc-arc main.m -o prog1 && ./prog1

The Objective-C code should run as expected.

Rust side:

// src/main.rs
use std::{ffi::c_void, ptr::NonNull};
use objc2::rc::Retained;
use objc2_core_foundation::*;
use objc2_core_media::*;

unsafe extern "C-unwind" fn free_block(
    ref_context: *mut c_void,
    block: NonNull<c_void>,
    size: usize,
) {
    let data = unsafe { Vec::from_raw_parts(block.as_ptr() as *mut char, 2, 2) };
    println!("Freeing block of size: {size}, {data:?}");
    let ref_value = unsafe { *(ref_context as *mut i32) };
    println!("Getting RefCon: {ref_value}");
}

fn main() {
    let data_size = 2;
    let data = vec!['A'; data_size];
    let data = data.leak(); // we free this at `free_block`

    let mut ref_data = 42;
    let mut source: CMBlockBufferCustomBlockSource = unsafe { std::mem::zeroed() };
    source.version = kCMBlockBufferCustomBlockSourceVersion;
    source.FreeBlock = Some(free_block);
    source.refCon = &mut ref_data as *mut _ as *mut c_void;

    let block_buffer: *mut CMBlockBuffer = std::ptr::null_mut();
    let status = unsafe {
        CMBlockBuffer::create_with_memory_block(
            None, // use default allocator
            data.as_mut_ptr() as *mut _,
            data_size,
            kCFAllocatorNull,
            &source,
            0,
            data_size,
            0,
            NonNull::from_ref(&block_buffer),
        )
    };
    assert_eq!(status, kCMBlockBufferNoErr); // fail to create block buffer
    let block_buffer = unsafe { Retained::from_raw(block_buffer).unwrap() };
    assert_eq!(block_buffer.retain_count(), 1);
}

• Create project

cargo new objc_custom_block_buffer && cd objc_custom_block_buffer

• Dependencies

cargo add objc2 objc2-core-foundation objc2-core-media

• Change "src/main.rs" to the example above. Run it with Cargo run we get a segfault.

---

Could it be the auto generated function signature does not match the original one here?

[madsmtm]

I found the issue: The problem is that the header has a #pragma pack(push, 4) at the top, which means we should have added #[repr(C, packed(4))] to CMBlockBufferCustomBlockSource. I'll see if I can fix it.

[madsmtm]

I've fixed this in 191b5bf, sorry for the trouble!

This is a fairly serious bug, so I think I'm going to do a patch release v0.3.2 (instead of waiting until v0.4.0), hopefully in a few weeks.