memcpy（）导致iOS中的EXC_BAD_ACCESS

Question

I am getting NSData on my socket receiving function and I'm trying to copy that data in a tempbuffer of my audio class, I am using external type global variable to do so.

This is my code:

memcpy([recorder tempBuffer].mdata,(__bridger const void *)data,data.length);

Here recorder is my extern type global variable of audio class.

When control reaches this line of code an exception is thrown, what possibly be the mistake.

Answer 1

There are really three possibilities here:

1. [recorder tempBuffer].mdata is not a valid pointer. (What type is it, for instance? If it's a NSMutableData , you should be accessing its mutableBytes property.)
2. [recorder tempBuffer].mdata is not a valid pointer of sufficient size ( data.length ).
3. (__bridger const void *)data is not a valid pointer of sufficient size.

Of the three, I can guarantee that #3 needs addressing. A NSData is not itself the data you want, but an object wrapping the data you want. Instead of using a bridge here, you should be using data.bytes .

The other two, I can't help you with. I don't know what type mdata is or where it was allocated.

Answer 2

If the destination buffer is really a buffer you allocated with malloc or a uint8_t (or equivalent) buffer, you should:

1. Check to make sure that the destination buffer is big enough to hold the entire data contents.

2. Don't try to cast the NSData to a (void *) pointer, but rather use:

    memcpy(destination, data.bytes, data.length); 

   If the NSData is not in a contiguous block (which in iOS 7 and later, it might not be), data.bytes will copy it to a contiguous buffer, which you can then use with memcpy .

3. Or better, you can avoid this redundant copy, by removing memcpy altogether:

    [data getBytes:destination length:data.length]; 

   This will, if the NSData is not in a contiguous block, avoid having data.bytes copy it to a contiguous buffer which you would then copy again with the memcpy .

   Bottom line, NSData has a rich interface that should eliminate the need to use low-level memcpy calls yourself.

From the question, it's not clear what [recorder tempBuffer].mdata is and how you allocated it, so perhaps you can clarify. Hopefully that's not another NSData object that you're trying to copy into.