Marcel Holtmann wrote:
>Hi Pedro,
>
>  
>>In the application I am developing, I want to log some information about 
>>Bluetooth devices, pretty much like "sdptool browse" and "hcitool info" 
>>do. In the code of these applications, there are calls to some library 
>>functions like hci_dflagstostr(), hci_lmtostr()... that create a string 
>>with information about the device (flags, features, etc.).
>>
>>The problem with these functions is that they call malloc(), expecting 
>>the caller to call free(). The problem with that is that this approach 
>>is not save, as it is not guaranteed that calling malloc() in a library 
>>function and free() in the main program works. In fact, I have seen a 
>>segmentation fault doing that.
>>    
>
>explain why this is not safe.
>  
I am not an expert in these issues, but I have read many times that 
there is no guarantee that the malloc/free implementation is the same in 
the main program and in the libraries. This is due to how the different 
modules are compiled: compiler version, compile flags, etc. But, as I 
said, I am not 100% sure. I am quite sure that I have read this for 
Windows, but maybe (probably) Linux is different.
>  
>>AFAIK, there are two alternatives: either the library functions expect a 
>>buffer (and its size) to be filled or malloc() and free() are both done 
>>within the library, which means creating a new library function call for 
>>freeing buffers allocated with malloc(). Are any of this alternatives 
>>implemented or on the roadmap? What workaround do you suggest?
>>    
>
>We can do that, but it is not on my roadmap. Send my patches for it and
>I will review them.
The easiest way to do it (and to continue with the actual API without 
modifications) would be to have a function call like:

void hci_str_free(char *str)
{
    free(str);
}

So after a call to hci_dflagstostr() (for example), the resulting 
pointer should be freed with hci_str_free(). In any case, there is one 
exception: hci_dtypetostr(); it returns a pointer to static data.

Cheers,

Pedro