malloc & free

Bluez-users Pedro Monjo Florit 3 participants 4 messages Mar 31, 2005 – Apr 6, 2005 Bluetooth drivers

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Pedro Monjo Florit

author

2005-03-31 16:30 UTC #0 lore raw

Hi Marcel!

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.

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?

Thanks,

Pedro Monjo

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Marcel Holtmann

maintainer

2005-04-03 18:58 UTC #1 lore raw

Hi Pedro,

Quoted 11 lines

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.

Quoted 5 lines

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.

Regards

Marcel

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Pedro Monjo Florit

author

2005-04-06 07:12 UTC #2 lore raw

Quoted 10 lines · Marcel Holtmann

Hi Pedro,
Quoted 4 lines
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.

Quoted 6 lines

Quoted 3 lines
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

Xavier Garreau

2005-04-06 07:33 UTC #3 lore raw

Quoted 8 lines

Quoted 7 lines
Quoted 3 lines
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.

Except if statically linked with the libc or if malloc and free have been re_#define_d, functions from the lib and the main program will uses the system default libc implementation.

Regards,
Xavier Garreau