[SOLVED] Why is 'size_t' in external C library interpreted wrong?

2020 5 22
1 个回答
更新时间：2020 6 26
9 次查看（30 天）

0 个投票

I'm running into odd behavior while interfacing with an external C library. For a function like the following,
int foo(size_t sz, uint8_t buf[8])
{
memset(buf, 0, 8);
memcpy(buf, &sz, sizeof(size_t));
return sizeof(size_t);
}
I get the error "Warning: The data type 'error' used by function foo does not exist" when loading the library. And, when I run foo, the contents of buf don't make sense to me. If I change the size_t to uint64_t (or any other suitable integral type), then I get what I expect out in buf.
How is Matlab treating size_t? Why does it appear to be treating it as an 'error' type?
SOLUTION: Added appropriate include (stddef.h) in library header.

请先登录，再回答此问题。

回答（1 个）

James Tursa
James Tursa 2020-5-22
编辑：James Tursa 2020-5-22

0 个投票

I am unaware of MATLAB treating size_t differently, but there is a potential error in your code snippet:
int foo(size_t sz, uint8_t buf[8])
:
return sizeof(size_t);
The sizeof( ) operator returns a size_t type, but your function returns an int. So on a 64-bit system these will probably not match since size_t is likely a 64-bit integer and int is likely a 32-bit integer. You should change the signature to:
size_t foo(size_t sz, uint8_t buf[8])
What version of MATLAB are you running? 64-bit? Are you using the loadlibrary function? Are you loading a third party library or a library you wrote? Is there a reference to 'error' in the header that you are using? Maybe a typedef?
As for your problem, I don't see a reference to an 'error' data type in your code, so there is nothing for us to analyze.

3 个评论
显示 1更早的评论 隐藏 1更早的评论

Tyson Lawrence
Tyson Lawrence 2020-5-26
Sorry for the slow response... holiday weekend.
I'm running 64-bit Matlab. I am not really concerned with the return statement. I probably should have left it out.
I'm working on building a Matlab API for a library I wrote. To interface with the library, I am simpliy using loadlibrary followed by calllibs.
What gets me is the memcpy. If I make foo something like the following (also changing the size_t in the memcpy),
void foo(uint64_t sz, uint8_t buf[8])
void foo(int32_t sz, uint8_t buf[8])
then the contents of buf are what I expect when I call it from Matlab (namely the bytes corresponding to integer value of sz). When the type of sz is size_t, the contents of buf don't make senes to me. For example, when sz is of type uint64_t and sz is set to uint64(2763306), then buf is [42,42,42,0,0,0,0,0]. This is correct. If sz is of type size_t and set to the same value, then buf is [208,166,244,85,37,127,0,0]. I get similar results when I change the type of sz on the Matlab size (in calllib... uint32(sz), int32(sz), uint64(sz), int64(sz), etc...). I also get similar results when I try to pass in 0 for sz.
The warning about the error data type (pasted below) occurs when loading the library.
Warning: The data type 'error' used by function foo does not exist.
> In loadlibrary
This warning only occurs when the data type of sz is size_t. If I change it to another numeric type, it goes away.
All of this makes me think Matlab is doing something different with size_t. There has got to be a reason for it, but I don't know it.
James Tursa
James Tursa 2020-5-26
编辑：James Tursa 2020-5-26
I am not sure what is going on yet. Can you double check to see that you have all necessary prototypes in your header file? E.g., if you are using memset( ) and memcpy( ) then you should have
#include <string.h>
in your header file to make sure the prototypes are present. This also defined size_t.
Can you post a small example complete library source code that reproduces the problem?
Tyson Lawrence
Tyson Lawrence 2020-6-26
编辑：Tyson Lawrence 2020-6-26
This is an insanely slow response... I got pulled away from this work for a stretch. I want to add the answer in case others have similar issues.
I resolved the problem by adding the appropriate includes in my library interface (stddef.h in this case). The problem didn't show up in my unit tests or Python API testing, so I missed it. Usually Matlab warns when something is missing.

请先登录，再进行评论。

类别

在 帮助中心 和 File Exchange 中查找有关 Matrix Indexing 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by