`sizeof` may give incorrect result in editor for structs containing bitfields

[ytxmobile98]

Bug type: Language Service

Describe the bug

• OS and Version: Ubuntu 20.04
• VS Code Version: 1.61.0
• C/C++ Extension Version: v1.7.0-insiders2
• Other extensions you installed (and if the issue persists after disabling them): N/A
• If using SSH remote, specify OS of remote machine: N/A
• A clear and concise description of what the bug is, including information about the workspace (i.e. is the workspace a single project or multiple projects, size of the project, etc).

Steps to reproduce

Code sample 1

#include <iostream>

struct test1 {
    unsigned int a : 4;
    unsigned int b : 4;
    unsigned char c;
    unsigned char d;
};

int main() {
    std::cout << sizeof(test1) << std::endl;
}

Expected behavior

• sizeof(test1) gives 4 when the program runs. (when memory alignment is set to 4 bytes)
• However in the editor it shows 8.

Code sample 2

This time I add #pragma pack(1).

#include <iostream>

#pragma pack(1)
struct test1 {
    unsigned int a : 4;
    unsigned int b : 4;
    unsigned char c;
    unsigned char d;
};

int main() {
    std::cout << sizeof(test1) << std::endl;
}

Expected behavior

• sizeof(test1) gives 3 when the program runs. (when memory alignment is set to 1 byte)
• However in the editor it shows 6.

[Colengms]

Hi @ytx21cn . Thanks for reporting this. I'm able to reproduce the same issue in VS (where I can configure for Linux GCC, using Open Folder, a CppProperties.json file, and setting intelliSenseMode to linux-gcc-x64). I will open an issue against VS internally. (The IntelliSense implementation is shared between VS and the C/C++ extension for VS Code).

[ytxmobile98]

Hi @ytx21cn . Thanks for reporting this. I'm able to reproduce the same issue in VS (where I can configure for Linux GCC, using Open Folder, a CppProperties.json file, and setting intelliSenseMode to linux-gcc-x64). I will open an issue against VS internally. (The IntelliSense implementation is shared between VS and the C/C++ extension for VS Code).

It doesn't look like it is a settings-dependent issue. When I place a single .cpp file to somewhere that any project would almost never live there (such as ~/Desktop), I observe the same result. Hence it is the fault of internal VS calculations that yield the wrong results.

[nickstoughton]

I see this when the structure in question has internal padding. For example:

struct test {
    unsigned short a;
    unsigned short b;
    unsigned short c;
    unsigned int d;
    unsigned int e;
};

With alignment 4 VScode v 1.77.3 shows the sizeof (struct test) to be 6, but at runtime it is 16. Arrays containing this structure are also shown incorrectly as a result.

[sean-mcmanus]

This is internal bug 1418475.

[ewaldc]

It is not fixed as of VScode 1.93.1, C/C++ IntelliSense v1.22.7 (pre-release).

enum rx_desc_len {
        RX_DESC_LEN_TYPE_1 = (sizeof(struct RxDesc)),
        RX_DESC_LEN_TYPE_3 = (sizeof(struct RxDescV3)),
        RX_DESC_LEN_TYPE_4 = (sizeof(struct RxDescV4))
};

All 3 sizes are wrong, the first two even come back as size 0. {