C++ ABI Interop with C, Strings issue | Together C & C++ | Page 1

dark barn Sep 20, 2024, 5:05 PM

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Hey guys, I'm currently trying to write a translation layer for the Steamworks SDK in pure C.

This SDK relies on C++ headers to link against, but has a C ABI included in the library. My goal has been to write a generator for a pure C header, including all the data structures needed to interface with the C ABI.

As part of my research for this project, I've run into an issue with calling C++ C ABI functions which return strings, illustrated in the example below:

// libabi.so
extern "C" {
    const char* getMessage() {
        return "Hello from the dynamic library!";
    }
}

// abi_test.c
int main() {
    const char *message = getMessage();
    printf("Message: %s\n", message);
    return 0;
}

(I have also tested this function returning a malloced char *, but the same problem occurs)

When running abi_test.c (properly linked against the .so), the memory at the pointer returned from getMessage() is corrupted.

(gdb) print message
$1 = 0x555592a0 <error: Cannot access memory at address 0x555592a0>

In my sanity checking, I tested creating a new C++ file to wrap the C++ ABI, and expose it as extern "C" within the same project as the test, and observed the memory being returned from the library un-corrupted in C++, and the same behavior with the pointer memory corrupting once it returns to C from the forward declaration.

I hope this isn't too much of a word soup, and I'm happy to clarify anything... This is a minimum recreated example of what I've experienced testing the Steamworks SDK directly, and if it helps I can upload a full CMake project example

surreal ibexBOT Sep 20, 2024, 5:05 PM

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flint stratus Sep 20, 2024, 5:12 PM

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This is a minimum recreated example
I love you

dark barn Sep 20, 2024, 5:22 PM

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Based on the ABI spec, they describe the return type as const char *, ex:

        {
          "methodname": "GetPersonaName",
          "methodname_flat": "SteamAPI_ISteamFriends_GetPersonaName",
          "params": [],
          "returntype": "const char *"
        },

I'm assuming that they are ensuring you have at least some amount of time to copy a heap string into whatever you want

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Also that snippet is indeed broken, since its returning a stack string, but just to illustrate a heap example:

// libabi.so
#include <cstring>
#include <cstdlib>

extern "C" {
    const char* getMessage() {
        const char* message = "Hello from the dynamic library!";
        char* heapMessage = (char*)std::malloc(std::strlen(message) + 1);
        std::strcpy(heapMessage, message);
        return heapMessage;
    }
}

Breakpoint 1, getMessage () at /home/ryan/repos/pure-c-steamworks/abi_demo/lib/src/main.cpp:6
6               const char* message = "Hello from the dynamic library!";
(gdb) n
7               char* heapMessage = (char*)std::malloc(std::strlen(message) + 1);
(gdb) 
8               std::strcpy(heapMessage, message);
(gdb) 
9               return heapMessage;
(gdb) print heap
heapMessage  heap_info    
(gdb) print heapMessage 
$1 = 0x5555555592a0 "Hello from the dynamic library!"
(gdb) n
10          }
(gdb) 
main () at /home/ryan/repos/pure-c-steamworks/abi_demo/test/src/main.c:7
7           printf("Message: %s\n", message);
(gdb) print message
$2 = 0x555592a0 <error: Cannot access memory at address 0x555592a0>

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My goal is to get as close to the native C ABI as possible for the Steamworks SDK, so ideally I could just write a generator to re-declare the existing C ABI function signatures and datatypes...

I'm assuming that valve would not go through the trouble of exposing a C ABI, just to make it inaccessible from pure C... It could very much be the case though

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I have tested exactly that though, but the implication of that is I need to write a more complex generator and change how the API is normally used

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if you have a minimum example for C++ C string return interop that would be awesome

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for this I have no header, I'm just implicitly linking against the sig

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but it's resolving properly, its just that the pointer memory corrupts once back in C

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is it even possible that the virtual address space is different for the lib and its getting unmapped when returning back to the main program?

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no (to the static link):

libabi

implemented in C++ with extern "C"
shared library

demo

links against libabi.so

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sounds good, if you could keep it platform independant that would be awesome

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I appreciate it

dark barn Sep 20, 2024, 7:19 PM

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dark barn Also that snippet is indeed broken, since its returning a stack string, but just...

tried just using malloc() rather than std::malloc() but same problem

dark barn Sep 20, 2024, 7:35 PM

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hmmm... this does indeed work for me

dark barn Sep 20, 2024, 8:35 PM

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So, I've been trying to apply this to the steamworks SDK again, and I've found that with a C++ wrapper just handing off the pointer to C I can get it to work, but not without such a wrapper...

working:

//internal.h
const char* GetPersonaName(void);

//bind.cpp
extern "C" const char* GetPersonaName()
{
    const char* personaName = SteamFriends()->GetPersonaName();
    return personaName;
}

//demo.c
#include "internal.h"
int main() {
    bool result = SteamAPI_InitSafe();
    printf("SteamAPI_InitFlat() result: %d\n",result);
    if(!result) {
        printf("SteamAPI_InitFlat() failed\n");
        return 1;
    }

    char * personaName = GetPersonaName();
    printf("Persona name: %s\n",personaName);

    SteamAPI_Shutdown();

    return 0;
}

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This is confusing, because I would expect to just be able to do something like this:

//internal.h
const char * SteamAPI_ISteamFriends_GetPersonaName(void);

//demo.c
#include "internal.h"
int main() {
    bool result = SteamAPI_InitSafe();
    printf("SteamAPI_InitFlat() result: %d\n",result);
    if(!result) {
        printf("SteamAPI_InitFlat() failed\n");
        return 1;
    }

    printf("Persona name: %s\n",SteamAPI_ISteamFriends_GetPersonaName());

    SteamAPI_Shutdown();

    return 0;
}

which does not work at all...

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but I guess technically the wrapper is not using the C ABI signature...

dark barn Sep 20, 2024, 9:04 PM

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That is a C ABI function exported in the steamworks SDK, of which the source code is not available.

the spec:

        {
          "methodname": "GetPersonaName",
          "methodname_flat": "SteamAPI_ISteamFriends_GetPersonaName",
          "params": [],
          "returntype": "const char *"
        },

There are headers included with the SDK for C++, and there is a "flat" header which describes a C ABI, of which the signatures dont appear to want to link. All of the flat symbols can be observed with nm or readelf, so I know they exist (and I can use the flat init function)

#

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Yeah the end goal is basically to parse that json into a pure C header so I have the declarations, but these smaller tests I'm doing now with the manual forward declarations are failing

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hmm

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statically linking isnt an option afaik

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hmmm... surely theres a way for the linker to automatically resolve these when compiling, no?

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hmm... for windows they provide a .lib but I'm not aware of an equivalent on linux

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hmm

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worth a shot i guess

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I took a mega long shot earlier and opened a steam support ticket, its kinda ridiculous how hard it is to get dev support... the only community is a steam group which has been closed (max members) for around a decade

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The guy who used to maintan CSteamworks (a C ABI before valve shipped one with the SDK) has no contact information, other than his linkedin

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pretty sure he is also the reason they ship the json spec, since he worked at valve for a year and rewrote their docs and improved the generators

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I do and that’s the plan… but I can’t get the ABI to work in the first place, so the header would be useless…

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Just seems like the linker already sees the symbols so why can’t it resolve them in the ELF?

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You’re right though

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no dice

    #include <dlfcn.h>

    const char *(*SteamAPI_ISteamFriends_GetPersonaName)(void);

    void *handle = dlopen("../lib/libsteam_api.so", RTLD_NOW);
    if (!handle) {
        fprintf(stderr, "Error opening library: %s\n", dlerror());
        return 1;
    }

    dlerror(); // Clear any existing error

    *(void **)(&SteamAPI_ISteamFriends_GetPersonaName) = dlsym(handle, "SteamAPI_ISteamFriends_GetPersonaName");
    const char *dlsym_error = dlerror();
    if (dlsym_error) {
        fprintf(stderr, "Error loading symbol 'SteamAPI_ISteamFriends_GetPersonaName': %s\n", dlsym_error);
        dlclose(handle);
        return 1;
    }

    printf("Successfully loaded 'SteamAPI_ISteamFriends_GetPersonaName' function.\n");

    const char *personaName = SteamAPI_ISteamFriends_GetPersonaName();
    if (personaName) {
        printf("Successfully retrieved persona name: %s\n", personaName);
    } else {
        printf("Failed to retrieve persona name.\n");
    }

    dlclose(handle);

    return 0;

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must be the impl inside the lib

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the flat header itself is quite odd, since it has the class the function belongs to in C++ as a parameter (which is not described in the json spec)

S_API const char * SteamAPI_ISteamFriends_GetPersonaName( ISteamFriends* self );

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idek. I'll leave it alone for today I guess

dark barn Sep 20, 2024, 10:17 PM

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what if you try to inkoke it after?

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it resolves for me too

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The invocation is the segfault

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It relies on C++ headers, the json describes the C ABI

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I don’t believe so, because the C++ api doesn’t, but maybe init_flat() is fundamentally different

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Thanks for trying

surreal ibexBOT Sep 20, 2024, 10:35 PM

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@dark barn Has your question been resolved? If so, type !solved :)

dark barn Sep 20, 2024, 10:49 PM

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Gonna mark as resolved since the original question has been answered

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Thanks again @regal hill

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!solved

surreal ibexBOT Sep 20, 2024, 10:50 PM

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Thank you and let us know if you have any more questions!

This thread is now set to auto-hide after an hour of inactivity

dark barn Sep 21, 2024, 5:21 PM

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update:
my goal would be impossible without wrapping certain things extensively in C++

this conversation with the SDK Docs author makes that pretty clear:
https://github.com/rlabrecque/CSteamworks/issues/11

and I found a hidden repo actually attempting to do so (incompletely):
https://github.com/EvilPudding/steam.candle
Also I opened a ticket with steam support (in a totally non developer channel) who did not have anything to add, other than telling me to get in contact via github (where there is no relevant repo) lol

#C++ ABI Interop with C, Strings issue

libabi

demo