[C and C++] Calling C++ function from c - Tilted Forum Project Discussion Community

madcow · 04-13-2004, 07:32 PM

Hello all,
I have a c program that is doing a few things with a MySQL database and I found a piece of code written in C++ to determine the amount of CPU usage on my machine. I would like to call the function written in C++ in my C program and then write this to the database, compiling goes fine until the linker fails saying:

Linking...
net_sql.obj : error LNK2001: unresolved external symbol _GetCpuUsage
release/netsql.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

Any idea why this is happening? Is there some way to do what I am trying to do?

I'm using Microsoft Visual C++ 6. The Cpu usage code is not a lib or dll, just cpp files.

KnifeMissile · 04-13-2004, 08:40 PM

How well do you know C++?

In C++, it's possible to have two functions with exactly the same function names, as long as they have a different signature. A function's signature is its parameters, their types, and the order they're declared (this includes no parameters, as well). How this is implemented in C++ is that the function's name is modified in a determinisitic and canonical way, depending on it's parameters. This process is often called name mangling although, officially, the function name is said to be decorated.
So, when your C linker goes looking into an object file made by your C++ compiler, it's looking for _GetCpuUage while the function has been "decorated" to something like _GetCpuUsage@@YA_NPAM@Z.

To stop C++ from decorating your function name, you use the extern "C" keyword(s). So, in the C++ header, you'd export the function like this:

Code:

extern "C" bool GetCpuUsage(float* percentage);

...and in your C++ source file, you'd have something like:

Code:

extern "C" bool GetCpuUsage(float* percentage) {&#10&#9// your implementation, here...&#10}

The function name GetCpuUsage won't get "mangled" and your C linker should be able to find it.

Actually, C++ programmers prefer to wrap the C++ function in a C function and export that so that they can both be used but I get the sense that you're not a C++ programmer, so this won't concern you.

I hope you found this interesting and helpful--so, happy programming!

madcow · 04-13-2004, 09:09 PM

Thank you so much for your elequent explanation and solution. I'm actually a very decent programmer when it comes to C and Java, I haven't touched C++ in years. It didn't surprise me that C++ allows you to have same functions with different signitures, Java does that all the time. I just hadn't considered that the compiler was doing that to the function name and have never heard of having to extern a function in that way. Thanks again!

04-13-2004, 07:32 PM	#1 (permalink)
madcow Crazy Location: The state of denial	[C and C++] Calling C++ function from c Hello all, I have a c program that is doing a few things with a MySQL database and I found a piece of code written in C++ to determine the amount of CPU usage on my machine. I would like to call the function written in C++ in my C program and then write this to the database, compiling goes fine until the linker fails saying: Linking... net_sql.obj : error LNK2001: unresolved external symbol _GetCpuUsage release/netsql.exe : fatal error LNK1120: 1 unresolved externals Error executing link.exe. Any idea why this is happening? Is there some way to do what I am trying to do? I'm using Microsoft Visual C++ 6. The Cpu usage code is not a lib or dll, just cpp files.

04-13-2004, 08:40 PM	#2 (permalink)
KnifeMissile Location: Waterloo, Ontario	How well do you know C++? In C++, it's possible to have two functions with exactly the same function names, as long as they have a different signature. A function's signature is its parameters, their types, and the order they're declared (this includes no parameters, as well). How this is implemented in C++ is that the function's name is modified in a determinisitic and canonical way, depending on it's parameters. This process is often called name mangling although, officially, the function name is said to be decorated. So, when your C linker goes looking into an object file made by your C++ compiler, it's looking for _GetCpuUage while the function has been "decorated" to something like _GetCpuUsage@@YA_NPAM@Z. To stop C++ from decorating your function name, you use the extern "C" keyword(s). So, in the C++ header, you'd export the function like this: Code: extern "C" bool GetCpuUsage(float* percentage); ...and in your C++ source file, you'd have something like: Code: extern "C" bool GetCpuUsage(float* percentage) {&#10&#9// your implementation, here...&#10} The function name GetCpuUsage won't get "mangled" and your C linker should be able to find it. Actually, C++ programmers prefer to wrap the C++ function in a C function and export that so that they can both be used but I get the sense that you're not a C++ programmer, so this won't concern you. I hope you found this interesting and helpful--so, happy programming!

04-13-2004, 09:09 PM	#3 (permalink)
madcow Crazy Location: The state of denial	Thank you so much for your elequent explanation and solution. I'm actually a very decent programmer when it comes to C and Java, I haven't touched C++ in years. It didn't surprise me that C++ allows you to have same functions with different signitures, Java does that all the time. I just hadn't considered that the compiler was doing that to the function name and have never heard of having to extern a function in that way. Thanks again! __________________ Smoke me a kipper, I'll be back for breakfast.