Linking to a C library in a C++ program

[JP]

I'm trying to link to a C library (libjpeg, not using irrlicht) from my C++ program but it won't link properly and brings up errors saying functions are not declared wheras it links fine if i'm linking to it from a C program.

I've seen some usage of 'extern "C"' as a wrapper around code for linking between the two languages but i couldn't get this to work either.

Any ideas?

[rogerborg]

jpeglib.h is indeed missing extern "C", but you should be able to simply wrap the include like this:

Code: Select all

extern "C"
{
#include <jpeglib.h>
}

Is that not working for you?

[JP]

No i initially tried that and it didn't work and then i tried putting it in jpeglib.h as well and that didn't help either.

Basically without extern "C" the linking errors i get are function_name(parameters) 'not found' but with the extern "C" added in it loses the parameters from the error and it's just the function name it complains about.

I'm using Visual Studio, is there possibly some compiler/project setting that might affect this?

[rogerborg]

Reaaally stupid question, but are you actually linking against the libjpeg library? I mean, in the configuration that you're building? I have a bad habit of adding libraries to the debug build and forgetting to add them to the release build.

[JP]

How dare you suggest i'd be so foolish!

*checks*

nope, all linked up properly as far as i can tell

Here's what happens without extern "C":
PS3_PPU_Release/JPEG.obj: In function `.Decompress_JPEG(jpeg_decompress_struct*, tImageJPG*)':
JPEG.cpp:(.text+0x24): undefined reference to `.jpeg_read_header(jpeg_decompress_struct*, unsigned char)'
....etc....
and with extern "C":
PS3_PPU_Release/JPEG.obj: In function `.Decompress_JPEG(jpeg_decompress_struct*, tImageJPG*)':
JPEG.cpp:(.text+0x24): undefined reference to `.jpeg_read_header'
....etc....

I even made sure that JPEG.cpp was indeed a C++ file and not a C file as that can cause problems like this.

[rogerborg]

Sorry, this Works For Me, using a libjpeg library swiped from ClanLib.

Code: Select all

#include <stdio.h> // Required by jpeglib.h

extern "C"
{
#include <C:/dev/jpeg-6b-msvc80/include/jpeglib.h>
}

#pragma comment(lib, "C:/dev/jpeg-6b-msvc80/lib/libjpeg-static-mt.lib")
#pragma comment(linker, "/NODEFAULTLIB:MSVCRTD.LIB")

int main(int argc, char* argv[])
{
	struct jpeg_decompress_struct decompressStruct;
	int result = jpeg_read_header(&decompressStruct, false);
	return result;
}

I think you need to sacrifice a chicken, possibly even a goat.

[JP]

Well i've snagged that lib aswell and i'll try it out with that one tomorrow. I had to build the lib myself so i could have made an error there (though it does work with C programs).

[JP]

No joy with that lib version either

Also tried out the pragma linking instead of in the project options but the compiler just boldly states that it's chosen to ignore the pragma comment...

[JP]

I'm gonna say that you can all stop worrying about this problem now and get a good nights sleep instead of lying awake fretting about me.

Turns out the PS3 SDK has jpg decoding support in it so i'm going to investigate this instead!

[rogerborg]

I NEED CLOSURE ON THIS ANECDOTE!

[FlyingIsFun1217]

If I remember my reading right, just append the letter c in front of the included header file. For example:

#include <cmath.h>

I've never run into this problem though, so I can't say whether or not it would work. Might also depend on your compiler

FlyingIsFun1217

[JP]

I think that only works for the standard c headers, such as math.h, as those are actually defined as cmath.h as well? I can't imagine changing the actual name of the header file wouldn't confuse the compiler.

As it turns out the PS3 implementation loads 16 jpg textures in 6.4 seconds whereas libjpeg did it in 8.4 seconds, so now i have a simpler method which is actually faster (due to the fact that it uses the SPUs, multi processors).

[hybrid]

<cmath> would be the correct way to include the math headers in C++. These are the versions which may take some advantage from being used only by C++ compilers such that templates and stuff can go into those headers. But external library headers are just as they are.
But Irrlicht also uses libjpeg and does just as rogerborg said. So for all those machines we are currently compiling on it works.