I'm trying to link some VTK libraries in my program...and It is not working like this
-ldl /usr/lib/libvtkIO.so, which is the way CMake does. But If I compile it manually using -L -lvtkIO, it works. What is the difference ?
Is dynamic versus static linking?
Thanks
CMake tries to use the full path to the library, rather than letting the linker search the library path(s). If you use ldd on the resulting binaries you will see they are linked to the same thing (dynamically in both cases). For things like VTK it is generally best to use CMake as it also tracks things like interface libraries for you.
I think you have misunderstood some of the arguments to the linker, -lvtkIO would link to a library in the library path called libvtkIO.so, -ldl would link to a library called libdl.so. The -L argument is used to add additional paths the linker should search for libraries.
Related
I am trying to compile OpenVDB, but I get a linker error telling me:
"cannot find -ldl"
That is the only linker I am getting. I have no idea what library -ldl belongs to. The makefile doesn't help either, so I'm guessing it is a standard lib. I am using Mingw-w64 on windows.
The -ldl is a linker option to link to libdl library. This library is used to perform dynamic library loading (.dll in Window's world) through dlopen, dlsym... functions.
Since this library is not available on Windows, I think you could remove the -ldl from your makefile.
Since Window's equivatent functions are accessible by kernel.lib, you do not need to add specific instruction in makefile.
I am a regular C programmer, and there's something I have wondered for some time about GNU GCC/LD internals.
I have noticed that, when passing a sequence of object files to GCC (e.g gcc main.o otherfile.o), GCC automatically links libc.a library file without my explicitly specifying -lc in the options. Similarly, when I compile a program using ncurses, I just need to specify -lncurses and libtinfo.a gets linked automatically (no need to specify -ltinfo). In other words, even though ncurses functions use functions from libtinfo (for instance, unctrl()), I don't need to explicitly link libtinfo.
How can it be possible?
Does GCC/LD have a list of "default libraries" where it looks for missing symbols when linking? If such a table exists, where is it and how can it be configured?
A direct answer is that you can see what libraries are linked by command line options by adding -v to the linking command line. This will show you the commands as they are executed.
The C library and the GCC support library or libraries are linked automatically; other libraries have to be specified manually.
The case of -lncurses and libtinfo.a (libtinfo.so?) is rather different. There, the link command used to build libncurses.so tells the linker that this library also needs -ltinfo, so it automatically picks up the extra library.
As I know, if I want to use pthread library in linux environment I must include pthread.h and compile the source code with -lpthread option.
But I don't understand why I should compile with -lpthread option. I think the option is redundant... because I already declared to include pthread.h header file so that gcc links pthread library. Why does gcc not link pthread library file automatically by reading #include?
Thanks in advance.
Well linking and compilation are two separate phases.
You include the header pthread.h so that the compiler understands the data types & symbol names, which you use in your source files but are defined/declared in the pthread library header file.
You link to the pthread libray using -lpthread so that the linker can actually find those symbols in the pthread library during the linking stage.
Having #include <pthread.h> in your code doesn't link in the library; it only includes the header for compilation. That allows the compiler to see the various structures, function declarations, etc. included. Having -lpthread actually causes the linking to be done by the linker. So the include tells the compiler what's available, and the -lpthread actually allows the program to call the functions within the library at runtime.
Because GCC doesn't do auto-linking of libraries triggered by header inclusion (as opposed to MSVC, or so I've been told).
The header file just declares what the pthread functions are and how they should be called. -lpthread links to the library itself, containing the actual functions.
The compiler has no idea how you're going to resolve the functions in pthread.h. You might want to use a static library, the one provided by the system, some compatible implementation - heck, you might implement them yourself in another source file. It's up to the linker, and doesn't concern the compiler.
By including the header files you tell the compiler which functions he's going to see. But if these functions are in an external library, like the pthread functions, you need to link this library to your program so it can actually access those functions. That's what -lpthread is doing.
Pthread.h header file is included in the posix thread program but you need
-lpthread while compiling because it links it with the library of pthread
NOTE: -lpthread -lpcap all are the switches with gcc compiler that can link particular library in our source code. (lpthread means "link pthread" library)
I am trying to link a C++ module using GCC, essentially like this:
gcc -c hello.c
g++ -c world.cpp
gcc -ohello -lstdc++ hello.o world.o
Note that I use -lstdc++ to link the C++ module in, so that I can use gcc instead of g++. The problem is that I'm getting the error:
undefined reference to `operator new(unsigned long)'
(Assuming that world.cpp contains at least one call to new.)
This error is fixed if I put -lstdc++ at the end of the linker line, like this:
gcc -ohello hello.o world.o -lstdc++
I am aware that this question has been asked many times here, but I have a special requirement. I am not directly calling GCC. I am using a build system for a different programming language (Mercury) which is calling GCC on my behalf, and I can't easily modify the way it calls GCC (though I can specify additional libraries using the LDFLAGS environment variable). So I have two additional requirements:
I cannot use g++ to link (only gcc) -- that is why I am doing the -lstdc++ trick above rather than simply linking with g++).
I don't think that I can control the order of the linker commands -- Mercury will put the .o files on the command-line after any libraries.
I understand the basic reason why the order is important, but what is baffling me is why did this break now? I just updated to Ubuntu 11.10 / GCC 4.6.1. I have been successfully compiling this program for years using precisely the above technique (putting -lstdc++ first). Only now has this error come up. An unrelated program of mine links against OpenGL using -lgl and that too broke when I upgraded and I had to move -lgl to the end of the command-line. I'm probably going to discover that dozens of my programs no longer compile. Why did this change? Is there something wrong with my new system or is that the way it is now? Note that these are ordinary shared libraries, not statically linked.
Is there anything I can do to make GCC go back to the old way, where the order of libraries doesn't matter? Is there any other way I can convince GCC to link libstdc++ properly without moving it after the .o files on the command-line?
If Mercury puts object files after libraries, Mercury is broken. Libraries belong after object files - always. You may sometimes get away with the reverse order, but not reliably. (Static libraries must go after the object files that reference symbols in the static library. Sometimes, a linker will note the symbols defined by a shared library even when none of the symbols are used; sometimes, the linker will only note the shared library symbols if the shared library provides at least one symbol.)
I'm having a problem with my compiler telling me there is an 'undefined reference to' a function I want to use in a library. Let me share some info on the problem:
I'm cross compiling with gcc for C.
I am calling a library function which is accessed through an included header which includes another header, which contains the prototype.
I have included the headers directory using -I and i'm sure it's being found.
I'm first creating the .o files then linking them in a separate command.
So my thought is it might be the order in which I include the library files, but i'm not sure what is the correct way to order them. I tried with including the headers folder both before and after the .o file.
Some suggests would be great, and maybe and explanation of how the linker does its thing.
Thanks!
Response to answers
there is no .a library file, just .h and .c in the library, so -l isn't appropriate
my understanding of a library file is that it is just a collection of header and source files, but maybe it's a collection of .o files created from the source?!
there is no library object file being created, maybe there should be?? Yes seems I don't understand the difference between includes and libraries...i'll work on that :-)
Thanks for all the responses! I learned a lot about libraries. I'd like to put all the responses as the accepted answer :-)
Headers provide function declarations and function definitions. To allow the linker find the function's implementation (and get rid of the undefined reference) you need to ask the compiler driver (gcc) to link the specific library where the function resides using the -l flag. For instance, -lm will link the math library. A function's manual page typically specifies what library, if any, must be specified to find the function.
If the linker can't find a specified library you can add a library search path using the -L switch (for example, -L/usr/local/lib). You can also permanently affect the library path through the LIBRARY_PATH environment variable.
Here are some additional details to help you debug your problem. By convention the names of library files are prefixed with lib and (in their static form) have a .a extension. Thus, the statically linked version of the system's default math library (the one you link with -lm) typically resides in /usr/lib/libm.a. To see what symbols a given library defines you can run nm --defined-only on the library file. On my system, running the command on libm.a gives me output like the following.
e_atan2.o:
00000000 T atan2
e_asinf.o:
00000000 T asinf
e_asin.o:
00000000 T asin
To see the library path that your compiler uses and which libraries it loads by default you can invoke gcc with the -v option. Again on my system this gives the following output.
GNU assembler version 2.15 [FreeBSD] 2004-05-23 (i386-obrien-freebsd)
using BFD version 2.15 [FreeBSD] 2004-05-23
/usr/bin/ld -V -dynamic-linker /libexec/ld-elf.so.1 /usr/lib/crt1.o
/usr/lib/crti.o /usr/lib/crtbegin.o -L/usr/lib /var/tmp//ccIxJczl.o -lgcc -lc
-lgcc /usr/lib/crtend.o /usr/lib/crtn.o
It sounds like you are not compiling the .c file in the library to produce a .o file. The linker would look for the prototype's implementation in the .o file produced by compiling the library
Does your build process compile the library .c file?
Why do you call it a "library" if it's actually just source code?
I fear you mixed the library and header concepts.
Let's say you have a library libmylib.a that contains the function myfunc() and a corresponding header mylib.h that defines its prototype. In your source file myapp.c you include the header, either directly or including another header that includes it. For example:
/* myapp.h
** Here I will include and define my stuff
*/
...
#include "mylib.h"
...
your source file looks like:
/* myapp.c
** Here is my real code
*/
...
#include "myapp.h"
...
/* Here I can use the function */
myfunc(3,"XYZ");
Now you can compile it to obtain myapp.o:
gcc -c -I../mylib/includes myapp.c
Note that the -I just tells gcc where the headers files are, they have nothing to do with the library itself!
Now you can link your application with the real library:
gcc -o myapp -L../mylib/libs myapp.o -lmylib
Note that the -L switch tells gcc where the library is, and the -l tells it to link your code to the library.
If you don't do this last step, you may encounter the problem you described.
There might be other more complex cases but from your question, I hope this would be enough to solve your problem.
Post your makefile, and the library function you are trying to call. Even simple gcc makefiles usually have a line like this:
LIBFLAGS =-lc -lpthread -lrt -lstdc++ -lShared -L../shared
In this case, it means link the standard C library, among others
I guess you have to add the path where the linker can find the libraray. In gcc/ld you can do this with -L and libraray with -l.
-Ldir, --library-path=dir
Search directory dir before standard
search directories (this option must
precede the -l option that searches
that directory).
-larch, --library=archive
Include the archive file arch in the
list of files to link.
Response to answers - there is no .a library file, just .h and .c in the library, so -l isn't approriate
Then you may have to create the libraray first?
gcc -c mylib.c -o mylib.o
ar rcs libmylib.a mylib.o
I have encountered this problem when building a program with a new version of gcc. The problem was fixed by calling gcc with the -std=gnu89 option. Apparently this was due to inline function declarations. I have found this solution at https://gcc.gnu.org/gcc-5/porting_to.html