Undefined reference to XXX problem cause

This is a creation in Article, where the information may have evolved or changed.

Original address: http://blog.csdn.net/cserchen/article/details/5503556

Linux under the compiler program, often encounter "undefined reference to XXX" error,

Here is a summary of some possible causes and solutions for the needy friends:

Said undefined reference error, first mention the Linux GCC link rules:

The search order when linking is:

Path specified by-L, looking from left to right
The path specified by the environment variable Library_path, using the ":" Split to find from left to right
/ETC/LD.SO.CONF the specified path order
/lib and/usr/lib (64-bit is/lib64 and/usr/lib64)

Lookup order for Dynamic library calls:

The-rpath parameter of the LD specifies the path, which is written dead in the code
The path specified by the LD script
Ld_library_path the specified path
/ETC/LD.SO.CONF the specified path
/lib and/usr/lib (64-bit is/lib64 and/usr/lib64)

General Link when we use the-l to specify the lookup path, call the dynamic link library when the Ld_library_path way to specify the link path.

Another concern is that as soon as the first one is found, it will be returned, and the latter will no longer be looked up. For example-L./A-L./B-LX in a libx.a B has libx.a and libx.so, this time will use in./a libx.a instead of following the principle of dynamic library precedence, because./A is found first, and there is no dynamic inventory with the same name

For dynamic-link libraries, the actual symbolic positioning is performed at run time. When compiling. So, if the library that it needs is not associated with him, for example libx.so needs to use Uldict, But forget in the compile libx.so when add-luldict words, in compiling libx.so time will not error, because this time libx.so is considered a library, it has some do not know the specific implementation of the symbol is legal, it can be specified during the run time or compile another binary program.

If you are using g++-LPATH-LX to compile, the linker will find that the required uldict symbol table can not be found to error, but if the program is loaded in Dlopen way, due to the operation period, the program in this place directly run the error. Another situation is that an external interface has been declared defined in the dynamic library, but it has been forgotten, and this time a similar error occurs.

If you report such errors at run time, be aware of the fact that some libraries are not linked in or that some interfaces are not implemented.

With the above foundation, it is not difficult to see that the reason for undefined reference error errors is:

0. The wrong name is very clear, but the definition code of the method you called is not found. Starting from here, it should be a clue. If this is still not possible, please refer to the next few.

1. The corresponding library (. o/.a/.so) is not specified using the entity defined in the library, but no library (-LXXX) is specified, or the library path (-LYYY) is not specified, which causes the error,
2. The order of the connection library parameters is not in the default case, for-l The requirement to use the library is that the more basic the library is to write in the back, whether static or dynamic
3. The GCC/LD version does not match the compatibility issue of the GCC/LD version, because of a problem with the compatibility of the GCC2 to gcc3 large version (actually gcc3.2 to 3.4 is somewhat problematic) This error can be caused by using a lower version of the machine on a higher version of the machine, which is more common in 32-bit environments, with the 32-bit environment accidentally using a 64-bit library or the reverse 64-bit environment using a 32-bit library.
4. C + + interdependence and linking GCC and g++ compilation results are required to ensure that the interface can be used on both sides of the extern "C", in our 64-bit environment, the GCC link g++ Library also needs to add-lstdc++, see the previous article for the description of mixed compilation
5. Run-time error This problem is basically due to the fact that the program uses the Dlopen mode to load. So, but. So does not link all the required libraries, specifically to the instructions for mixing static and dynamic libraries in the previous section