I have the following makefile
CXXFILES = pthreads.cpp
CXXFLAGS = -O3 -o prog -rdynamic -D_GNU_SOURCE -L./libmine
LIBS = -lpthread -ldl
all:
$(CXX) $(CXXFILES) $(LIBS) $(CXXFLAGS)
clean:
rm -f prog *.o
I am trying to include the ./libmine library within CXXFLAGS, but it seems like it is not the right way to include a static library, because when I compile the program, I get many undefined references error. So what is actually the right way to include a static library in the makefile?
use
LDFLAGS= -L<Directory where the library resides> -l<library name>
Like :
LDFLAGS = -L. -lmine
for ensuring static compilation you can also add
LDFLAGS = -static
Or you can just get rid of the whole library searching, and link with with it directly.
Say you have main.c, fun.c and a static library libmine.a.
Then you can just do in your final link line of the Makefile
$(CC) $(CFLAGS) main.o fun.o libmine.a
CXXFLAGS = -O3 -o prog -rdynamic -D_GNU_SOURCE -L./libmine
LIBS = libmine.a -lpthread
Make sure that the -L option appears ahead of the -l option; the order of options in linker command lines does matter, especially with static libraries. The -L option specifies a directory to be searched for libraries (static or shared). The -lname option specifies a library which is with libmine.a (static) or libmine.so (shared on most variants of Unix, but Mac OS X uses .dylib and HP-UX used to use .sl). Conventionally, a static library will be in a file libmine.a. This is convention, not mandatory, but if the name is not in the libmine.a format, you cannot use the -lmine notation to find it; you must list it explicitly on the compiler (linker) command line.
The -L./libmine option says "there is a sub-directory called libmine which can be searched to find libraries". I can see three possibilities:
You have such a sub-directory containing libmine.a, in which case you also need to add -lmine to the linker line (after the object files that reference the library).
You have a file libmine that is a static archive, in which case you simply list it as a file ./libmine with no -L in front.
You have a file libmine.a in the current directory that you want to pick up. You can either write ./libmine.a or -L . -lmine and both should find the library.
The -L merely gives the path where to find the .a or .so file. What you're looking for is to add -lmine to the LIBS variable.
Make that -static -lmine to force it to pick the static library (in case both static and dynamic library exist).
Addition: Suppose the path to the file has been conveyed to the linker (or compiler driver) via -L you can also specifically tell it to link libfoo.a by giving -l:libfoo.a. Note that in this case the name includes the conventional lib-prefix. You can also give a full path this way. Sometimes this is the better method to "guide" the linker to the right location.
Related
I am trying to understand static libraries and shared objects in C. I am trying to understand whether one type of library can depend on other type.
Consider a scenario:
libA.so has a function foo_A_dyn():
libA.so ---> foo_A_dyn()
foo_A_dyn() uses a function foo_B_static() which is defined in libB.a which is a static library.
libB.a ---> foo_B_static()
I have built my libraries in the following way:
gcc -c foo_B.c -o foo_B.o
ar -cvq libB.a foo_B.o
gcc -fPIC -c foo_A.c -o foo_A.o
gcc -shared libA.so foo_A.o -I.
gcc main.c -lA -lB -L. -I. -o EXE
Note: main.c makes call to foo_A_dyn() and does NOT call foo_B_static() directly.
And now when I am trying to build my executable EXE, I am getting the error "undefined reference to foo_B_static".
I think the error seems genuine but I am not able to decode the rationale behind this and put it to words.
Can someone please help?
From gcc link options:
-llibrary
-l library
...
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file foo.o but before bar.o. If bar.o refers to functions in ‘z’, those functions may not be loaded.
Try:
gcc main.c -lB -lA -L. -I. -o EXE
Here's what the linker is doing. When we link our executable ('EXE' above) it has some symbols (functions and other things) that are unresolved. It will look down the list of libraries that follow in sequential order, trying to resolve unresolved symbols. Along the way, it finds that some of the symbols are provided by libB.so, so it notes that they are now resolved by this library. While going through libB.so it finds some symbols which are unresolved and it tries to resolve them by looking up the library that follows.
When we are ordering the libraries like:
gcc main.c -lA -lB -L. -I. -o EXE
Linker is not able to lookup for the definition of symbols used in libB into libA. Reason could be that backward reference is not available.
I have also figured out that:
shared object can depend on a static archive,
a static archive can depend on a shared object, and
one static archive can depend on another static archive
Please let me know if I have erred somewhere.
I have a program and a static library:
// main.cpp
int main() {}
// mylib.cpp
#include <iostream>
struct S {
S() { std::cout << "Hello World\n";}
};
S s;
I want to link the static library (libmylib.a) to the program object (main.o), although the latter does not use any symbol of the former directly.
The following commands do not seem to the job with g++ 4.7. They will run without any errors or warnings, but apparently libmylib.a will not be linked:
g++ -o program main.o -Wl,--no-as-needed /path/to/libmylib.a
or
g++ -o program main.o -L/path/to/ -Wl,--no-as-needed -lmylib
Do you have any better ideas?
Use --whole-archive linker option.
Libraries that come after it in the command line will not have unreferenced symbols discarded. You can resume normal linking behaviour by adding --no-whole-archive after these libraries.
In your example, the command will be:
g++ -o program main.o -Wl,--whole-archive /path/to/libmylib.a
In general, it will be:
g++ -o program main.o \
-Wl,--whole-archive -lmylib \
-Wl,--no-whole-archive -llib1 -llib2
The original suggestion was "close":
How to force gcc to link unreferenced, static C++ objects from a library
Try this: -Wl,--whole-archive -lyourlib
I like the other answers better, but here is another "solution".
Use the ar command to extract all the .o files from the archive.
cd mylib ; ar x /path/to/libmylib.a
Then add all those .o files to the linker command
g++ -o program main.o mylib/*.o
If there is a specific function in the static library that is stripped by the linker as unused, but you really need it (one common example is JNI_OnLoad() function), you can force the linker to keep it (and naturally, all code that is called from this function). Add -u JNI_OnLoad to your link command.
I'm trying to compile my C code but I need to tell the GCC compiler where two file are.
The two files are located here
/usr/local/ssl/include/
/usr/local/ssl/lib/
I want to add this to my gcc -o file file.c so that my program can work.
In gcc, the -I option is used for adding a directory to the set of directories to search for header files, and the -L option is used for adding a directory to the set of directories to search for libraries. Since you're not explicitly linking in any libraries, you shouldn't need the -L option in this case.
gcc -I/usr/local/ssl/include -o file file.c
If you were linking in libraries, something like the following format should work, assuming that file.c calls a function in libmyLib.a:
gcc -I/usr/local/ssl/include -o file file.c -L/path/to/my/library -lmyLib
See this question for more details regarding library linking order.
I am interested in including C++ language in my c based program. I have two systems (Host and Target) which target system is limited, especially in memory. So, in my host machine, I have to statically link some libraries which will be needed by the program on target side. However, in my makefile I have been manipulating gcc patch (using a variety of gcc flags, during both compiling and linking steps).
Everything is ok until using C++ language;
For example, By including iostream header file in .cpp file, there exist an error on the target system showing that : "can not resolve symbol 'wctob' "
I must indicate my makefile as bellow:
in the compiling step:
$(CXX) -O0 -I<Headers dir> $(LIBSC) -Wno-write-strings -Wno-narrowing -Wno-return-type -Wno-abi -Wno-unused-variable -DNDEBUG -Wa, -c -fmessage-length=0 -I<Headers dir> -march=4kec -EL -o "$#" "$<"
in the linking step:
$(CXX) -lpthread -Wl,-Map,output.map $(MyFlags) $(LIBS) $(STRIP) -muclibc -march=4kec -EL -o
"output.elf" $(OBJS) $(LIBS) $(MyFlags) -Wl,-rpath -Wl,<LIBDIR> -L<LIBDIR>
------some definitions
MyFlags = -lc -static-libstdc++ -static-libgcc;(such flags used in the linking step)
LIBDIR = is the address of needed libraries
LIBSC = -nostdinc++ ;(such flags used in the compiling step)
Headers dir = is the address of needed both c and c++ header files included by the programmer.
LIBS and OBJS are also the list of needed libraries and objects.
Also, I added LIBDIR to ld.so.conf and LD_LIBRARY_PATH, but the problem is not solved far away. (My g++ version is 4.7.3.)
Can anybody help me out?
You have to tell C++ compiler to use C naming and symbols. so :
extern "C" {
// your cpp codes
}
I am working on embedded software for a ARM microcontroller (SAM7) and using Yagarto toolchain.
My code currently links libc.a. However I'd like to use a custom implementation of the builtin function memcpy that my code already has.
I have tried using -fno-builtin and/or -fno-builtin-memcpy as specified in the GCC Manual but the linker still complains will the following warning:
contiki-crazy-horse.a(flashd_efc.o): In function `memcpy':
C:\Users\Melvin\GitRepo\projects\Amatis_Project\SAM7_Contiki\examples\er-rest-example/../../cpu/arm//at91sam7s-x/./flashd_efc.c:669: multiple definition of `memcpy'
c:/toolchains/yagarto/bin/../lib/gcc/arm-none-eabi/4.6.2/../../../../arm-none-eabi/lib\libc.a(lib_a-memcpy.o):C:\msys\1.0\home\yagarto\newlib-build\arm-none-eabi\newlib\libc\string/../../../../../newlib-1.19.0/newlib/libc/string/memcpy.c:78: first defined here
collect2: ld returned 1 exit status
make: *** [rest-server-example-nosyms.crazy-horse] Error 1
../../cpu/arm/at91sam7s-x/Makefile.at91sam7s-x:181: recipe for target `rest-server-example-nosyms.crazy-horse' failed
What is the correct way to use custom implementations of certain gcc built-in functions?
Edit 1: Adding the linking command I am using. In the code below Porject.a is an archive file created with all the project's object files.
CC = arm-none-eabi-gcc
CFLAGSNO = -I. -I$(CONTIKI)/core -I$(CONTIKI_CPU) -I$(CONTIKI_CPU)/loader \
-I$(CONTIKI_CPU)/dbg-io \
-I$(CONTIKI)/platform/$(TARGET) \
${addprefix -I,$(APPDIRS)} \
-DWITH_UIP -DWITH_ASCII -DMCK=$(MCK) \
-Wall $(ARCH_FLAGS) -g -D SUBTARGET=$(SUBTARGET)
CFLAGS += $(CFLAGSNO) -O -DRUN_AS_SYSTEM -DROM_RUN -ffunction-sections
LDFLAGS += -L $(CONTIKI_CPU) --verbose -T $(LINKERSCRIPT) -nostartfiles -Wl,-Map,$(TARGET).map
$(CC) $(LDFLAGS) $(CFLAGS) -nostartfiles -o project.elf -lc Project.a
If it is finding memcpy() in libc.a, then it is not conflicting with any "built-in", but rather with the newlib implementation. You may need also to specify -nostdlibs option and explicitly link libc.a and libm.a as necessary.
Object (.o) files are linked before library archives (.a) files are searched, so if a symbol is resolved by an object file, it will not be searched for in the archives. If you place your overrides in an static-link library, then you simply list it ahead of the standard library (or any other libraries that use the standard library) on the linker command line.
[Added] The following was originally a "comment" but should probably be in the answer; it is in response to "Edit 1" in the question, and the comment below about link order:
Change -nostartfiles -o project.elf -lc Project.a to -nostdlib -o project.elf -start-group Project.a -lc -end-group. The switch -nostdlib disables default linking of both start-up files (i.e. -nostartfiles) and standard libraries. The library grouping causes the libraries in the group to be searched iteratively until no further symbols can be resolved, allowing out-of-order and circular dependencies like yours to be resolved. An alternative form for the grouping switches is -( Project.a -lc -).