I am writing a plugin system which is separated other 3 modules:
plugin_system.c - the core of the system
list.c - contains a linked list implementation for plugins' storage
plugin_interface.h - contains the declaration needed by plugins, has no source file associated with
plugin_interface.h only contains only of types and the function:
extern int plugin_register(PluginManager *plug_manager, const char *name, Plugin *plug);
which is defined in plugin_system.c
When loading a plugin, the plugin system look for a funcion init_plugname() and call it, that function must call plugin_register to register the plugin.
The program is compiled with complex recursive Makefiles (not the best idea), but what I try to achieve is:
I compile the plugin system object in the main program folder, it is then linked with the main program. From make execution:
gcc -Wall -O2 -std=gnu99 -D DEBUG -g -fPIC -c -o /home/kowa/code/reseaux/projet/ringo/c/bin/list.o list.c
gcc -Wall -O2 -std=gnu99 -D DEBUG -g -fPIC -c -o /home/kowa/code/reseaux/projet/ringo/c/bin/plugin_system.o plugin_system.c
A plugin is compiled with gcc -fPIC -c -o plugname.o plugname.c plug_system.o followed by gcc -o plugname.so plugname.o plug_system.o -shared
I try to load the plugin in my main program and get this error:
symbol lookup error: ./plugins/zyva.so: undefined symbol: exists
exists is a function in the list module used by the plugin_system module to store plugins, the plugin_register function mentionned aboved calls it.
I've never done that kind of system before and I'm not an expert in shared library, I guess the problem is from how I compile the whole project, I may miss some linkage...
I just forgot to link the plugin with list.o which is used by the plugin_system.o...
Related
I found a useful library on github for my project, after building this later I tried to use some predefined function on it. I couldn't compile my project because there is some header file missing like this one :
In file included from main.c:2:0:
ptask.h:11:19: fatal error: ptime.h: No such file or directory
I compiled my project using this command :
gcc main.c -L. -lptask
This is all the files in project folder :
libptask.a main.c ptask.h
This is the library content:
$ ar -t libptask.a
pbarrier.c.o
pmutex.c.o
ptask.c.o
ptime.c.o
rtmode.c.o
tstat.c.o
libdl.c.o
dle_timer.c.o
calibrate.c.o
Do I need to add all the headers of this files or just link the lib when compiling ?
Your main.c #include-s ptask.h which in turn #include-s ptime.h. Having compiled static libs alone is not enough (that's the linker's job), you still need to have all used header files (which is the compiler's job), both the ones you use and their dependencies, recursively applicable.
Normally you need to be sure that the header files are in your "include path", something that a lot of compilers define with -I as a command-line option. You'll need to include the source directory of that library, or if it has a make install option, then the place where they got installed.
regarding:
gcc main.c -L. -lptask
this is performing the compile step and the link step in one command.
It is also not enabling the warnings, which should always be enabled during the compile step.
Suggest something similar to the following to compile
gcc -Wall -Wextra -Wconversion -pedantic -std=gnu11 -g -c main.c -o main.o -I.
and when you have fixed all the warnings, then use something similar to the following to link
gcc main.o -o main -L. -lptask
I am a Unix/Linux newbie who is trying to run a shell script written by a person who left no documentation and has since demised. This script contains line:
./search $opt1 $arg1 < $poly 2>&1 | tee $output
Which is trying to get the file $poly and call program ./search and divert the output to $output.
When I get to this line, I am given message: ./search: cannot execute binary file: Exec format error
search is a C program called from the script and is in the same folder as various other C programs to do with this project. Script and C programs were developed and originally executed on a Unix/Linux box which is no longer available, so I have been asked to try to resurrect this project but under Windows using gcc in NetBeans and cygwin.
The message : ./search: cannot execute binary file: Exec format error is most likely to do with the fact there is no executable file for search. When I try to build the C programs I get the following output:
C:\cygwin64\bin\make.exe -f Makefile
gcc -ansi -g -c cbuild.c
gcc -ansi -g -c complex.c
gcc -ansi -g -c mylib.c
gcc -ansi -g -c poly.c
gcc -ansi -g -c real.c
gcc -ansi -g -c zero.c
gcc -lgmp -lm -lrt -o cbuild cbuild.o complex.o mylib.o poly.o real.o zero.o
real.o: In function `rabs':
/cygdrive/c/../progs/real.c:9: undefined reference to `__imp___gmpf_abs'
/cygdrive/c/../progs/real.c:9:(.text+0x1e): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `__imp___gmpf_abs'
real.o: In function `radd':
I assume that R_X86_64_PC32 refers to the environment I am using. I am using a 64 bit version of Netbeans with gcc 5.4.0 in a 64 bit version of cygwin on Windows 10.
Can anyone advise what I must to to resolve this so that I can build the C programs?
The problem is this:
gcc -lgmp -lm -lrt -o cbuild cbuild.o complex.o mylib.o poly.o real.o zero.o
By default, the linker will link libraries and objects in the order specified on the command line, and, when linking a library, will only include symbols needed by things before it on the command line. Since -lgmp is first, there are (as yet) no outstanding symbols (except main), so nothing is included from the library. When later objects need the symbols from it, they won't see them.
Change the order to
gcc -o cbuild cbuild.o complex.o mylib.o poly.o real.o zero.o -lgmp -lm -lrt
and it should work. Alternately, use the -Wl,--as_needed linker option to get the linker to remember earlier libraries and relink them if more symbols from them are referenced by later object files (requires a recent version of the GNU linker -- I have no idea if it works with cygwin).
This kind of misordering is usually a symptom of a broken Makefile. The normal Makefile structure has a bunch of variables that are set to control the default rules that know how to compile source files and link object files. The two variables relevant for linking are LDFLAGS and LDLIBS, and the difference is that LDFLAGS comes before all the object files on the command line and LDLIBS comes after all the object files.
So in order to make things work, you need to ensure that all of the -l options and other libraries are in LDLIBS:
LDLIBS = -lgmp -lrt -lm
and NOT in LDFLAGS
Im creating a runtime loaded shared object in linux that impliments some JSON using jannson.h.
This is how I'm building the .so:
gcc -Wall -fPIC -c device_simulator_json.c
gcc -shared -Wl,-soname,device_simulator_json.so.1 -o device_simulator_json.so.1.0 device_simulator_json.o
When my application tries to load the .so, I get an error indication:
undefined symbol: json_object
I'm guessing that my .so has to include in it the jansson object. But I'm not sure which one or how. I can see the following jansson objects in my /usr/local/lib:
./usr/local/lib/libjansson.so.4.7.0
./usr/local/lib/libjansson.so.4
./usr/local/lib/libjansson.a
./usr/local/lib/libjansson.so
Because the .so I'm building is dynamically loaded at runtime (using dlopen), doesn't the jansson object I use also have to be built with PIC.
Thanks.
You probably should link libjansson.so inside your shared library:
gcc -L/usr/local/lib -shared -Wl,-soname,device_simulator_json.so.1 \
-o device_simulator_json.so.1.0 device_simulator_json.o -ljansson
Alternatively, you might link -ljansson into the main program (doing the dlopen). You'll better then link that program with -rdynamic
BTW, you probably should compile your shared object with all warnings and debug info:
gcc -Wall -Wextra -g -fPIC -c device_simulator_json.c
once all is debugged you might optimize with -O2
Im wondering why Im getting no undefined references when compiling as a shared library using -shared as an option for gcc. Consider the following case:
#include <confuse.h>
int
main(int argc, char **argv)
{
cfg_opt_t opts[1];
cfg_t *cfg = cfg_init(opts, CFGF_NOCASE);
return 0;
}
libconfuse is needed here in order to run the program properly. If im compiling it as a "normal" application without specifying that libconfuse is needed (-lconfuse) im getting the following (from my perspective regular) error:
$ gcc -Wall -Wno-unused-variable test.c -o test
/tmp/ccTVz6an.o: In function `main':
test.c:(.text+0x20): undefined reference to `cfg_init'
collect2: error: ld returned 1 exit status
If im compiling the same code as a shared library im not getting any error message regarding the library:
$ gcc -Wall -Wno-unused-variable test.c -o test.so -shared
$ echo $?
0
Can someone please bring light into darkness?
You are compiling a (shared) library, not a program, and libraries are expected not to be complete, so undefined references do not prevent the shared library from building.
Yes, it may sound a bit surprising, particularly if you come from a OS (Windows?) where the shared libraries are always fully linked, but that's how ELF works.
If you want to ensure that all your references are are resolved when building a shared library you can use the linker option --no-undefined:
gcc -Wall -Wno-unused-variable test.c -o test.so -shared -Wl,--no-undefined
Note that libraries used to resolve references when linking a shared library will be added to the header of that library, as NEEDED entries, and then linked automatically when using that library. See objdump -p for details.
With shared libraries (not static ones), you could link a library libA.so into your other library libB.so.
So you could compile your shared library as:
gcc -Wall -Wno-unused-variable -fPIC test.c \
-o libmytest.so -shared -lconfuse
Later you would link that library to some main.c with
gcc -Wall main.c -lmytest -L. -o myprog
or you could load it at runtime, using dlopen(3) on ./libmytest.so (read the man page about why ./ is significant to dlopen)
Simple explanations are in the Program Library HowTo. But Drepper's paper: How to Write Shared Libraries is the best reference.
And it is permitted for a shared library (or even a static one) to have undefined references, since quite often the user of that library would explicitly link the lower level libraries needed by it.
Here is the error:
symbol lookup error: ./src/libprog3.so: undefined symbol: pow
Everything else in the library works fine, even functions that use math.h, but when I call the function that needs pow(), it crashes. It worked perfectly fine when it was a shared library. I am using gcc and a makefile to compile. The only change I made to the makefile was adding -ldl for the library. I still have -lm in it. I changed the driver program to support the change to the dll instead of the shared library, but the function causing the error hasn't changed.
Is there more to the makefile that I need to change?
Here is the makefile:
testlib: src/driver.o src/prog3.o
gcc -fPIC -Wall -c src/prog3.c -o src/prog3.o
gcc -Wall -shared -o src/libprog3.so src/prog3.o
gcc -Wall -o $# src/driver.c -ldl -lm -Lsrc -lprog3
You need to be doing:
gcc -Wall -shared -o src/libprog3.so src/prog3.o -lm
That is, shared libraries must be built with their dependencies. If you omit the -lm, you're telling the linker that unresolved symbols in libprog3.so are intended to be resolved by symbols in the main program or libraries it has already caused to be loaded. I see you did put -lm on the link command line for the main program, but my guess is that the main program did not actually use any symbols from libm.so and thus it did not get added to the DT_NEEDED table for the main program and therefore is not already loaded.
In any case, making dependencies explicit like this is almost always best.