I try to write simple mongo c client. Source file (a.c):
#include <stdio.h>
#define MONGO_HAVE_STDINT
#include <mongo.h>
void mongo_init_c(mongo *con)
{
mongo_init(con);
}
int main() {
return 0;
}
And i try to compile it with:
gcc -I/usr/local/include -L/usr/local/lib -lmongoc a.c
But get an error:
a.c:(.text+0xd): undefined reference to `mongo_init'
Files /usr/local/include/mongo.h and /usr/local/lib/libmongoc.so exists
How can I correctly compile a.c?
p.s. mongo-2.0.4, gcc-4.6, mongo-c-driver - pulled from github
update
$ nm /usr/local/lib/libmongoc.so | grep init
000034e0 T _init
0000dd10 T bson_init
0000c740 T bson_init_data
0000c7b0 T bson_init_finished_data
0000dc10 T bson_init_size
0000d060 T bson_iterator_init
0000a5e0 T gridfile_init
00009af0 T gridfile_writer_init
000095e0 T gridfs_init
00010a18 R initialBufferSize
00005f40 T mongo_cursor_init
00008da0 T mongo_env_sock_init
00005d90 T mongo_init
000057b0 T mongo_init_sockets
00004800 T mongo_md5_init
00005e40 T mongo_replica_set_init
00005f00 T mongo_replset_init
00005b80 T mongo_write_concern_init
$ gcc -I/usr/local/include -L/usr/local/lib -Wall -Werror -lmongoc a.c
/tmp/cccuNEp1.o: In function `mongo_init_c':
a.c:(.text+0xd): undefined reference to `mongo_init'
Try linking the library after the source file, like gcc a.c -lmongoc. This is because you're using a traditional single-pass linker, which expects to satisfy dependencies with subsequent, not previous, objects specified on the command line.
Related
I have a statically linked library, containing a global variable barvar. I can compile the library with no problems with either gcc-10 or clang (this is on macOS Catalina). Interestingly, the behavior differs between the two when I try to link it into a program that uses the library. Here's the code:
In globvars.h, int barvar is declared:
#ifndef H_GLOBVARS_H
#define H_GLOBVARS_H
extern int barvar;
#endif
In globvars.c, int barvar is defined:
#include "globvars.h"
int barvar;
In foo.c, the function foo sets and prints barvar:
#include <stdio.h>
#include "globvars.h"
void foo()
{
barvar = 10;
printf("barvar is: %d\n", barvar);
return;
}
Here's test.c, the program that uses the library:
void foo();
int main(int argc, char **argv)
{
foo();
return 0;
}
When I compile and link with gcc-10, no problems:
gcc-10 -c foo.c -o foo.o
gcc-10 -c globvars.c -o globvars.o
gcc-10 -c test.c -o test.o
gcc-ar-10 rcs liblinktest.a foo.o globvars.o
gcc -o testlinkrun test2.o -L. -llinktest
When I compile and link with clang, I get an undefined symbol error at the last step:
cc -c foo.c -o foo.o
cc -c globvars.c -o globvars.o
cc -c test.c -o test.o
ar rcs liblinktest.a foo.o globvars.o
cc -o testlinkrun test2.o -L. -llinktest
with error:
Undefined symbols for architecture x86_64:
"_barvar", referenced from:
_foo in liblinktest.a(foo.o)
Any ideas? Interestingly, it appears the only step that has to be done with gcc-10 is compiling globvars.c. I can use clang and the clang linker for all other steps, and everything is fine. Is it possible that clang is optimizing away all the variables in globvars.c? How can I prevent this?
As #EricPostpischil observed in this comment, the issue is that clang defaults to treating barvar as a common symbol. Either changing int barvar; to int barvar = 0;, or compiling with -fno-common, fix the issue.
Beginning with gcc-10, gcc's default behavior is -fno-common instead of -fcommon.
Why is my following code not throwing duplicate symbol conflict?
I referred to name mangling, but that seems only when there is difference in parameters. But, here there is not difference in parameters. But, still it does not throw conflict. Why?
good.c
#include <stdio.h>
void printGood() {
printf("I am good");
}
perfect.c
#include <stdio.h>
void printGood() {
printf("I am perfect");
}
A.c
extern void printGood();
void bringGood() {
printGood();
}
B.c
extern void printGood();
void bringPerfect() {
printGood();
}
orchestrator.c
#include <stdio.h>
void bringGood();
void bringPerfect();
int main() {
printf("bringing good");
bringGood();
printf("bringing perfect");
bringPerfect();
return 1;
}
compile line:
gcc -g -c good.c
gcc -g -c perfect.c
gcc -g -c A.c
gcc -g -c B.c
gcc -g -c orchestrator.c
ar rcs libA.a perfect.o A.o
ar rcs libB.a good.o B.o
gcc -o orchestrator orchestrator.o -L. -lA -lB
Why is my following code not throwing duplicate symbol conflict?
The linker looks for undefined symbols in the libraries in the order in which they are specified in the linker line. When it finds a symbol in a library, it uses that definition and stops. It does not check whether that symbol is defined in any of the other libraries specified in the linker line.
In your case, if the linker finds a symbol in A.lib, it stops there. It does not look for the symbol in B.lib.
With your commands, the linker will find function printGood() in object perfect.o in library A. It will not use the function of the same name in good.o from library B. So you effectively link orchestrator.o, A.o, B.o and perfect.o. That's why the executable program prints I am perfect twice and not I am good.
Multiple definition errors are reported only when the object files used in the linker line contain multiple definitions.
You will see the error if you use:
gcc -o orchestrator orchestrator.o a.o b.o perfect.o good.o
I create a .so file with the code below, but when I compile a file that invokes functions in the .so file with GCC, I get an "undefined reference to 'outlib1'" error.
What's wrong with my code or my command? Thanks.
OS Ubuntu 11.10
gcc 4.6.1
//file name outscreen.c
#include <stdio.h>
void outlib1(void)
{
printf("out screen func1\n");
}
//file name main.c
int main(int argc, char* argv[])
{
outlib1();
}
gcc outscreen.c -fPIC -shared -o outscreen.so
gcc main.c -L. -loutscreen -o call
./call
Try:
$ gcc outscreen.c -fPIC -shared -o liboutscreen.so
$ gcc main.c -L. -loutscreen -o call
(note the change to the first line - the second line is unchanged)
What is the output of this?
nm outscreen.so | grep outlib1
Perhaps it is exporting with an underscore.
In libname.h:
int add_libname(int, int);
In libname.c:
#include "libname.h"
int add_libname(int a, int b)
{
return a+b;
}
I can build the shared library this way:
gcc -shared -fPIC libname.c -o libname.so
But I can't use it in another programe test.c:
#include <stdio.h>
#include "libname.h"
int main(int argc, char* argv[])
{
printf("%d\n", add_libname(1,5));
}
Reporting undefined reference to add_libname when I try to build it..
What's wrong here?
Because add_libname takes (int, int) you're giving it (1+5 = 6) or just (int)
I think you meant
add_libname(1, 5);
Also to compile it correctly you must use gcc like so
gcc -o myapp test.c -L. -lname
the lib part of libname is ignored as it is implicit
To create a shared library use these
gcc -fPIC -c libname.c
it gives warning: position independent code and libname.o file is generated.
and now type these command,
gcc -shared libname.so libname.o
libname.so ( the shared library is created with .so extension). To use the shared library
gcc -I/give the path of libname.h sourcefile.c /give the path of your .so file
example if your c file is file.c and the header file libname.h is in c:\folder1\project and your libname.so (shared library) is in c:\folder\project2
then
gcc -I/cygdrive/c/folder1/project file.c /cygdrive/c/folder/project/libname.so
this is the gcc command to be used while using the shared library.
Thank you.
How can I include foo() function of foo.c in this small program (sorry for my noob question):
In my foo.h file:
/* foo.h */
#include <stdio.h>
#include <stdlib.h>
int foo(double largeur);
In foo.c:
/* foo.c */
#include <stdio.h>
#include <stdlib.h>
#include "foo.h"
int foo(double largeur)
{
printf("foo");
return 0;
}
And in main.c:
/* main.c */
#include <stdio.h>
#include <stdlib.h>
#include "foo.h"
int main(int argc, char *argv[])
{
printf("Avant...");
foo(2);
printf("Apres...");
return 0;
}
After compiling:
$ gcc -Wall -o main main.c
I get this error:
Undefined symbols: "_foo",
referenced from:
_main in ccerSyBF.o ld: symbol(s) not found collect2: ld
returned 1 exit status
Thanks for any help.
$ gcc -Wall -o main main.c foo.c
GCC doesn't know to look for foo.c if you don't tell it to :)
Creating a program in C requires two steps, compiling and linking. To just run the compiling part, use the -c option to gcc:
gcc -c main.c
This creates an object file, main.o (or main.obj on Windows). Similarly for gcc -c foo.c. You won't get the error message above at this stage. Then you link these two object files together. At this stage, the symbol foo is resolved. The reason you got the error message was because the linker couldn't find the symbol, because it was only looking at main.o and not foo.o. The linker is usually run from gcc, so to link your object files and create the final executable file main, use
gcc -o main main.o foo.o
You have to compile foo.c also because it is another module. Let me see how they do it in gcc:
$ gcc -Wall main.c foo.c -o main
You could also do this in your MakeFiles, like this:
APP_NAME = Foo
Foo_HEADERS = foo.h
Foo_FILES = main.c foo.c
If you're not so much familiar with MakeFiles i suggest you to take a look at Make Docs, but this is a simple example, APP_NAME sets the name of the compiled executable(in this case is Foo), Foo_HEADERS will set the headers used by your application, Foo_FILES you will set the source files of your applications, remember to put the APP_NAME(in this case Foo) at the beginning of _HEADERS and _FILES. I suggest you to use MakeFiles because they will organize you application build process and will be better for the end-user.