I am trying to write a C program that uses dlysm, and I keep getting an undefined reference to dlysm. I think I need to set my -ldl flags but I have no idea how to do this.
I am very new to linux and setting variables. If this is what I need to do can someone help me out with the commands?
-l library options are used at link time.
If you compile just one source file (gcc -o program program.c), then you both compile and link in one go. Just add the -ldl.
If you compile multiple object (*.o) files, and then link them together, specify the -ldl option to the linker (ld).
See also man ld and man cc
Pass -ldl as a parameter to the compiler.
Example:
gcc myprog.c -o app -ldl
Related
I am trying to build a shared library lib_test.so from 'test.c' & test.exp files. This lib_test.so file will be used as a extension to another application.
The application doc specifies generation of tle lib_test.so file directly in a single pass by the following command:
`gcc -q64 -o lib_test.so test.c -bM:Sre -bE:test.exp -bnoentry`
But my requirement is to build the library in two passes:
Compile to generate test.o file using gcc command.
Link to generate the library lib_test.so using ld command.
I tried this as follows:
Executed compile step as follows: gcc -q64 -c -o test.o test.c.
Create lib_test.so as follows: ld -bM:Sre -bE:test.exp -bnoentry -o lib_test.so test.o
But it is not generating a proper lib_test.so file.
I am using Ubuntu 16.04 LTS 64-Bit with latest GCC
Can you please suggest the correct way to split the process into two passes...
Thanks & Regards.
You rarely ever want to use ld to perform the linking. The gcc frontend does the better job of setting the right flags etc. So, use gcc.
i.e. Instead of
ld -bM:Sre -bE:test.exp -bnoentry -o lib_test.so test.o
do
gcc -bM:Sre -bE:test.exp -bnoentry -o lib_test.so test.o
in your second step.
The big difference between linking with the GCC frontend program gcc and with the actual linker ld is that the GCC frontend adds a few libraries to be linked with. Most notably the GCC runtime library (-lgcc_s or -lgcc) and the actual standard C library (-lc).
When you invoke ld directly you do not tell it to link with those libraries.
There might also be other libraries and flags the GCC frontend passes to ld without your knowledge. For the "one pass" build, pass the flag -v to gcc for verbose output and see what arguments, flags and libraries it uses.
I'm trying to compile and link some .c file. I have been using Eclipse IDE for C/C++ developers, and in my local machine i can compile without problems. However, when i try to compile and link the same file in a RedHat OS (gcc version is 4.9.2-6 in this OS) i'm having problems. I get some warnings at compile time, but those are fine, i think, i just ignored and the application still runs fine. Here are the commands i executed and the associated output:
gcc -O0 -g3 -Wall -c -fmessage-length=0 -std=c99 -MMD -MP -MF"example.d" -MT"example.d" -o "example.o" "example.c"
warning: suggest parentheses around assignment used as truth value [-Wparentheses]
warning: implicit declaration of function ‘wait’ [-Wimplicit-function-declaration]
This generates two files, example.d and example.o. Then, i try to link them, without luck, with the following command:
gcc -Xlinker -L/usr/lib -lrt -static -pthread example.o -o example
/usr/bin/ld: cannot find -lrt
/usr/bin/ld: cannot find -lpthread
/usr/bin/ld: cannot find -lc
collect2: error: ld returned 1 exit status
The commands are taken directly from the ones that Eclipse generates, and work just fine in my local computer (Ubuntu OS) but not in the RedHat environment. The last command didn't work, with and without the -L option. I suppose the directory in -L is fine, as i run, for example,
locate libpthread.so
And one of the locations i get is /usr/lib (also /usr/lib64, but neither work).
Any help will be greatly appreciated!! :)
If you try to link a static executable, it will look for the *.a versions of the libraries, not what you usually want. Remove the -static flag. Or you can install the static libraries if you really want to. It also should not be necessary to add -L/usr/lib explicitly.
I have got the object-file from source code using MinGW.
But on linking:
ld -o test.exe test.o
I get errors, for example the following:
undefined reference to printf
First, why are you using ld directly?
The following is an excerpt from the "GCC and Make" tutorial found at http://www3.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html.
Compile and Link Separately
The above command compile the source file into object file and link with other object files (system library) into executable in one step. You may separate compile and link in two steps as follows:
// Compile-only with -c option
> g++ -c -Wall -g Hello.cpp
// Link object file(s) into an executable
> g++ -g -o Hello.exe Hello.o
Note g++ (you can substitute gcc if you are using C and not C++) is used both for compiling and linking. ld is not used at all.
The benefit of using g++ or gcc to link is that it will link with default libraries, such as the one you need to link with for printf, automatically.
To link with other libraries, you specify the library name with the -l parameter, as in -lmylib.
We can view commands ran by compiler via command
c99 -v test.o
We'll get some text. All after string which contains "COLLECT_CGG_OPTIONS" will be arguments of ld.
But size of executable file is much more then size of file got by previous way.
I'm trying to compile some c code via make with gcc, but I keep getting:
/usr/bin/ld: cannot find -lnetlink
I have -lnetlink included in the gcc make parameters. Using locate netlink is able to find multiple items. I've even told gcc exactly where to find one by using -L/usr/include/linux, but it still gives the error.
The gcc command arguments below:
gcc -g -ggdb -Wall -Wextra -o mt6d mt6d.o address_worker.o tunnel_worker.o mt6d_assoc.o addr_gen.o send_utils.o if_utils.o tun_utils.o icmp_utils.o utils.o -lcrypto -lssl -lnetlink -lpthread -lnetfilter_queue
I've also been having errors with libnetlink.h, but was able to get them resolved, but I've included that here if that might be related and this error appears immediately after the other was fixed. Fixed by using C_INCLUDE_PATH
I've recently upgraded to Ubuntu 13.04 and was using 11.04 before that.
Let me know if you need any more information. Any help would be greatly appreciated!
Thanks,
-Alan
I'm trying to compile some c code via make with gcc, but I keep getting:
Technically, this stage is called linking. That difference may seem subtle at first, but it's a really important one and can help frame the investigation when problems like this arise.
You should not reference /usr/include paths with -L. -L adds to the search path for libraries and generally only header files should show up under /usr/include.
libnl enables pkg-config, so you should use that.
For example (assuming you have libnl-3-dev installed):
gcc -o my_executable $(pkg-config --libs libnl-3.0) my_foo.o my_bar.o
In a Makefile, you could do the following to achieve that effect:
LDLIBS+=$(shell pkg-config --libs libnl-3.0)
CFLAGS+=$(shell pkg-config --cflags libnl-3.0)
This is the header file and its C file:
cs50.h and
cs50.c
Now I use them in the following example http://www.paste.ubuntu.com/576370/ — which is no longer available.
I already put the header file in /usr/bin/include or something like that and when I try to compile my code using gcc -o xxx xxx.c, it doesn't work, so tried to fix this and the following way worked: http://www.paste.ubuntu.com/576371/ — which is no longer available.
Now I want to do something to make the 'make' command work as the gcc does.
What do I need to do?
The following was the old topic:
I was using gcc command to
compile C programs but after a period
of time I got a problem. I need
to compile a new header file and use
it as a library.
The header file called cs50.h.
so after doing it and it's ok I can
compile using the following
gcc -o xxx xxx.c -lcs50
It works but now I want to use 'make'
command and I can't get it to work.
It just don't compile the header file
and library as gcc was before I edit
it to accept the cs50 library.
So now I want to add to the 'make'
command the following: -lcs50
Can anyone help me in this please?
Near the top of your Makefile, add the line:
LDLIBS = -lcs50
If you are using Make's default (implicit) rules for the building, then that is all you need to do. If you are using explicit rules, you will need to add $(LDLIBS) to your explicit rules.
If there is no Makefile, then make is using default rules, and you can either just create a makefile with
echo LDLIBS = -lcs50 > Makefile
or tell make to link with certain libraries by specifying the required libraries in LDLIBS in the environment. For example, if you are using a sh-derived shell (anything other than csh or tcsh) you can do:
LDLIBS=-lcs50 make target
If you are using a csh variant, you can do:
env LDLIBS=-lcs50 make target
or just do (again, for non-csh shells)
export LDLIBS=-lcs50
before running make. (For csh variants, do setenv LDLIBS -lcs50)
You can use below “make” command to link library and include header directories,
make <.c or .cpp source filename_without_extension> LDLIBS="-l<lib1> -l<lib2>"
suppose you have server.cpp file to compile using make command,
make server LDLIBS="-lcpprest -lpthread -lssl -lcrypto" LDFLAGS="-L/usr/lib/" CXXFLAGS="-I/usr/include/"
Output will expand the compilation command as,
g++ -I/usr/include/ -L/usr/lib/ server.cpp -lcpprest -lpthread -lssl -lcrypto -o server
Did you forget that you have to tell gcc in what directory the CS50 library is located?
gcc … -L/directory/for/cs50_library -lcs50