This problem has bothered me for days.
After I compile and install ffmpeg , I try to build a demo using it, but it always fails.
The demo is:
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
int main(int argc,char *argv[]) {
av_register_all();
return 1;
}
With gcc main.c -o main.o, an error occurs: undefined reference to 'av_register_all'
Building with: gcc main.c -o main.o -lm -ld -lz -lavcodec -lavformat -lavutil, another error occurs: /usr/bin/ld: cannot find -ld
How can I resolve this?
Putting includes within extern "C" block may work.
extern "C"{
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
}
As mentioned here
the order of libraries matters
so in your case the following hopefully should work:
gcc main.c -o main.o -lavformat -lavcodec -lavutil -lz -lm -lpthread
gcc filename.c -o outputfilename -lavformat -lavcodec -lavutil -lz -lm -lpthread -I'/usr/local/include' -lswresample
The above command will help to compile properly.
Have a look at how this project builds sample code.
The Makefile is pretty simple to follow.
It's work for me:
gcc <your source code> -o main -lavformat -lavcodec -lavutil -lm -lpthread -I'/usr/local/include' -lswresample
Related
Minimal example:
// file: main.cpp
#include "pch.h"
int main()
{
std::cout << "test" << std::endl;
return 0;
}
--
// file: pch.h
#include <iostream>
Works fine and as expected if I compile this with
g++ pch.h
g++ main.cpp -Winvalid-pch
However once I change the last line to:
g++ main.cpp -fopenmp -Winvalid-pch
usage of the precompiled header is disabled:
warning: pch.h.gch: not used because `_REENTRANT' is defined [-Winvalid-pch]
How can I still use precompiled headers while linking to OpenMP? Why does the _REENTRANT define conflict with using a precompiled header at all?
You must generate .pch and compile sources with identical flags. -fopenmp implies #pragma omp and -pthread.
g++ -fopenmp pch.h
g++ main.cpp -fopenmp -Winvalid-pch
Or at least
g++ -pthread pch.h
g++ main.cpp -fopenmp -Winvalid-pch
Given the following file print.cpp
#include <stdio.h>
int main() {
printf("asdf\n");
}
I can link this statically like this
g++ -static print.cpp
or like this
g++ -static-libgcc -Wl,-Bstatic -lc print.cpp -o print
But now let's add a little OpenMP and call the file print_omp.cpp
#include <omp.h>
#include <stdio.h>
int main() {
printf("%d\n", omp_get_num_threads());
}
I can link this statically like this (I checked it with ldd)
g++ -fopenmp -static print_omp.cpp
However, this does not work
g++ -fopenmp -static-libgcc -Wl,-Bstatic -lc print_omp.cpp -o print
I have tried various combinations of -Wl,--whole-archive -lpthread -Wl,--no-whole-archive and -lgomp -lpthread but no luck (I get various problems linking to pthreads). Can someone explain how I can do this without using the -static option?
GCC says
On glibc-based systems, OpenMP enabled applications cannot be statically linked due to limitations of the underlying pthreads-implementation
However, since g++ -fopenmp -static print_omp.cpp works just fine this does not make sense to me.
Edit:
I figured this out. The library GOMP comes with GCC whereas pthreads and libc come from GLIBC. So I can link GOMP statically like this
ln -s `g++ -print-file-name=libgomp.a`
g++ foo.cpp -static-libgcc -static-libstdc++ -L. -o foo -O3 -fopenmp
ldd shows
linux-vdso.so.1 => (0x00007fff71dbe000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fc231923000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fc23155c000)
/lib64/ld-linux-x86-64.so.2 (0x00007fc231b5c000)
However, if I the try this
ln -s `g++ -print-file-name=libpthread.a`
g++ foo.cpp -static-libgcc -static-libstdc++ -L. -o foo -O3 -fopenmp
It won't link. Pthreads and libc must be linked statically together. So once I add
ln -s `g++ -print-file-name=libc.a`
g++ foo.cpp -static-libgcc -static-libstdc++ -L. -o foo -O3 -fopenmp
ldd returns
not a dynamic executable
I really don't get why you may want to link only libgomp statically, but having separate compilation and linking commands may help. For instance assume main.cpp contains:
#include <omp.h>
#include <stdio.h>
int main() {
#pragma omp parallel
{
printf("%d\n", omp_get_thread_num());
}
}
Then:
~/tmp$ ls
main.cpp
~/tmp$ g++ -Wall -Werror -pedantic -fopenmp main.cpp -c
~/tmp$ ls
main.cpp main.o
~/tmp$ locate libgomp.a
${SOME_PATH_TO_LIBGOMP}/libgomp.a
~/tmp$ g++ -Wall -Werror -pedantic main.o -o main.x ${SOME_PATH_TO_LIBGOMP}/libgomp.a -pthread
~/tmp$ ls
main.cpp main.o main.x
~/tmp$ ldd main.x
linux-gate.so.1 => (0xb7747000)
libstdc++.so.6 => /production/install/gnu/compiler/gcc/lib/libstdc++.so.6 (0xb765c000)
libm.so.6 => /lib/i386-linux-gnu/libm.so.6 (0xb75fa000)
libgcc_s.so.1 => /production/install/gnu/compiler/gcc/lib/libgcc_s.so.1 (0xb75de000)
libpthread.so.0 => /lib/i386-linux-gnu/libpthread.so.0 (0xb75c2000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb7413000)
/lib/ld-linux.so.2 (0xb7748000)
The most clean solution I've found for this is by modifying the libgomp.spec. Mine is at /usr/local/lib64/libgomp.spec. Change the content as follow:
*link_gomp: -l:libgomp.a %{static: -ldl }
I am having trouble developing a makefile for a code using the CImg library. I have 3 files:
mainProgram.cpp
program.cpp
program.h
CImg.h // CImg library
In the mainProgram.cpp
#include "program.h"
In the program.cpp
#include "program.h"
In the program.h
#ifndef PROGRAM_H
#define PROGRAM_H
#include "CImg.h"
#include <vector>
#include <string>
#include <iostream>
#include <sstream>
using namespace std;
using namespace cimg_library;
I am using a MAC and it suggested to compile it using: g++ -o snake mainSnake.cpp -O2 -lm -lpthread -I/usr/X11R6/include -L/usr/X11R6/lib -lm -lpthread -lX11
But, I am having difficulty communicating this to a makefile. Can anyone assist me?
The simplest make file would be
all:mainProgram.cpp program.cpp program.h
g++ -o snake mainSnake.cpp -O2 -lm -lpthread -I/usr/X11R6/include -L/usr/X11R6/lib -lm -lpthread -lX11
If you read some basics of makefile writing then you can first create the object files then create the final snake executable
May be this would help you get started with. The similar example is given but in c.
I have searched high and low for answer to this issue. I am using boost 1.48 and the program is extremely simple, since I have broken it down to its simplest form in order to solve this issue.
#include <boost/filesystem.hpp>
int main(int argc, char **argv) {
return 0;
}
The g++ command executed from my Makefile is as follows:
g++ -m32 -Wall -o mapnik-test -L/usr/lib -I/usr/include -I/usr/include/freetype2 -lpthread -lboost_system mapnik-test.cpp
The complete list of errors during linking is as follows:
/tmp/ccIbmuee.o: In function `__static_initialization_and_destruction_0(int, int)':
mapnik-test.cpp:(.text+0x49): undefined reference to `boost::system::generic_category()'
mapnik-test.cpp:(.text+0x53): undefined reference to `boost::system::generic_category()'
mapnik-test.cpp:(.text+0x5d): undefined reference to `boost::system::system_category()'
collect2: ld returned 1 exit status
make: *** [mapnik-test] Error 1
I have found many people suffering from the same issue, but in most cases, the solution has been to provide the boost_system library in the LDFLAGS. As you can see from the g++ command line, I already have this specified. I have even tried explicitly linking against the libboost_system.a library to no avail. Am I the only person with this complaint?
Put the source file at the beginning of the command line.
Try
g++ -m32 -Wall mapnik-test.cpp -o mapnik-test -L/usr/lib -I/usr/include -I/usr/include/freetype2 -lpthread -lboost_system
The libraries should be specified only after the source file so that the linker can resolve the undefined references in the source file.
after upgrading to Ubuntu 11.10, I've found that many of my old and current developments can't be compiled anymore. I've reduced the problem to a simple example:
#include <X11/Xlib.h>
int main() {
Display* display = XOpenDisplay(":0.0");
XCloseDisplay(display);
return 0;
}
Compiling it using:
g++ -lX11 test.cpp
or
g++ -c -o test.o test.cpp
g++ -lX11 -o test test.o
Causes a failure to happen:
/tmp/ccBAOpzy.o: In function `main':
test.cpp:(.text+0x11): undefined reference to `XOpenDisplay'
test.cpp:(.text+0x21): undefined reference to `XCloseDisplay'
Any ideas? I've found that some linker stuff has changed in 11.10:
https://wiki.ubuntu.com/NattyNarwhal/ToolchainTransition
But still doesn't explain these problems.
g++ -lX11 -o test test.o
Above command is incorrect. Try this instead:
g++ test.o -lX11
Explanation of why the order matters here.
Also, you should never call your executables test on UNIX.