How can I make sure `iostream` is available to the linker? - c++

I have the following C++ code in a file called helloworld.cpp:
#include<iostream>
int main()
{
std::cout << "Hello, World!\n";
}
I would like to compile this manually so I can really understand how the compilation steps work with gcc, namely:
Preprocessing
Compilation
Assembly
Linking
This article gives some useful information about how to break up the compilation steps. And so I came up with the following:
Preprocessing
cpp helloworld.cpp > helloworld.i
Compilation
g++ -S helloworld.i
Assembly as -o helloworld.o helloworld.s
Linking
ld -o helloworld helloworld.o
Everything seems to work except for the last step, as outlined by the article:
ld -o hello hello.o ...libraries...
The libraries argument above is a long list of libraries that you need to find out. I omitted the exact arguments because the list is really long and complicated, and depends on which libraries g++ is using on your system. If you are interested to find out, you can run the command g++ -Q -v -o hello hello.cpp and take a look at the last line where g++ invokes collect2
And so I tried running g++ -Q -v -o helloworld helloworld.cpp, but the result is extremely verbose.
I'm still unsure how to complete ld such that iostream can be available to the linker when I invoke it. How can I make sure iostream is available to the linker?

I'm still unsure how to complete ld such that iostream can be available to the linker when I invoke it.
You should never use ld to link any user-level programs, only when you are linking something esoteric, like an OS kernel or a boot loader.
Instead, (for user-level program) always use appropriate compiler driver (g++ here).
While you can find out how the compiler driver eventually invokes ld internally (e.g. using strace -fe execve -s 1024 g++ ...), note that that command may change from version to version, and depends on a multitude of flags (-pie vs. -no-pie, -shared, -static etc.) and you chances of continuing to use correct ld command after a few months are nil.

Based on the comments and posted answer I realized that the blog from which I was copying those commands makes things more complicated than they really need to be for my purposes. It's definitely possible to isolate every step of the compilation process using solely the g++ command. Here's a Makefile I came up with:
all: preprocess compile assemble link
# helloworld.i contains preprocessed source code
preprocess:
#echo "\nPREPROCESSING\n"; g++ -E -o helloworld.i helloworld.cpp
# compile preprocessed source code to assembly language.
# hello.s will contain assembly code
compile:
#echo "\nCOMPILATION\n"; g++ -S helloworld.i
# convert assembly to machine code
assemble:
#echo "\nASSEMBLY\n"; g++ -c helloworld.s
# links object code with the library code to produce an executable
# libraries need to be specified here
link:
#echo "\nLINKING\n"; g++ helloworld.o -o test
clean:
#find -type f ! -name "*.cpp" ! -name "*.h" ! -name "Makefile" -delete
Now I can compile my C++ programs in such a way that I can track whether the preprocessor, compiler, assembler or linker is generating the error.

Related

C++ makefile does not compile

Apologies for such a beginner question, but I have been stuck on making Makefile work on my c++ files for quite a while
My makefile contains two .cpp files and one .h files and are as follows
example.o: example.cpp example.h
g++ -c example.cpp
main.o: main.cpp example.h
g++ -c main.cpp
main: main.o example.o
g++ main.o example.o -o main
and it outputs the following error when I try to make main
Undefined symbols for architecture x86_64: *(with large pieces of code)*
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [main] Error 1
However, it works perfectly fine when I just do a compilation of the program by using
g++ main.cpp example.cpp -o main
Is there any reason why Makefile doesn't work but just compiling works? Thanks a lot for any replies!
Please show the output of make including the compiler lines that were invoked, not just some of the error output. Also it would be helpful if you provided at least SOME of the symbols that were not found so we could get an idea of what is missing.
There is no reason from the info you've provided why this should happen, so the cause must be related to some detail that you haven't provided.
One difference between the command line and the makefile is that the makefile builds object files then turns them into an executable, while the command line simply compiles the sources directly into an executable.
Maybe you have some .o files lying around from a different compilation? If you use rm -f *.o before you run make do you still see the same errors?
Also, it's a little odd that you're invoking g++ but the error output says clang which is a completely different compiler. Are you trying to build on MacOS? You should always provide your operating system info. If you're building on MacOS you should just use clang directly, unless you've explicitly install GCC and want to use that.

C++ file compiling: -L and -I arguments don't work for boost library

There are similar questions but their answers did not work for my issue.
I have a c++ program with #include <boost/test/unit_test.hpp> on top (among other includes).
To compile correctly, if I understood, I should do the command:
g++ -g -L/path_to_boost_lib -lboost_lib myprog.cpp -o myprog.exe
If i do a locate, I get /usr/lib/x86_64-linux-gnu/libboost_unit_test_framework.so.
Hence I edited my call to g++ by doing:
g++ -g -L/usr/lib/x86_64-linux-gnu -lboost_unit_test_framework myprog.cpp -o myprog.exe
But I still get errors of the type undefined reference to boost::unit_test.
I also tried the option -I/usr/include/ which contains the boost folder, without success.
It's because of the order. The GCC linker goes through the artifacts left-to-right, and every unknown symbol it encounters in an object file must be resolved by an artifact occurring afterwards.
The right command is thus:
g++ -g myprog.cpp -L/usr/lib/x86_64-linux-gnu -lboost_unit_test_framework -o myprog.exe
See this answer for a more thorough explanation.
I suggest using a build tool like CMake that takes care of such low-level details for you.

Embedding libmicrohttpd code in C++ OR how to compile this example?

I have a requirement of creating a C++ program which exposes certain functions through HTTP. For that reason I was trying to use libmicrohttpd for the same. Now this library is written in C. However I am kind of new to C++ and am trying to compile this C and C++ code given here. (Which can be git cloned from here)
Now I need help in understanding how g++ may be used to compile a program which is not written completely in C++. And/or how to compile the above linked code.
PS: Working in linux
And finally if someone can point to an easier alternative than libmicrohttpd - I am all ears.
Edit to Edit:
Finally got it working. Compiled the individual cpp files with gcc and then linked everything using g++. I have no clue how this came to work, maybe some one can reply below.
I have made the following script to compile and link:
LOC="path/to/directory"
gcc -c httphandler.cpp -o httphandler.o -I $LOC
gcc -c strutil.cpp -o strutil.o -I $LOC
gcc -c api.cpp -o api.o -I $LOC
gcc -c executor.cpp -o executor.o -I $LOC
g++ -o out httphandler.o strutil.o api.o executor.o -lmicrohttpd -lboost_regex
But in the final step I am getting the following error:
/usr/bin/ld: strutil.o: undefined reference to symbol '__cxa_free_exception##CXXABI_1.3'
/usr/bin/ld: note: '__cxa_free_exception##CXXABI_1.3' is defined in DSO /usr/lib/x86_64-linux-gnu/libstdc++.so.6 so try adding it to the linker command line
/usr/lib/x86_64-linux-gnu/libstdc++.so.6: could not read symbols: Invalid operation
What gives?
For starters, don't compile the C code with g++, use gcc instead. Then just include the header file and use the functions normally. When linking don't forget to link with the object file(s) generated from compiling the libmicrohttpd source file(s).

compiling a c++ program including mysql

I'm new to gcc, and trying to compile a c++ program which includes mysql.h using the command:
g++ -o test test.cpp -L/usr/include/mysql -lmysqlclient -I/usr/include/mysql
It works without issue, but I was wondering if someone could explain the arguments to me. I don't like using commands I don't understand.
Thanks
-o test means the output file is to be named "test".
test.cpp is your source file, of course.
-L/usr/include/mysql means to look for libraries in /usr/include/mysql, as well as in the usual link path. (It probably isn't finding any libraries here; my libmysqlclient.a is in the standard library directory /usr/lib. So I don't think you need this option.)
-lmysqlclient means to link with the mysqlclient library (actually named libmysqlclient.a)
-I/usr/include/mysql means to look for #include files in /usr/include/mysql, as well as in the usual include path.
try "man g++" for a full description of what the various options mean.
man gcc will give you the details of all these options.
g++ -o test test.cpp -L/usr/include/mysql -lmysqlclient -I/usr/include/mysql
g++ : the compiler
-o test : name the resulting binary "test"
test.cpp : your source file
-L : the directory to look in for libraries (that are specified by -l)
-l : named library to link against (looks for it in -L)
-I : the directory to look in for #included header files

C++, the "Old Fashioned" way

I have been learning C++ in school to create small command-line programs.
However, I have only built my projects with IDEs, including VS08 and QtCreator.
I understand the process behind building a project: compile source to object code, then link them into an executable that is platform specific (.exe, .app, etc). I also know most projects also use make to streamline the process of compiling and linking multiple source and header files.
The thing is, although IDEs do all this under the hood, making life very easy, I don't really know what is really happening, and feel that I need to get accustomed to building projects the "old fashioned way": from the command line, using the tool chain explicitly.
I know what Makefiles are, but not how to write them.
I know what gcc does, but not how to use it.
I know what the linker does, but not how to use it.
What I am looking for, is either an explanation, or link to a tutorial that explains, the workflow for a C++ project, from first writing the code up to running the produced executable.
I would really like to know the what, how, and why of building C++.
(If it makes any difference, I am running Mac OS X, with gcc 4.0.1 and make 3.81)
Thanks!
Compiling
Let's say you want to write a simple 'hello world' application. You have 3 files, hello.cpp hello-writer.cpp and hello-writer.h, the contents being
// hello-writer.h
void WriteHello(void);
// hello-writer.cpp
#include "hello-writer.h"
#include <stdio>
void WriteHello(void){
std::cout<<"Hello World"<<std::endl;
}
// hello.cpp
#include "hello-writer.h"
int main(int argc, char ** argv){
WriteHello();
}
The *.cpp files are converted to object files by g++, using the commands
g++ -c hello.cpp -o hello.o
g++ -c hello-writer.cpp -o hello-writer.o
The -c flag skips the linking for the moment. To link all the modules together requires running
g++ hello.o hello-writer.o -o hello
creating the program hello. If you need to link in any external libraries you add them to this line, eg -lm for the math library. The actual library files would look something like libm.a or libm.so, you ignore the suffix and the 'lib' part of the filename when adding the linker flag.
Makefile
To automate the build process you use a makefile, which consists of a series of rules, listing a thing to create and the files needed to create it. For instance, hello.o depends on hello.cpp and hello-writer.h, its rule is
hello.o:hello.cpp hello-writer.h
g++ -c hello.cpp -o hello.o # This line must begin with a tab.
If you want to read the make manual, it tells you how to use variables and automatic rules to simplify things. You should be able to just write
hello.o:hello.cpp hello-writer.h
and the rule will be created automagically. The full makefile for the hello example is
all:hello
hello:hello.o hello-writer.o
g++ hello.o hello-writer.o -o hello
hello.o:hello.cpp hello-writer.h
g++ -c hello.cpp -o hello.o
hello-writer.o:hello-writer.cpp hello-writer.h
g++ -c hello-writer.cpp -o hello-writer.o
Remember that indented lines must start with tabs. Not that not all rules need an actual file, the all target just says create hello. It is common for this to be the first rule in the makefile, the first being automatically created when you run make.
With all this set up you should then be able to go to a command line and run
$ make
$ ./hello
Hello World
More advanced Makefile stuff
There are also some useful variables that you can define in your makefile, which include
CXX: c++ compiler
CXXFLAGS:
Additional flags to pass to the
compiler (E.g include directories
with -I)
LDFLAGS: Additional flags to
pass to the linker
LDLIBS: Libraries
to link
CC: c compiler (also used to
link)
CPPFLAGS: preprocessor flags
Define variables using =, add to variables using +=.
The default rule to convert a .cpp file to a .o file is
$(CXX) $(CXXFLAGS) $(CPPFLAGS) -c $< -o $#
where $< is the first dependancy and $# is the output file. Variables are expanded by enclosing them in $(), this rule will be run with the pattern hello.o:hello.cpp
Similarly the default linker rule is
$(CC) $(LDFLAGS) $^ -o $# $(LDLIBS)
where $^ is all of the prerequisites. This rule will be run with the pattern hello:hello.o hello-writer.o. Note that this uses the c compiler, if you don't want to override this rule and are using c++ add the library -lstdc++ to LDLIBS with the line
LDLIBS+=-lstdc++
in the makefile.
Finally, if you don't list the dependancies of a .o file make can find them itself, so a minimal makefile might be
LDFLAGS=-lstdc++
all:hello
hello:hello.o hello-writer.o
Note that this ignores the dependancy of the two files on hello-writer.h, so if the header is modified the program won't be rebuilt. If you're interested, check the -MD flag in the gcc docs for how you can automatically generate this dependancy.
Final makefile
A reasonable final makefile would be
// Makefile
CC=gcc
CXX=g++
CXXFLAGS+=-Wall -Wextra -Werror
CXXFLAGS+=-Ipath/to/headers
LDLIBS+=-lstdc++ # You could instead use CC = $(CXX) for the same effect
# (watch out for c code though!)
all:hello # default target
hello:hello.o hello-world.o # linker
hello.o:hello.cpp hello-world.h # compile a module
hello-world.o:hello-world.cpp hello-world.h # compile another module
$(CXX) $(CXXFLAGS) -c $< -o $# # command to run (same as the default rule)
# expands to g++ -Wall ... -c hello-world.cpp -o hello-world.o
A simple example is often useful to show the basic procedure, so:
Sample gcc usage to compile C++ files:
$ g++ -c file1.cpp # compile object files
[...]
$ g++ -c file2.cpp
[...]
$ g++ -o program file1.o file2.o # link program
[...]
$ ./program # run program
To use make to do this build, the following Makefile could be used:
# main target, with dependencies, followed by build command (indented with <tab>)
program: file1.o file2.o
g++ -o program file1.o file2.o
# rules for object files, with dependencies and build commands
file1.o: file1.cpp file1.h
g++ -c file1.cpp
file2.o: file2.cpp file2.h file1.h
g++ -c file2.cpp
Sample Makefile usage:
$ make # build it
[...]
$ ./program # run it
For all the details you can look at the Gnu make manual and GCC's documentation.
I know what Makefiles are, but not how to write them.
The make syntax is horrible, but the GNU make docs aren't bad. The main syntax is:
<target> : <dependency> <dependency> <dep...>
<tab> <command>
<tab> <command>
Which defines commands to build the target from the given dependencies.
Reading docs and examples is probably how most people learn makefiles, as there are many flavors of make with their own slight differences. Download some projects (pick something known to work on your system, so you can actually try it out), look at the build system, and see how they work.
You should also try building a simple make (strip out a bunch of the harder features for your first version); I think this is one case where that will give you a much better grasp on the situation.
I know what gcc does, but not how to use it.
Again, man g++, info pages, and other documentation is useful, but the main use when you call it directly (instead of through a build system) will be:
g++ file.cpp -o name # to compile and link
g++ file.cpp other.cpp -o name # to compile multiple files and link as "name"
You can also write your own shell script (below is my ~/bin/c++ simplified) to incorporate $CXXFLAGS so you won't forget:
#!/bin/sh
g++ $CXXFLAGS "$#"
You can include any other option as well. Now you can set that environment variable ($CXXFLAGS, the standard variable for C++ flags) in your .bashrc or similar, or redefine it in a particular session, for working without a makefile (which make does do just fine, too).
Also use the -v flag to see details on what g++ does, including...
I know what the linker does, but not how to use it.
The linker is what takes the object files and links them, as I'm sure you know, but g++ -v will show you the exact command it uses. Compare gcc -v file.cpp (gcc can work with C++ files) and g++ -v file.cpp to see the difference in linker commands that often causes the first to fail, for example. Make also shows the commands as it runs them by default.
You are better off not using the linker directly, because it is much simpler to use either gcc or g++ and give them specific linker options if required.
Just to throw this out there, the complete gcc documentation can be found here: http://www.delorie.com/gnu/docs/gcc/gcc_toc.html
compiler takes a cpp and turns into an object file which contains native code and some information about that native code
a linker takes the object files and lays out an excutable using the extra information in the object file.... it finds all the references to the same things and links them up, and makes and image useful for the operating system to know how to load all the code into memory.
check out object file formats to get a better understanding of what the compiler produces
http://en.wikipedia.org/wiki/Object_file (different compilers use different formats)
also check out (for gcc)
http://pages.cs.wisc.edu/~beechung/ref/gcc-intro.html on what you type at the command line
You might also look into Autoproject, which sets up automake and autoconf files, which makes it easier for people to compile your packages on different platforms: http://packages.debian.org/unstable/devel/autoproject
I like this quirky intro to building a hello world program with gcc, Linux-based but the command-line stuff should work fine on OS/X. In particular, it walks you through making some common mistakes and seeing the error messages.
Holy Compilers, Robin, the darn thing worked!
This is what has helped me to learn the autoconf, automake, ...:
http://www.bioinf.uni-freiburg.de/~mmann/HowTo/automake.html
It is a nice tutorial progresses from a simple helloworld to more advanced structures with libraries etc.