Libraries and Compilation Environment (I) Computadors Grau en Ciència i Enginyeria de Dades Q2

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Libraries and Compilation Environment (I) Computadors Grau en Ciència i Enginyeria de Dades 2018-2019 Q2 Facultat d Informàtica de Barcelona Departament d Arquitectura de Computadors Applications

1 Libraries and Compilation Environment (I) Computadors Grau en Ciència i Enginyeria de Dades Q2 Facultat d Informàtica de Barcelona Departament d Arquitectura de Computadors Applications running on a computer system use pre-built components in the form of libraries, that include most of the common functionalities that applications need. Libraries are part of the compilation environment, where the compiler has the main role to transform the source code provided onto a binary executable file that implements a specific application. During this lab session, we will work with the structure of the binary files, the typical locations of the libraries and header files, and the way programs are built by the compiler. We will use a simple example program using common libraries to generate values from Math functions that we can represent using GNUplot. The structure of binary programs Use the $ objdump <options> <file> Command to look into the structure of the following commands/libraries in your system: /bin/mkdir /bin/ls /usr/lib64/libx11.so (hint: use the -h option to see the full list of sections, and their properties) (hint: use the -s option to see the full contents of all sections) (hint: use the -p option to see the private headers, and the libraries needed to complete the loading of these binary files) Exercise 1 For the previous three binary files, list: -The contents of the.interp section, if present - The size of the.text,.data, and.bss sections - The binary files on which each one depends on Write your answers in the answers.txt file 1

2 Basic system libraries In our OS, there are several hundreds of library files, let s see a few of them: - libc.so, system calls and basic C support functions (files, FILE, processes, sockets ) - pthread.so, basic POSIX thread support - libm.so, Math functions (sqrt, log, exp, trigonometric functions ) - libstd++, basic C++ support functions (vector, list, deque, iostreams ) - libx11.so, libxaw.so, graphics support (X-Windows ) - libgomp.so, support for OpenMP parallelism, usually from GCC - libmpi.so, support for Message Passing Interface (MPI) - Libraries usually reside in the /lib or /usr/lib directories. When systems support both 32- and 64-bit applications, they can also be resident in /lib32, /usr/lib32, or /lib64, /usr/lib64. In modern OS versions they can also be under an specific machine-architecture directory: /lib/x86_64-linux-gnu/ /lib/i386-linux-gnu/ /usr/lib/x86_64-linux-gnu/ /usr/lib/i386-linux-gnu/ For locally installed services and applications, libraries can also be in directories like /usr/local/lib or /opt/lib. Libraries are binary files, and the compiler cannot understand them. Only the linker can use them to link with the symbols they contain. For this reason, we also need a set of include or header - files, that are written in a source language (C, C++), and that the compiler understands. They provide the type definitions, constants, and external symbol definitions contained in the binary libraries. These header files usually are installed in the directory: /usr/include For header files there is a single version, supporting both 32- and 64-bit installations. The reason is that header files come prepared for both types of compilation environments, and sometimes more. The C/C++ preprocessors implements this feature: #if defined( x86_64 ) // i386 powerpc powerpc64 arm arm64 #endif Exercise 2 Count the number of shared object files (.so) present in your system library directories: /lib /lib64 /usr/lib /usr/lib64 (hint: use wc l to count the number of lines in a text output) (hint: use find <directory> -name \*.so print to search for all files ending in.so in a directory hierarchy, including subdirectories) Write your answers in the answers.txt file. 2

3 Compilation steps The most simple way to compile a program is: $ g++ -o <executable> sources.cc By default, if no o option is given, a a.out file is generated: $ g++ sources.cc A common way of compiling applications is doing it in two steps: Compilation phase: Linking step: $ gcc c sources.cc Each source file generates a.o object file. $ gcc o <executable> sources.o Usually, the stdc++, math, and c libraries are automatically added into the linking process, so all symbols from them get automatically resolved. Common compilation options - -c, compile to object file - -o, name the output file - -S, compile to assembly file - -S o output.s, compile to assembly file and give the output the name output.s - -I<dir>, while pre-processing, search for header files in <dir> - -L<dir>, while linking, search for library files in <dir> - -l<name>, link against the lib<name>.so or lib<name>.a library o lc -lm -lstdc++ -lx11 -lxaw - See the full GCC compiler options using: o g++ --help --verbose less Example: C++ Vectors We are interested on knowing the shape of the curve generated by the expression y = sin(x) + sqrt(x). We are providing the program vectors.cpp that generates the first values out of this formula. Download the program package from the Racó: FilesS5.tar.gz Edit the program (vectors.cpp and vectors.h), and understand how it works. Compile (use make) and run the program (make run), to see the values generated, as printed in the terminal. You should see a sequence of values starting with: 3

4 Now run: $ make s run > values.dat The s option indicates to make that it should not print the commands executed in the standard output. This way, the output printed in values.dat is useful for GNUplot. Use GNUplot to visualize the data: $ gnuplot p fullplot.gp Visualize also a section of the values, from x= 10 to x= 50 $ gnuplot p 10-50plot.gp With this plot, you should be able to appreciate that the visualization is actually a little sharp, while the addition of the two continuous functions sin(x) and sqrt(x) should be softer. The reason is that we are visualizing only the values at the integer points (0, 1, 2, 3...). Let s try to improve the visualization of the function by doing two changes in the program: Exercise 3 Change the program so that it prints also the X coordinate of each point. The output should now look like this: Write the change you do in the cout line in the answers.txt file. 4

5 Exercise 4 Change the program so that it computes the formula with a step of 0.1 units, that is 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, When done, you should optain an output like this: Now use GNUplot again, and you should be able to see the difference on the shape of the graph. This is a much better representation of the formula. Which function calls am I using? Sometimes, it may be interesting to see which function calls are issued by a program, when running. In Linux, the execution environment allows to obtain this information. We are providing the ltrace command to display it. Exercise 5 Try: $ make ltrace And follow the instructions provide by the Makefile when running this target. Compare the two listings provided by $ make ltrace from the programs sin.cc, and sqrt.c. Look at the programs, and try to match the output provided by each program with its implementation. Write to the answers.txt file the reason why they are so different, if the programs are so similar. Exercise 6 Modify the Makefile: replace the line reading LTRACE = ltrace/ltrace -n 4 -C -F ltrace/ltrace.conf Adding the S flag to ltrace, this way: LTRACE = ltrace/ltrace -S -n 4 -C -F ltrace/ltrace.conf Re-run the $ make ltrace command and now list into the answers.txt file the library functions that you can identify because they have been explained in our theory classes. Upload the Deliverable When done with the lab session, to save the changes you can use the tar command as follows: #tar czvf session5.tar.gz answers.txt Makefile *.cc *.h Now go to RACO and upload this recently created file to the corresponding session slot. 5

6 Annex A: Compiling ltrace in your account You can work on this section after you have submitted your deliverable for this session. Download the ltrace com.tar.bz2 file, which contains the sources of the ltrace command. We are going to install the command in your account, so that you can use it in the next laboratory sessions. We will follow the usual procedure to configure, compile and install this application. Follow these steps: 1. Download the package from RACO (a given notification of the COM course) 2. Unpack the package tar jxvf ltrace com.tar.bz2 3. Enter the ltrace com directory cd ltrace com 4. Configure the package./configure --prefix=$home/dades/linux/ltrace-install 5. Compile it make 6. Install it: make install 7. Change your.tcshrc and.bashrc files to add PATH to the application installed:.tcshrc setenv PATH $HOME/dades/linux/ltrace-install/bin:$PATH.bashrc export PATH=$HOME/dades/linux/ltrace-install/bin:$PATH 6

Linux Operating System Environment Computadors Grau en Ciència i Enginyeria de Dades Q2

Linux Operating System Environment Computadors Grau en Ciència i Enginyeria de Dades 2017-2018 Q2 Facultat d Informàtica de Barcelona This first lab session is focused on getting experience in working