Learn Linux Series (#3) - Introduction to programming

• Libraries and flags
  
  
  Libraries are a collection of code ready for use by applications. When compiling programs that use libraries, you must remember that you usually need to install, besides the libraries themselves, packages with header files, static libraries, etc. in addition to attaching the appropriate header files in the sources, you must also provide the appropriate flags for the linker so that he can consolidate the output program with the appropriate libraries. In the case of gcc, this flag is -llibraryname, so for the libjpeg library listed above, the compilation of the sample program will look like:

gcc -Wall -o test test.c -ljpeg

A useful tool is the pkg-config program, which gives the necessary flags, which should be passed to the program. Using the program is simple:

pkg-config --cflags library
pkg-config --libs library

The first call returns the flags for the compiler, the second flags for the linker. Calls can be combined, as well as using the `, operator in bash, which inserts the result of one command into the second command:

gcc -Wall `pkg-config --cflags --libs gtk+-2.0` -o hellogtk hellogtk.c

• Specialized tools
  
  make - a tool for building applications.
  
  Manually entering commands that call the compiler can be a very tiring task. Fortunately, there are tools that help in building programs. On Unix systems, the make program plays such a role. Make uses scripts, called Makefiles describing the process of building applications. Sample script:
  

CC=gcc
CFLAGS=-O3 -Wall
LIBS=-Wall -lm

SOURCES=
    gfx.c 
    main.c 
    menu.c 

HEADERS=
    gfx.h 
    menu.h


OBJS=$(SOURCES:.c=.o)

all: myapp

.c.o:
    $(CC) -c $(CFLAGS) $< -o $@

$(OBJS): $(HEADERS)

myapp: $(OBJS)
    $(CC) -c $(CFLAGS) $< -o $@

The first lines declare variables, respectively compiler, compiler flags, linker flags, files with sources, header files. They can be freely edited. The next one defines the OBJS variable, resulting from the SOURCES variable.



gdb - the GNU system debugger

Debugger is a tool that allows you to track the execution of the program, "preview" the values of variables and, in general, to facilitate the search for errors in the software, hence the name. In Linux, we have the gdb program, the GNU system debugger. Starting work with the debugger, you must remember that the program you want to examine should be compiled by giving the compiler the -g flag, which adds additional information needed to gdb to the output file.

Run the debugger:

gdb programname

doxygen - a tool for creating documentation

Doxygen is a tool that creates technical documentation of the program based on the comments contained in the program's code. The programmer in source files places specially formatted commentators that are read by Doxygen (doxygen package), which on the basis of this creates documentation containing a description of all functions, classes or methods of a given program, in several different formats, including HTML, PDF or man page manual pages. Document each function or class:

/**
 * brief Class representing a rectangle
 **/
class Rectangle
{
    private:
        int width;  /** < Rectangle width */
        int height; /** < Rectangle height */

    public:
        Rectnagle (int w, int h);
        int Area ();
};


/**
 * brief Rectangle constructor
 * param w Rectangle initial width
 * param h Rectangle initial height
 **/
Rectangle::Rectangle (int w, int h)
    : width (w), height (h)
{
}


/**
 * brief Compute rectnagle area
 * return Rectangle area
 **/
int Rectangle::Area ()
{
    return width * height;
}

In the main project directory, place the Doxyfile file which can be generated using doxywizard (doxywizard package). next, we execute the

doxygen

command, and in the doc directory we will find the generated documentation in various formats.

Curriculum

• Part 1 - TCP/IP Computer Adaptation
• Part 2 - Proftpd management and configuration