Linux programming (i) Getting Started

1. Linux Programs

Linux applications behave as two special types of files: executables and script files .

Executable: is a program that calculates the ability to run directly, equivalent to a Windows. exe file.

Script file: A collection of sets of instructions. These directives are executed by another program (that is, the interpreter), which is equivalent to a. bat file, a. cmd file under Windows.

Use (:) to separate entries for the path variable. MS-DOS and Windows use semicolons (;).

Linux uses forward slashes (/) to separate directory names in filenames.

2. C language Compiler

Compile, link, and run the Hello program:

[Email protected]:~/mypro$ gcc-o Hello hello. C[email protected]-virtualbox:~/mypro$./!

The program name (hello) plus "./" is a program that instructs the shell to execute the given name under the current directory.

The-o name option tells the compiler where to place the executable program. If you do not, the compiler places the program in a file called a.out (assembler output, which is the assembly outputs).

3. Development system Guidance

3.1 Applications

(1)/usr/bin: The program provided by the system for normal use, including tools for program development, can be found in the directory/usr/bin.

(2)/usr/local/bin or/OPT: A program that the system administrator adds to a particular host or network.

(3) The system administrator prefers to use the/usr/local directory, as it separates the vendors from the applications that the vendor provides and subsequently adds files to the system itself.

3.2 Header Files

(1) For C language, the header file is in the/usr/include directory and its subdirectories. Header files that depend on a particular Linux version are usually found in/usr/include/sys and/usr/include/linux.

(2) When invoking the C language compiler, you can use the-I flag to include an include file that is saved in a subdirectory or in a nonstandard location.

gcc-i/usr/openwin/include fred.c

The previous example indicates that the header files contained in the FRED.C are also found in the/usr/openwin/include directory, not only in the standard location.

3.3 Library files

A library is a set of pre-compiled functions.

Standard system library files are typically stored in the/lib and/usr/lib directories.

. A: Traditional library of static functions.

. So: Shared function library.

-L: Flag indicates the library file to be searched by the compiler.

-L: The compiler adds a search path to the library.

3.4 Static Libraries

Static libraries, also known as Archive Files (archive).

Static Library Experiment:

(1) Create a source file for two functions, ADD.C and SUB.C:

Add.c
#include <stdio.h>

int add (int a, int b) {
return a + B;
}

// sub.c#include <stdio.h>int sub (intint  b) {        return A - b;}

(2) Compile these functions separately, producing the target files to be included in the library file.

[Email protected]:~/mypro$ gcc-C add.c sub.c[email protected]-virtualbox:~/mypro$ lsadd.c  ADD.O  hello.c  sub.c  sub.o

-C: Prevents the compiler from creating a complete program. (-c Compile and assemble, but does not link)

(3) Create a header file that will declare the functions in the library file.

// Lib.h int Add (intint); int Sub (intint);

(4) The Calling program (PROGRAM.C).

//PROGRAM.C#include <stdio.h> #include"Lib.h"intMain () {printf ("5 + 4 =%d\n", Add (5,4)); printf ("5-4 =%d\n", Sub (5,4)); return 0;}

(5) Compile and test the program.

[Email protected]:~/mypro$ gcc-c program.c[email protected]-virtualbox:~/mypro$ gcc-o program PROGRAM.O ADD.O Sub.o[email protected]-virtualbox:~/mypro$ lsadd.c  add.o  hello.c  lib.h  Program  program.c  program.o  sub.c  sub.o[email protected]-virtualbox:~/mypro$./5  495)41

(6) Create and use a library file.

Use the AR program to create an archive file and add the target file.

[Email protected]:~/mypro$ lsadd.c  add.o  hello.c  lib.h  program  program.c  PROGRAM.O  SUB.C  sub.o[email protected]-virtualbox:~/-- sub.o[email protected]- virtualbox:~/mypro$ lsadd.c  add.o  hello.c  libcalc.a  lib.h  program  PROGRAM.C  program.o  sub.c  sub.o

(7) To successfully use the library, you also need to generate a table of contents for the library of functions.

Randlib LIBCALC.A

Add the library file to the list of files in the compiler command line to create our program:

[Email protected]:~/mypro$ lsadd.c  add.o  hello.c  libcalc.a  lib.h  program.c  PROGRAM.O  SUB.C  sub.o[email protected]-virtualbox:~/mypro$ gcc-o program PROGRAM.O libcalc.a[email protected ]-virtualbox:~/mypro$ lsadd.c  add.o  hello.c  libcalc.a  lib.h  Program PROGRAM.C  program.o  sub.c  sub.o[email protected]-virtualbox:~/mypro$./program   549541

NM command: View the functions contained in the target file, function library, or executable program.

3.5 Shared libraries

When a program uses a shared library, it is linked in such a way that it does not contain function code by itself, but rather refers to shared code that is accessible at run time.

Comparison of Linux and Windows files:

Project	Unix	Windows
Target Module	. o	. obj
static function library	. A	. lib
Program	Program (a.out)	Program.exe
Shared libraries	. So	. dll

Reference: The 3rd edition of Linux Programming

Linux programming (i) Getting Started