Simple howtos

This collection of mini howtos is a dynamic copy of the Unix Toolbox. This page extracts the XML content directly from the original XHTML DOM and displays only the requested node. Source code here (use "save as").
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C basics

strcpy(newstr,str)                        /* copy str to newstr */
expr1 ? expr2 : expr3                     /* if (expr1) expr2 else expr3 */
x = (y > z) ? y : z;                      /* if (y > z) x = y; else x = z; */
int a[]={0,1,2};                          /* Initialized array (or a[3]={0,1,2}; */
int a[2][3]={{1,2,3},{4,5,6}};            /* Array of array of ints */
int i = 12345;                            /* Convert in i to char str */
char str[10];
sprintf(str, "%d", i);

C example

A minimal c program simple.c:
#include <stdio.h>
main() {
    int number=42;
    printf("The answer is %i\n", number);  
Compile with:
# gcc simple.c -o simple
# ./simple
The answer is 42

C++ basics

*pointer                                  // Object pointed to by pointer
&obj                                      // Address of object obj
obj.x                                     // Member x of class obj (object obj)
pobj->x                                   // Member x of class pointed to by pobj
                                          // (*pobj).x and pobj->x are the same

C++ example

As a slightly more realistic program in C++: a class in its own header (IPv4.h) and implementation (IPv4.cpp) and a program which uses the class functionality. The class converts an IP address in integer format to the known quad format.

IPv4 class


#ifndef IPV4_H
#define IPV4_H
#include <string>

namespace GenericUtils {                          // create a namespace
class IPv4 {                                      // class definition
    IPv4(); ~IPv4();
    std::string IPint_to_IPquad(unsigned long ip);// member interface
} //namespace GenericUtils
#endif // IPV4_H


#include "IPv4.h"
#include <string>
#include <sstream>
using namespace std;                              // use the namespaces
using namespace GenericUtils;

IPv4::IPv4() {}                                   // default constructor/destructor
IPv4::~IPv4() {}
string IPv4::IPint_to_IPquad(unsigned long ip) {  // member implementation
    ostringstream ipstr;                          // use a stringstream
    ipstr << ((ip &0xff000000) >> 24)             // Bitwise right shift
          << "." << ((ip &0x00ff0000) >> 16)
          << "." << ((ip &0x0000ff00) >> 8)
          << "." << ((ip &0x000000ff));
    return ipstr.str();

The program simplecpp.cpp

#include "IPv4.h"
#include <iostream>
#include <string>
using namespace std;
int main (int argc, char* argv[]) {
    string ipstr;                                 // define variables
    unsigned long ipint = 1347861486;             // The IP in integer form
    GenericUtils::IPv4 iputils;                   // create an object of the class
    ipstr = iputils.IPint_to_IPquad(ipint);       // call the class member
    cout << ipint << " = " << ipstr << endl;      // print the result

    return 0;
Compile and execute with:
# g++ -c IPv4.cpp simplecpp.cpp                # Compile in objects
# g++ IPv4.o simplecpp.o -o simplecpp.exe      # Link the objects to final executable
# ./simplecpp.exe 
1347861486 =
Use ldd to check which libraries are used by the executable and where they are located. Also used to check if a shared library is missing or if the executable is static.
# ldd /sbin/ifconfig                           # list dynamic object dependencies
# ar rcs staticlib.a *.o                       # create static archive
# ar t staticlib.a                             # print the objects list from the archive
# ar x /usr/lib/libc.a version.o               # extract an object file from the archive
# nm version.o                                 # show function members provided by object

Simple Makefile

The minimal Makefile for the multi-source program is shown below. The lines with instructions must begin with a tab! The back slash "\" can be used to cut long lines.
CC = g++
OBJS = IPv4.o simplecpp.o

simplecpp: ${OBJS}
	${CC} -o simplecpp ${CFLAGS} ${OBJS}
	rm -f ${TARGET} ${OBJS}