Mar 20, 2011

POSIX Threads - A simple Example

In our earlier post, we have written multithreaded program using win32 apis. This post deals with writing the multithreaded programs using POSIX thread apis. The important POSIX thread apis are namely pthread_create, pthread_exit and pthread_join

Create a thread

int pthread_create
(
    pthread_t *thread,
    const pthread_attr_t *attr,
    void *( *start_routine)(void *), 
    void *arg
);

Terminate a thread
void pthread_exit (void *retval);

Wait on a thread
int pthread_join (pthread_t thread, void **status);

The Complete Example
#include <pthread.h>
#include <iostream>

//File: SimpleThreadExample.cpp

#define MAX_THREADS 3
using namespace std;

void* func(void *args)
{
    int tp = *(int *)args * 10;
    while(tp--)
    {
        cout << *(int *) args;
        fflush(stdout);
        sleep(1);
    }

    cout << endl;
    pthread_exit((void *)10);
    return (void *)10;
}

int main(void)
{
    pthread_t tid[MAX_THREADS] = {0};
    int param[MAX_THREADS] = { 1, 2, 3 };

    int rc;
    for(int i = 0; i < MAX_THREADS; ++i)
    {
        rc = pthread_create(&tid[i], NULL, func, ¶m[i]);
        if(0 != rc)
        {
            cout << "ERROR: Thread Creation failed" << endl;
        }
        cout << "Thread[" << tid[i] << "] Created" << endl;
    }

    void *status[3];
    for(int i = 0; i < MAX_THREADS; ++i)
    {
        rc = pthread_join(tid[i], &status[i]);
    }

    for(int i = 0; i < MAX_THREADS; ++i)
    {
        cout << "Thread[" << tid[i] << "] returned with exit code=" << (long) status[i] << endl;
    }

    cout << "Main thread terminating" << endl;
    return 0;
}
The makefile for the project
#-----------------------------------------------------------------------
#                   A simple thread example
#-----------------------------------------------------------------------
PROJECT := SimpleThreadExample

SRCEXT := .h .cpp
OBJEXT := .o
EXEEXT := .exe

#compilers
CC := gcc
CXX := g++

#flags
CFLAGS := -o
CXXFLAGS := -o
LDFLAGS := -lpthread -o

#disable implicit suffix rules
.SUFFIXES:
.SUFFIXES: $(SRCEXT) $(OBJEXT) $(EXEEXT)

SRC := \
    SimpleThreadExample.cpp

OBJ := \
    $(SRC:.cpp=.o)

EXE := \
    $(PROJECT)$(EXEEXT)

#define dummy targets
.PHONY: all clean compile link

all : compile link

clean :
    @echo

    @echo "Cleaning Temporary Files..."
    $(RM) $(OBJ) $(EXE)

compile : $(OBJ)

link : $(EXE)

$(OBJ) :
    @echo
    @echo "Compiling Source Files..."
    $(CXX) $(CXXFLAGS) $(OBJ) $(SRC)

$(EXE) :
    @echo
    @echo "Linking..."
    $(LD) $(OBJ) $(LDFLAGS) $(EXE)

run :
    @echo
    @$(EXE)
Building the project. (The output is from Cygwin Environment. You may need to modify the makefile to compile it on a linux environment)
$ gmake clean all

Cleaning Temporary Files...
rm -f SimpleThreadExample.o SimpleThreadExample.exe

Compiling Source Files...
g++ -o SimpleThreadExample.o SimpleThreadExample.cpp

Linking...
ld SimpleThreadExample.o -lpthread -o SimpleThreadExample.exe
Output:

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Command prompt and setting environment variable properties

Working on multiple projects which require different compiler versions can be a daunting task especially if you are using windows command prompt cmd.exe to build the projects. You need to export the compiler specific environment variables and set the PATH variable appropriately each time you launch the command window to build each project. Here is a small tip that can help save some time

Tip
Write a batch file which will set the necessary environment variables and the path variable before launching the command window
A simple batch script which sets JAVA_HOME to jdk1.6.0_21 and launches the command prompt window
set JAVA_HOME=
set JAVA_HOME=c:\Program Files\Java\jdk1.6.0_21

set PATH=%JAVA_HOME%\bin;%PATH%
start cmd /k

On executing the batch file, command prompt window is launched with the JAVA_HOME environment variable set to the desired version (jdk1.6.0_21)

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Mar 15, 2011

Customize Cygwin/Linux command prompt

From the bash manual, the following environment variables control the display of prompt in a Cygwin or Linux terminal namely PS1, PS2, PS3, PS4.
Note: For referring the bash manual type info bash

Key Points
1. PS1 : The value of this parameter is expanded and used as the primary prompt string. It's default value is '\s-\v\$ '
2. PS2 : The value of this parameter is expanded as with PS1 and used as the secondary prompt string. The default value is >
3. PS3 : The value of this variable is used as the prompt for select command. If this variable is not set, select command prompts with #?
4. PS4 : The prompt is used when you have tracing enabled for debugging the bash scripts. The PS4 value is printed each time the command is echoed on the screen. The default value is +

a) PS1 demo
1. Identify the current prompt setting in your Cygwin/Linux terminal by typing echo $PS1
2. Make the modifications to PS1 and export the new PS1 variable. Your prompt should change immediately


b) PS2 demo
When executing interactively, bash displays the primary prompt PS1 when it is ready to read a command, and the secondary prompt PS2 when
it needs more input to complete a command.


c) PS3 demo
This prompts user to provide an input.
#!/bin/bash

LINUX=linux
WIN32=win32
HPUX=hpux
SUNOS=sunos
AIX=aix
QUIT=quit

OPTIONS="\
    $LINUX \
    $WIN32 \
    $HPUX \
    $SUNOS \
    $AIX \
    $QUIT"

select option in $OPTIONS;
do
  echo "${option}"  
  if [ "${option}" = "quit" ]; then
    exit
  fi
done
Output:


d) PS4 demo
Enable tracing by typing the following command set -x


Bash allows these prompt strings to be customized by inserting a number of backslash-escaped special characters that are decoded as follows:

\a     an ASCII bell character (07)
\d     the date in "Weekday Month Date" format (e.g., "Tue May 26")
\D{format}
    the format is passed to strftime(3) and the result is inserted into the prompt  string;  an  empty  format  results  in  a locale-specific time representation.  The braces are required
\e     an ASCII escape character (033)
\h     the hostname up to the first `.'
\H     the hostname
\j     the number of jobs currently managed by the shell
\l     the basename of the shell's terminal device name
\n     newline
\r     carriage return
\s     the name of the shell, the basename of $0 (the portion following the final slash)
\t     the current time in 24-hour HH:MM:SS format
\T     the current time in 12-hour HH:MM:SS format
\@     the current time in 12-hour am/pm format
\A     the current time in 24-hour HH:MM format
\u     the username of the current user
\v     the version of bash (e.g., 2.00)
\V     the release of bash, version + patch level (e.g., 2.00.0)
\w     the current working directory, with $HOME abbreviated with a tilde
\W     the basename of the current working directory, with $HOME abbreviated with a tilde
\!     the history number of this command
\#     the command number of this command
\$     if the effective UID is 0, a #, otherwise a $
\nnn   the character corresponding to the octal number nnn
\\     a backslash
\[     begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt
\]     end a sequence of non-printing characters

References:
1. bash manual

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Mar 14, 2011

Anatomy of a process stack

A process is a program in execution. A process may have one or more threads executing different sections of the program. Every thread in the process maintains it's own separate stack, registers and program counter.

Key Points
A stack is a collection of stack frames where each stack frame refers to a function call

Stack Frame
When a function call is made, a block of memory is set aside which stores information about the following:
1. Arguments passed to the function
2. Return address of the caller function
3. Local variables used by the function and
4. Other temporary variables needed by the compiler
This block of memory is called a stack frame. When the function returns, the stack frame is turned invalid and reclaimed.

Registers used in a Stack Frame
1. ESP (Extended Stack Pointer) : 32 bit register that points to the top of the stack. All the addresses lower than the stack pointer are considered unused or garbage and all the higher addresses are considered valid
2. EBP (Extended Base Pointer) : Also known as Frame Pointer, this 32 bit register is used to reference all the function parameters and the local variables in the current stack frame

ebp + 4 + 4 * n points to the address at which the nth argument of the function is stored
ebp + 4 points to the address at which return address of the caller function is stored
ebp is the frame pointer of the callee function
ebp - 4n refers to the address where the nth local variable is stored

3. EIP (Extended Instruction Pointer) : This register holds the address of the next instruction to be executed. It is saved on to the stack as part of the CALL instruction. Also known as Program Counter
4. EAX, EBX, ECX, EDX : General purpose registers for storing intermediate results

Following figure shows how stack frames are laid out in memory during execution


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Mar 6, 2011

A simple Makefile example

Continuing our series on makefile tutorial, below is a simple makefile which compiles the Hello World program Main.cpp into an object file Main.o and generates the binary executable Main.exe

#-----------------------------------------------------------------------
#                   A Simple Makefile example
#-----------------------------------------------------------------------
PROJECT := Main

SRCEXT := .h .cpp
OBJEXT := .o
EXEEXT := .exe

#compilers
CC := gcc
CXX := g++


#flags
CFLAGS := -o
CXXFLAGS := -o
LDFLAGS := -o


#disable implicit suffix rules
.SUFFIXES:
.SUFFIXES: $(SRCEXT) $(OBJEXT) $(EXEEXT)

SRC := \
    Main.cpp

OBJ := \
    $(SRC:.cpp=.o)

EXE := \
    $(PROJECT)$(EXEEXT)

#define dummy targets
.PHONY: all clean compile link

all : compile link
    
clean :
    @echo
    @echo "Cleaning Temporary Files..."
    $(RM) $(OBJ) $(EXE)

compile : $(OBJ)

link : $(EXE)

$(OBJ) :
    @echo
    @echo "Compiling Source Files..."
    $(CXX) $(CXXFLAGS) $(OBJ) $(SRC)

$(EXE) :
    @echo
    @echo "Linking..."
    $(LD) $(OBJ) $(LDFLAGS) $(EXE)

run :
    @echo
    @$(EXE)

The Hello World program
#include <iostream>

int main()
{
    printf("Hello World");
    return 0;
}

Output

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Mar 4, 2011

Makefile tutorial for beginners

The purpose of the make utility is to determine automatically which pieces of your large program needs to be re-compiled, and issue the commands to recompile them. The make program uses the makefile instructions and the last modified time of the files to decide which of the files needs to be updated.
A makefile tells make what to do. The make utility executes commands in the makefile to update one or more targets. If -f option is not provided, make looks for the makefiles GNUmakefile, makefile, and Makefile, in that order.

Key Points
1. Make is a software engineering tool which helps simplifies the process of software development
2. Makefiles can be used to automate the build process (Generation of binary from source files)
3. The programmer does not need to type the complex compiler commands and flags to be used during compilation

1. Variables
1. A variable in a makefile can be a recursively expanded variable NAME = value or a simply expanded variable NAME := value
2. The variable's value can be accessed using $(NAME) or ${NAME}
3. As a convention, the variable names are always written in uppercase.
4. Variable names are case-sensitive and should not contain other than letters, numbers and underscores as they may have a special meaning
5. Setting a variable if not already set use ?= e.g. NAME ?= value

Note: You cannot append a value at the end of the recursively expanded variable using CC = $(CC) -O2. This will end up in an infinite loop as make will continue expanding this variable until it cannot be expanded anymore. To overcome this problem, we use simply expanded variables to append values at the end as shown

CC := g++ -o
CC += $(CC) -O2

2. Explicit Rules
Explicit rules tell make which files depend on the compilation of other files and the commands required to compile those files. They take the following form:

targetfile : dependentfiles
[TAB]commands
...
[TAB]commands 

3. Implicit Rules or Suffix Rules
Implicit rules are similar to explicit rules except that they are listed without commands. The make utility makes uses of the file suffixes to determine what commands to execute on the source files

hello.o : hello.c
Running make will cause the following command to be executed cc -c -o hello.o hello.c

Common variables used by implicit rules:

AR  : archive program, default ar
CC  : compiling c program, default cc
CXX : compiling c++ program, default g++
CPP : c preprocessor, default $(CC) -E
RM  : remove, default rm -f

Flags used by the programs above

ARFLAGS  : flags for the archive program, default rv
CFLAGS   : flags for the c compiler
CXXFLAGS : flags for the c++ compiler
CPPFLAGS : flags for the c preprocessor
LDFLAGS  : flags for the linker

Some of the old fashioned suffix rules for C and C++ are:

Compiling C Programs
file.o is automatically built from file.c. $(CC) -c $(CPPFLAGS) $(CFLAGS)

Compiling C++ Programs
file.o is automatically built from file.cc. $(CXX) -c $(CPPFLAGS) $(CXXFLAGS)

Linking Object Files
file is automatically generated from file.o by running the linker (ld). $(CC) $(LDFLAGS) file.o $(LOADLIBS) $(LDLIBS)

The default suffix list for common C and C++ programs include .out, .a, .o, .c, .cc, .C, .def, .h. The complete list can be found out here

The above catalogue of implicit rules are always available unless makefile explicitely overrides them. Running make with -r or --no-builtin-rules option cancels all predefined rules

Implicit rules can also be disabled by disabling default suffixes and having only the suffixes you need

.SUFFIXES:    # Delete the default suffixes
.SUFFIXES: .c .cpp .o .h       # Define our suffix list

Inference Rules: Inference rules are rules distinguished by the use of the character “%” in the dependency line. The “%” (rule character) is a wild card, matching zero or more characters. As an example, here is an inference rule for building .obj files from .c files:
%.obj : %.c
        $(CC) $(CFLAGS) –c $(.SOURCE) 

4. Phony Targets
A phony target is one that is not really the name of the target file. It is a request for executing a set of commands that should not create the target file name. As the target file will never exist, the commands will be executed always e.g.

clean:
        rm *.o temp

You can also explicitely declare a target as phony using the special target .PHONY

.PHONY = clean

5. Comments
A comment starts with # character in a makefile

6. Substitution References
A substitution reference has the form $(file:.c=.o). The below example sets OBJFILES to a.o b.o c.o

SRCFILES = a.c b.c c.c
OBJFILES = $(SRCFILES:.c=.o)
SRCFILES = a.c b.c c.c
OBJFILES = $(SRCFILES:%.c=%.o)

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Mar 2, 2011

Producer and Consumer Problem revisited with Events

The producer and consumer problem we visited earlier used CRITICAL_SECTION for synchronization and the threads were spinning and checking over the queue size to add or remove the elements. In this post, we are going to use Windows Event objects for synchronizing the Producer and Consumer threads.

Key Points
1. Events are kernel objects which contain an usage count, flag indicating the type of event (manual-reset or auto-reset) and another flag indicating the state (signaled or non-signaled)
2. Applications use Event Objects to notify the waiting threads that an operation has been completed
3. An event is signaled using the call
SetEvent(Handle hEvent)

4. When a manual-reset event is signaled, all threads waiting on the event become schedulable. When an auto-reset event is signaled, only one of the threads waiting on the event becomes schedulable

Event uses five functions CreateEvent, OpenEvent, SetEvent, ResetEvent, and PulseEvent. The functions I will be using in my examples are:

HANDLE CreateEvent(
  LPSECURITY_ATTRIBUTES lpEventAttributes,
  BOOL bManualReset,
  BOOL bInitialState,
  LPCTSTR lpName
);
BOOL SetEvent(
  HANDLE hEvent
);
BOOL ResetEvent(
  HANDLE hEvent
);

The Event class below is a wrapper over windows manual-reset Event Object, provides easy interface to use and also takes care of the cleanup activities for the event handles.
#ifndef _Event_H_
#define _Event_H_

//File: Event.h

#include <windows.h>
#include <process.h>
#include <iostream>

namespace examples
{
    class Event : public NonCopyable
    {
    public:
        Event()
        {
            m_evt = (HANDLE) ::CreateEvent(0, true, false, 0);
            if(NULL == m_evt)
            {
                std::cout <<  "ERROR: Cannot create event" << std::endl;
            }
        }

        ~Event()
        {
            ::CloseHandle(m_evt);
        }

        bool wait(size_t timeout)
        {
            bool retval = false;

            switch(::WaitForSingleObjectEx(m_evt, timeout == 0 ? INFINITE : timeout, false))
            {
            case WAIT_OBJECT_0:
                retval = true;
                break;
            default:
                std::cout << "ERROR: Wait Failed " << ::GetLastError() << std::endl;
                break;
            }
            ::ResetEvent(m_evt);

            return retval;
        }

        bool signal()
        {
            return ::SetEvent(m_evt);
        }

    private:
        HANDLE m_evt;
    };
}

#endif //_Event_H_
The Shared Message Queue Class
#ifndef _MQ_H_
#define _MQ_H_

//File: MQ.h

#include <iostream>
#include <deque>
#include "Lock.h"
#include "Event.h"

using namespace std;

namespace examples
{
    class MQ
    {
    public:
        MQ(size_t size = 10) : m_max_size(size){}

        ~MQ()
        {
            m_q.clear();
        }

        void add(const string& elem)
        {
            m_qlock.acquire();
            while(isFull())
            {
                m_qlock.release();
                m_evtProducer.wait(0);
                m_qlock.acquire();
            }
            m_q.push_back(elem);
            debug_print("Producer");
            m_evtConsumer.signal();
            m_qlock.release();
        }

        void remove()
        {
            m_qlock.acquire();
            while(isEmpty())
            {
                m_qlock.release();
                m_evtConsumer.wait(0);
                m_qlock.acquire();
            }
            m_q.pop_front();
            debug_print("Consumer");
            m_evtProducer.signal();
            m_qlock.release();
        }

        bool isEmpty() const
        {
            return m_q.size() == 0;
        }

        bool isFull() const
        {
            return m_q.size() >= m_max_size - 1;
        }

    private:

        void debug_print(const std::string& str) const
        {
            std::cout << str.c_str() << "  [" << ::GetCurrentThreadId() << "] Size=[" << m_q.size() << "]";
            std::cout << endl;
        }

        const size_t   m_max_size;
        deque<const string>   m_q;
        Lock           m_qlock;
        Event          m_evtProducer;
        Event          m_evtConsumer;
    };

}

#endif //_MQ_H_

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