how to periodically read contents on stdout redirected to file descriptor using dup2

Question

Hi, I am struggling to continuously read contents of stdout which is redirected to file. please help me to resolve issue.

My problem in detail is that i have two thread in first thread , i have to execute a c file which is continously writing output to stdout(i.e writing hello world in infinite loop) and in second thread i have to retrieve line by line output periodically from stdout and store in buffer. For second thread implementation i created pipe and redirected stdout to the write end of this pipe and tried to continuously read content of this pipe line by line but i didnt get expeced result. code for second thread is as below:

int redirectPipe[2];
//creation of pipe
pipe(redirectPipe);
//redirection of pipe
dup2(redirectPipe[1],STDOUT_FILENO);
//reading continuously line by line from pipe
// some code here

Can somebody please help me in coding how to continuously read line by line from this pipe?

Answer 1

Below is the code: please suggest me is my approach right? and how to implement entryOpThread() method so that it will read contents of stdout periodically whether should i use read() system call or getLine()or any other method to read line by line from pipe(stdout is redirected to this pipe). In this example exec thread is executing test.c which is continuouslly printing hello world on stdout,so according to my requirement i have to read this output line by line("Hello world") continuouslly in Opthread from pipe and store it in buffer

//************CODE********************************

include

include

include

include

const int MAX_LENGTH=100; using namespace std;

//! file descriptor to duplicate the system standard input //! int inputPIPE[2];

//! file descriptor to duplicate the system standard output int OutputPipe[2];

//!Thread to retrieve output //! pthread_t OpThread;

//!Thread to execute script //! pthread_t ExecThread;

//! entry point of the exec thread void *entryExecThread(void * obj) { //create a child process to launch the script int lPid; lPid=fork(); //connect read end of the pipe to the standard input if(dup2(inputPIPE[0],STDIN_FILENO)==-1) { //throw exception }

//connect write end of the pipe to the standard output
if(dup2(OutputPipe[1],STDOUT_FILENO)==-1)
{
    //throw exception
}
if(lPid==0)
{



    execvp("./test",NULL);
}

else
{
    int mProcessId=lPid;
    waitpid(mProcessId,NULL,0);
}
return NULL;

}

//! entry point of the output thread void *entryOpThread(void * obj) { //read contents of stdout periodically /* char *lMssg=new char[MAX_LENGTH]; read(OutputPipe[0],lMssg,MAX_LENGTH);

FILE *fd=fdopen(OutputPipe[0],"r");
int lBufferLength=MAX_LENGTH;
 while(true)
    {
        getline(&lMssg,(size_t * )&lBufferLength,fd);
        // push message to the output buffer
        if (lMssg !=NULL)
        {
            pthread_mutex_lock( &mOutputBufferMutex);
            mOutputBuffer.push(lMssg);
            pthread_mutex_unlock( &mOutputBufferMutex);
        }
    }
 */

} int main() {

//call to pipe system call
if(pipe(inputPIPE)==-1)
{
    printf("ERROR IN OPENING FILE \n");
}
if(pipe(OutputPipe)==-1)
{
    printf("ERROR IN OPENING FILE\n ");
}

//CREATE OUTPUT THREAD TO RETRIEVE OUTPUT

if (pthread_create(&OpThread,0,entryOpThread,NULL)!=0)
{

    //throw exception
    printf("Creation of Output thread  failed for Script with ID\n");
}

//Create execution thread to launch script
pthread_t execThread;
if (pthread_create(&ExecThread,0,entryExecThread,NULL)!=0)
{

    //Stop output thread
    pthread_cancel(OpThread);
    //throw exception
    printf("Creation of Execution thread  failed for Script with ID\n");
}

} //************************************************888 test.c

include

include "string.h"

//using namespace std;

int main() { int i=0; while(i<500) { printf("!!!Hello World!!! \n" ); // prints !!!Hello World!!! //extracting header and data from message sleep(5); i++; }
return 0; } //***********************************************