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Full Discussion: C++ stuck with dup2 and pipe
Top Forums Programming C++ stuck with dup2 and pipe Post 302938399 by ramono on Sunday 15th of March 2015 03:12:59 PM
Old 03-15-2015
C++ stuck with dup2 and pipe
What this code should do is: there are parent.cpp and child.cpp. Parent will send whatever is in the buffer to child and child will send back whatever received to parent. I do not know what I am doing wrong. I am confused what is missing in the parent and what else I should include into the child. Thanks.

Code:
//parent.cpp
//Check for fork error
 if ( (pid = fork()) < 0 )
{
    cerr << "FORK ERROR" << endl;
    return -3;
}
else  if (pid == 0)  // Child 
{
    close(fd1[1]);//Close parent's stdout-write
    close(fd2[0]);//Close child's stdin-read
    if (fd1[0] != STDIN_FILENO)//Make sure file desc. matches
    {
        if (dup2(fd1[0], STDIN_FILENO) != STDIN_FILENO)
        {
            cerr << "dup2 error to stdin" << endl;
        }
        close(fd1[0]);
    }
    if (fd2[1] != STDOUT_FILENO)//Make sure file desc. mathces
    {
        if (dup2(fd2[1], STDOUT_FILENO) != STDOUT_FILENO)
        {
            cerr << "dup2 error to stdout" << endl;
        }
        close(fd2[1]);
    }
    if ( execl("./child", "child", (char *)0) < 0 )
    {
        cerr << "system error" << endl;
        return -4;
    }
    return 0;
}//end of child
else //parent
{
    int rv;
   close(fd1[0]);//Close parent's read
   close(fd2[1]);//close child's write
   if ( write(fd1[1], buffer, strlen(buffer)) != strlen(buffer))
    {
        cerr << "Write ERROR FROM PIPE" << endl;
    }
    if ( (rv = read(fd2[0], buffer, MAXLINE)) < 0 )
    {
        cerr << "READ ERROR FROM PIPE" << endl;
    }
    else if (rv == 0)
    {
        cerr << "Child Closed Pipe" << endl;
        return 0;
    }
    cout << "Output of child is: " << buffer;
    return 0;
}//end of parent

//child.cpp
char line[1000];
int MAXLEN=1001;    
read(STDIN_FILENO, line, MAXLEN);

 

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LEARN ABOUT BSD

fork

FORK(2) 							System Calls Manual							   FORK(2)

NAME

       fork - create a new process

SYNOPSIS

       pid = fork()
       int pid;

DESCRIPTION

       Fork causes creation of a new process.  The new process (child process) is an exact copy of the calling process except for the following:

	    The child process has a unique process ID.

	    The child process has a different parent process ID (i.e., the process ID of the parent process).

	    The child process has its own copy of the parent's descriptors.  These descriptors reference the same underlying objects, so that, for
	    instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the  child
	    process  can affect a subsequent read or write by the parent.  This descriptor copying is also used by the shell to establish standard
	    input and output for newly created processes as well as to set up pipes.

	    The child processes resource utilizations are set to 0; see setrlimit(2).

RETURN VALUE

       Upon successful completion, fork returns a value of 0 to the child process and returns the process ID of the child process  to  the  parent
       process.   Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
       indicate the error.

ERRORS

       Fork will fail and no child process will be created if one or more of the following are true:

       [EAGAIN]       The system-imposed limit on the total number of processes under execution would be exceeded.  This limit	is  configuration-
		      dependent.

       [EAGAIN]       The  system-imposed limit MAXUPRC (<sys/param.h>) on the total number of processes under execution by a single user would be
		      exceeded.

       [ENOMEM]       There is insufficient swap space for the new process.

SEE ALSO

       execve(2), wait(2)

3rd Berkeley Distribution					   May 22, 1986 							   FORK(2)
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