Unable to send message to parent process after multiple forks - c++

Question

I have a program that forks off four processes and calls execlp() to run different code for the child. I pass the child a number as an id. So far, all the child does is try to pass the id back to the parent process. The pipes work, if i put a string though the stream it prints out in the parent process. However, when i try to put the id as an int thought the stream, it does not work. I dont even get to the line of code after the fprintf() and fflush() command in the child.
I made some changes for how i created the file descriptors and added more code for an example. Now, in the child, i am unable to create the FILE* out. However, if i create out on file descriptor 1, it does print to the screen. I tried creating out on file descriptor 3 and the program just sits there and waits for input from the child that never comes.
Here is my parent:
Mom::Mom():childCount(0)
{
pipeCount = fileCount = 0;
int fd[2];
srand(time(NULL));
for(int c=0; c<NUMJOBS; ++c) jobs[c] = newJob();
//createFileDescriptors(fd);
ret = pipe(fd);
if(ret < 0) fatal("Error creating pipes");
//cout << fd[0] << "\t" << fd[1] << endl;
pipes[fileCount++] = fdopen(fd[0], "r");
fcntl( 3, F_SETFD, 0 );
//close(fd[1]);
//for(int c=3; c<FILEDESCRIPTORS; c+=2) pipes[pipeCount++] = fdopen(c, "w");
createChildren();
for(int c=0; c<4; c++)
{
int tmp = -1;
//cout << "About to read from children, tmp = " << tmp << endl;
ret = fscanf(pipes[0], "%d", &tmp);
//char* buffer = (char*) malloc(80*sizeof(char));
//char buffer[80];
//read(3, buffer, 80);
cout << ret << "\t" << tmp << endl;
//cout << ret << " " << tmp << endl;
//free(buffer);
}
//sleep(5);
}
/*------------------------------------------------------------------------------
Create all the children by using fork() and execlp()
----------------------------------------------------------------------------*/
void Mom::createChildren()
{
int fd[2];
fcntl( fd[IN], F_SETFD, 0 );
for(int c=0; c<NUMCHILDREN; c++)
{
ret = pipe(fd);
if(ret < 0) fatal("Error creating pipes");
int pid = fork();
//cout << pid << endl;
if(pid == 0)
{
setupChild(c, fd);
}
else
{
//close(fd[1]);
}
}
}
/*------------------------------------------------------------------------------
set up the child and call exec to run ChildMain
----------------------------------------------------------------------------*/
void Mom::setupChild(int count, int fd[])
{
//cout << "Creating child with id: " << count << endl;
char cnt = '0' + count;
string id_str (&cnt + '\0');
fcntl( fd[0], F_SETFD, 0 );
pipes[fileCount++] = fdopen(fd[1], "w");
//execlp("ChildMain", "ChildMain", id_str.c_str(), NULL);
execlp("ChildMain", id_str.c_str(), NULL);
}
And here is the child code:
int main(int argc, char* argv[])
{
//cout << argv[argc-1] << endl;
if(argc < 1) fatal("Not enough arguments provided to ChildMain");
int id = atoi(argv[argc-1]);
//cout << *argv[1] << " " << id << endl;
//redirect STDIN and STDOUT
/*int c_in = dup(0);
close(0);
dup((2*id) + 5);
int c_out = dup(1);
close(1);
dup(4);*/
/////////////////////////////
//Child kid((int) *argv[1]);
FILE* out = fdopen(4, "w");
if(out == NULL)
cout << "Error opening stream to parent in child: " << id << endl;
//char childID = '0' + id;
//char buf[80];
//strcpy(buf, "Child ");
//strcat(buf, &childID);
string buf ("Child");
//cout << tmp << " " << childID << endl;
//write(4, buf.c_str(), buf.length()+1);
//cout << id << endl;
int ret = fprintf(out, "%d", id);
fflush(out);
//fclose(out);
//cout << id << " " << ret << endl;
//ch.push_back((char) id);
//put STDIN and STDOUT back to correct file descriptors
/*close(1);
dup(c_out);
close(0);
dup(c_in);*/
////////////////////////////////////////////////////////
return 0;
}
I am very confused why this works for the first child, with id 0, but no the others. Does anyone know what is wrong with my code?

Answer 1

execlp(3) is expecting null terminated strings as it's args. &cnt won't be null terminated.
Simple fix:
void Mom::setupChild(int count, int fd[])
{
char cnt[2];
cnt[0] = '0' + count;
cnt[1] = '\0';
fcntl( fd[(2*count)+3], F_SETFD, 0 );
execlp("ChildMain", "ChildMain", &cnt, NULL);
}
This doesn't scale to 10 processes though, so I'd probably use a buffer and just sprintf() into it.

Answer 2

Here is a small example on how to implement the suggestion in my comment:
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
int main()
{
/* Need two sets of pipes: one for child stdin, one for child stdout */
int pipefds1[2];
int pipefds2[2];
pipe(pipefds1);
pipe(pipefds2);
int rc = fork();
if (rc == -1)
perror("fork");
else if (rc == 0)
{
/* In child */
/* Close the old stdin and stdout */
close(STDIN_FILENO);
close(STDOUT_FILENO);
/* Create new stdin/stroud from the pipes */
dup2(pipefds1[0], STDIN_FILENO);
dup2(pipefds2[1], STDOUT_FILENO);
/* Close the unneeded pipe handles */
close(pipefds1[1]);
close(pipefds2[0]);
/* Now pass control to the new program */
execl("/bin/ls", "ls", "-l", "/", NULL);
}
else
{
/* In parent */
/* Close the uneeded pipe handles */
close(pipefds1[0]);
close(pipefds2[1]);
/* We want to use stdio functions */
FILE *fp = fdopen(pipefds2[0], "r");
/* Read all from the child */
char buffer[128];
while (fgets(buffer, sizeof(buffer), fp))
{
printf("Input from child: %s\n", buffer);
}
fclose(fp);
/* Wait for child to exit */
wait(NULL);
}
return 0;
}
Hopefully this will be enough for you to build on.
The error handling is non-existant, but it is tested.