sorting random numbers sent to child using exec - c++

Question

What I am trying to do is send random numbers generated by a parent and then sent to the child, who then execs "sort -nr", and then sends back the sorted numbers back to the parent. I found this question had already been asked and answered here pretty similar to mine: how to redirect output of "sort" program from child to parent, and I thought I did everything that it said to get it to work, but I am not able to get the sorting to actually happen. I've even checked to see if it errors out, but I have gotten nothing.
Both Pipes send and receive the same numbers, but they never come out sorted. What Am I missing?
int pipe1[2], pipe2[2];
pid_t childID;
if (pipe(pipe1) < 0 || pipe(pipe2) < 0) {
perror("pipe");
exit(EXIT_FAILURE);
}
childID = fork();
if (childID < 0) {
//Child Process Failure
perror("fork");
exit(EXIT_FAILURE);
}
else if (childID == 0){
//Child Process Instructions
cout << "Sent Numbers: " << endl;
//Closes Unused Pipes
close(pipe1[WRITE_END]);
close(pipe2[READ_END]);
//Dups Over the Others, then closes them
dup2(pipe1[READ_END], STDIN_FILENO);
close(pipe1[READ_END]);
dup2(pipe2[WRITE_END], STDOUT_FILENO);
close(pipe2[WRITE_END]);
int fail = execlp("sort", "sort", "-nr", (char *)NULL);
cout << fail << endl;
}
else {
//Parent Process Instructions
//Close Unused Pipes
close(pipe1[READ_END]);
close(pipe2[WRITE_END]);
srand(randSeed);
cout << "Random Numbers: " << endl;
for (int i = 0; i < nWorkers; i++){
//Generate nWorker numbers, then Write
randNumbers[i] = rand() % (sleepMax - sleepMin + 1) + sleepMin;
write(pipe1[WRITE_END], &randNumbers[i], sizeof(randNumbers[i]));
cout << randNumbers[i] << endl;
}
close(pipe1[WRITE_END]);
wait(NULL);
cout << "SORTED NUMBERS:" << endl;
double sortedNumbers[nWorkers];
int n;
for(int k = 0; k < nWorkers; k++) {
n = read(pipe2[READ_END], &sortedNumbers[k], sizeof(sortedNumbers[k]));
cout << sortedNumbers[k] << ", " << n << endl;
}
}

Answer 1

sort(1) expects its input to be ASCII strings, not raw binary numbers. When you're passing it the data with write(2), that's writing out the raw binary representation of the numbers to the pipes, which is not what you want. You need to convert the numbers to their string representations.
One way to do that would be to open a stdio stream on top of the pipe with fdopen(3). You could then use fprintf to write the formatted data:
FILE *childInput = fdopen(pipe1[WRITE_END], "w");
if (childInput == NULL) { /* Handle error */ }
for (...)
{
...
fprintf(childInput, "%d\n", randNumbers[i]);
}
fclose(childInput);
Likewise, you need to do the same thing when reading back in the output from the child:
FILE *childOutput = fdopen(pipe2[READ_END], "r");
if (childOutput == NULL) { /* Handle error */ }
while (fscanf(childOutput, "%d", &sortedNubers[i]) == 1)
{
...
}
fclose(childOutput);