How to send std::string array between processes C++ - c++

Question

In C++, I need to start a secondary program from a primary program, sending the second some arguments. I need to return the data produced by the secondary program to the primary program. In this case, the data happens to be a two-dimensional std::string array; we'll call it stringArray. This is easy enough to do:
// snippet from Primary
std::string executionString ("./secondaryProgram arg1 arg2 arg3");
system(executionString);
What I don't know how to do is get the data that Secondary Program produces back to the Primary program (short of writing to a temporary file from Secondary and then reading the file from Primary).
In other words, it would be great if I could do something like:
// snippet from Primary
std::string stringArray[2][3];
stringArray = system(executionString);
I'm not hoping for a solution as simple as this or working code from anyone, any nudge in the right direction is appreciated.
I cannot use sockets for this purpose. I have not been able to figure out how to build a pipe between std::cout and std::cin that works for this case. My only real constraint is that my solution involve system() somehow.

Answer 1

system() does not create pipes to the child process. The child process inherits the parent's standard in, standard out, and standard error descriptors.
On Linux, you can use popen() if you want access to the child's stdin or stdout.
Since you have to use system(), you could
have the secondary program store its results in a file. Your main program would then open the file after the system completes. Sort of like this:
std::string executionString ("./secondaryProgram arg1 arg2 arg3 > output_file.txt");
system(executionString);
std::ifstream result("output_file.txt");
while( result >> str) {
result_vector.push_back(str);
}

Answer 2

Take a look at boost.interprocess. It contains many utilities that can be used for IPC in a portable way.
If you don't want to rely on boost, you can do something like this. Compile with C++11 mode and -pthread GCC option.

Answer 3

Why don't you write the relevant information in the second process to a file, and then read that file in the first process. Seems weird to do it this way, but I think it meets your professor's criteria, at least the parts you've shared with us.

Answer 4

Boost Interprocess should work for you. It supports message queues between threads on different processes.

Answer 5

You can use pipes to communicate. The link provided has examples for linux, but it's very similar to what you would write for windows.
If you need to send arbitrary data that might change at run time you might consider serializing the data sent over the pipes and deserializing it at the receiver. You might use XML, JSON, or something like Protobuf. If you make it human readable that adds the opportunity to reuse components or debug what's happening using the eyeball mark 1

Answer 6

Alright here's what I ended up doing.
"translate"
#include <iostream>
#include <fstream>
#include <vector>
#include <sstream>
#include <algorithm>
#include <cstdlib>
#include <unistd.h>
std::vector<std::string> sortTerms(int n, char* argv[]) {
std::vector<std::string> sortedTerms (n);
for (int i = 0; i < n; i++) {
sortedTerms[i] = argv[i+1]; // first term argv is program name
}
std::sort(sortedTerms.begin(),sortedTerms.end());
return sortedTerms;
}
std::vector<std::string> splitString(int n,std::string str) {
std::vector<std::string> stringVector (n);
std::istringstream iss(str);
for (int i = 0; i < n; i++)
std::getline(iss, stringVector[i], ' ');
return stringVector;
}
int main(int argc, char** argv) {
const int NUM_TERMS = (argc - 1); // number of words to translate
std::string output[NUM_TERMS][2]; // used to store a translated word alongside the English equivalent
std::string stringBuffer; // used to start dictionary with arguments
std::vector<std::string> stringVector (NUM_TERMS); // used as a buffer
std::ofstream outputFile("translated.txt"); // file to write translations to
const bool VERBOSE = true;
stringBuffer.clear();
stringBuffer.append("./dictionary ");
// Sort English words and load them into output
stringVector = sortTerms(NUM_TERMS, argv);
for (int i = 0; i < NUM_TERMS; i++) {
output[i][0] = stringVector[i];
stringBuffer = stringBuffer.append(stringVector[i]);
stringBuffer = stringBuffer.append(" ");
}
int pipeStatus;
int pipeOutput[2]; // file descriptor
pipeStatus = pipe(pipeOutput); // create output read/write pipe ends
if (pipeStatus < 0) {
std::cerr << "ERROR CREATING PIPE" << std::endl;
exit(1);
}
int pid = 0;
pid = fork();
if (pid == 0) { // dictionary
// Connect the pipes
dup2 (pipeOutput[1],1);
// Execute the program
system(stringBuffer.c_str());
// Close pipes
close(pipeOutput[0]);
close(pipeOutput[1]);
exit(0);
}
else if (pid > 0) { // Original process
char* buffer = new char[1024]; // input buffer
// Receive string from dictionary
read(pipeOutput[0],buffer,1024); // read in from output of dictionary
stringBuffer = buffer; // I'd rather work with a std::string
stringVector = splitString(NUM_TERMS, stringBuffer);
for (int i = 0; i < NUM_TERMS; i++)
output[i][1] = stringVector[i];
// Close pipes
close(pipeOutput[0]);
close(pipeOutput[1]);
if (VERBOSE) {
for (int i = 0; i < NUM_TERMS; i++)
std::cout << output[i][0] << " -> " << output[i][1] << std::endl;
}
// write translationString to file
for (int i = 0; i < NUM_TERMS; i++) {
outputFile.write(output[i][0].c_str(),output[i][0].length());
outputFile.write(" -> ",4);
outputFile.write(output[i][1].c_str(),output[i][1].length());
outputFile.write("\n",1);
}
outputFile.close();
exit(0);
}
else if (pid == -1) {
std::cerr << "ERROR FORKING PROCESS" << std::endl;
exit(1);
}
return 0;
}
"dictionary"
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
std::vector<std::string> splitString(std::string str)
{
std::vector<std::string> stringVector (2);
std::istringstream iss(str);
std::getline(iss, stringVector[0], ' ');
std::getline(iss, stringVector[1], ' ');
return stringVector;
}
int main(int argc, char* argv[]) {
const int NUM_TERMS = (argc - 1);
std::string stringBuffer;
std::string returnString[NUM_TERMS];
std::vector<std::string> stringVector;
std::ifstream dictionaryFile ("./dictionary.txt");
// There must be at least one arguement
if (argc <= 1)
std::cout << "Nothing to translate..." << std::endl;
for (int i = 0; i < NUM_TERMS; i++) {
while (dictionaryFile) {
getline(dictionaryFile,stringBuffer);
stringVector = splitString(stringBuffer);
if (stringVector[0] == argv[i+1]) { // wut
returnString[i] = stringVector[1];
break;
}
}
}
// clear string buffer
stringBuffer.clear();
// Form translated words string
for (int i = 0; i < NUM_TERMS; i++) {
stringBuffer.append(returnString[i]);
if (i < (NUM_TERMS - 1))
stringBuffer.append(" "); // append a space after each but the last term
}
// print translated words
std::cout << stringBuffer << std::endl;
dictionaryFile.close();
return 0;
}
"dictionary.txt"
Apple Apfel
Banana Banane
Blackberry Brombeere
Blueberry Heidelbeere
Cherry Kirsche
Fruit Obst
Grape Traube
Lemon Zitrone
Lime Limone
Orange Orange
Peach Pfirsich
Pear Birne
Plum Zwetschge
Raspberry Himbeere
Strawberry Erdbeere
meant to be run like $ ./dictionary Apple Orange Strawberry
produces "translated.txt"
Apple -> Apfel
Orange -> Orange
Strawberry -> Erdbeere
I've still got a bit of polishing to do before I turn it in, but that's the gist of it. Thanks guys!