posix_spawnp and piping child output to a string

posix_spawn is interesting and useful, which makes this question worth necromancing -- even if it is no longer relevant to the OP.

There are some significant bugs in the code as posted. I suspect that some of these were the result of hacking in desperation, but I don't know which was the original bug:

1. The args array does not include the argv[0] that would represent the executable name. This results in the echo program never seeing the intended argv[1] ("bla").
2. The read() function is called from different places in a way that just doesn't make sense. A correct way to do this would be to only call read as part of the control expression for the while loops.
3. waitpid() is called before reading from the pipes. This prevents the I/O from completing (in non-trivial cases at least).
4. A more subtle issue with this code is that attempts to read all of the child's stdout before reading anything from stderr. In principle, this could cause the child to block while attempting to write to stderr, thus preventing the program from completing. Creating an efficient solution to this is more complicated as it requires that you can read from whichever pipe has available data. I used poll() for this. Another approach would be to use multiple threads.

Additionally, I have used sh (the command shell, i.e. bash) as the child process. This provides a great deal of additional flexibility, such as running a pipeline instead of a single executable. In particular, though, using sh provides the simple convenience of not having to manage the parsing of the command-line.

/*BINFMTCXX: -std=c++11 -Wall -Werror*/#include <spawn.h> // see manpages-posix-dev#include <poll.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <string.h>#include <sys/wait.h>#include <iostream>#include <string>#include <vector>using namespace std;int main(){  int exit_code;  int cout_pipe[2];  int cerr_pipe[2];  posix_spawn_file_actions_t action;  if(pipe(cout_pipe) || pipe(cerr_pipe))    cout << "pipe returned an error.\n";  posix_spawn_file_actions_init(&action);  posix_spawn_file_actions_addclose(&action, cout_pipe[0]);  posix_spawn_file_actions_addclose(&action, cerr_pipe[0]);  posix_spawn_file_actions_adddup2(&action, cout_pipe[1], 1);  posix_spawn_file_actions_adddup2(&action, cerr_pipe[1], 2);  posix_spawn_file_actions_addclose(&action, cout_pipe[1]);  posix_spawn_file_actions_addclose(&action, cerr_pipe[1]);//string command = "echo bla"; // example #1  string command = "pgmcrater -width 64 -height 9 |pgmtopbm |pnmtoplainpnm";  string argsmem[] = {"sh","-c"}; // allows non-const access to literals  char * args[] = {&argsmem[0][0],&argsmem[1][0],&command[0],nullptr};  pid_t pid;  if(posix_spawnp(&pid, args[0], &action, NULL, &args[0], NULL) != 0)    cout << "posix_spawnp failed with error: " << strerror(errno) << "\n";  close(cout_pipe[1]), close(cerr_pipe[1]); // close child-side of pipes  // Read from pipes  string buffer(1024,' ');  std::vector<pollfd> plist = { {cout_pipe[0],POLLIN}, {cerr_pipe[0],POLLIN} };  for ( int rval; (rval=poll(&plist[0],plist.size(),/*timeout*/-1))>0; ) {    if ( plist[0].revents&POLLIN) {      int bytes_read = read(cout_pipe[0], &buffer[0], buffer.length());      cout << "read " << bytes_read << " bytes from stdout.\n";      cout << buffer.substr(0, static_cast<size_t>(bytes_read)) << "\n";    }    else if ( plist[1].revents&POLLIN ) {      int bytes_read = read(cerr_pipe[0], &buffer[0], buffer.length());      cout << "read " << bytes_read << " bytes from stderr.\n";      cout << buffer.substr(0, static_cast<size_t>(bytes_read)) << "\n";    }    else break; // nothing left to read  }  waitpid(pid,&exit_code,0);  cout << "exit code: " << exit_code << "\n";  posix_spawn_file_actions_destroy(&action);}