Example of how to intercept messages to stdout/stderr

intercept-stdio.cc

#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <assert.h>

#if defined(__unix__) || defined(__unix) || \
    (defined(__APPLE__) && defined(__MACH__))
#include <unistd.h>
#endif

#ifdef WIN32
#include <io.h>
#define pipe(X) _pipe(X, 4096, O_BINARY)
#define fileno _fileno
#define dup _dup
#define dup2 _dup2
#define read _read
#endif

#include "uv.h"

uv_thread_t stdout_thread;
uv_thread_t stderr_thread;
static bool process_alive_ = true;


static void stdio_dup(void* arg) {
  FILE* std_io = static_cast<FILE*>(arg);
  char buf[256];
  int fds[2];
  int res;

  int saved_stdio = dup(fileno(std_io));

  res = pipe(fds);
  assert(res == 0);

  res = dup2(fds[1], fileno(std_io));
  assert(res != -1);

// Windows does not buffer calls.
#if defined(__unix__) || defined(__unix) || \
    (defined(__APPLE__) && defined(__MACH__))
  setvbuf(std_io, nullptr, _IONBF, 0);
#endif

  while (process_alive_) {
    res = read(fds[0], buf, sizeof(buf) - 1);
    assert(res >= 0 && res < sizeof(buf));
    buf[res] = '\0';
    dprintf(saved_stdio, "buf => %s\n", buf);

    if (!process_alive_) break;
  }

  dup2(saved_stdio, fileno(std_io));
  close(saved_stdio);
  close(fds[1]);
  close(fds[0]);

  // TODO(trevnorris): Restore vbuf settings for std_io.
}


int main() {
  uv_thread_create(&stdout_thread, stdio_dup, stdout);
  uv_thread_create(&stderr_thread, stdio_dup, stderr);

  sleep(1);
  printf("sleep 1");
  sleep(1);
  printf("sleep 2");
  sleep(1);
  printf("sleep 3");

  process_alive_ = false;
  printf("\nbye stdout");
  fprintf(stderr, "\nbye stderr");

  uv_thread_join(&stdout_thread);
  uv_thread_join(&stderr_thread);

  printf("all done\n");

  return 0;
}

Good job @trevnorris!

But two threads? It's not like you're having to deal with Java or something ... ;-)