buffer-then-echo.cpp

#include <cstdio>
#include <iostream>
#include <vector>

static const int BUFFER_READ_SIZE = 8;
static const int MAX_READ_SIZE = 4 * 1024 * 1024; // 4MiB

int main() {
  std::vector<char> echoData;
  char stdinBuffer[BUFFER_READ_SIZE];
  FILE *standardInput = fdopen(0, "rb");

  int bufferBytesToSave;

  // Keep looping while we are within the MAX_READ_SIZE and there are bytes in
  // the buffer
  while ((echoData.size() < MAX_READ_SIZE) &&
         (bufferBytesToSave = fread(&stdinBuffer[0], sizeof stdinBuffer[0],
                                    BUFFER_READ_SIZE, standardInput))) {

    // Loop until we have written all the bytes written to the buffer AND this
    // buffer doesn't take us beyond MAX_READ_SIZE. I am intentionally checking
    // in both places. There is probably an optimization to calculate and use:
    //      (MAX_READ_SIZE - (echoData.size + bufferBytesToSave))?
    for (int i = 0; i < bufferBytesToSave && echoData.size() < MAX_READ_SIZE;
         i++) {
      echoData.push_back(stdinBuffer[i]);
    }
  }

  // trim trailing newline
  if (echoData.back() == 0x0a) {
    echoData.pop_back();
  }

  // write echoData
  for (const auto &datum : echoData) {
    std::cout << datum;
  }

  return 0;
}