text: /*
   * Copyright 2016 Facebook, Inc.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  #include <folly/portability/SysUio.h>
  
  #include <errno.h>
  #include <stdio.h>
  
  #include <folly/ScopeGuard.h>
  #include <folly/portability/SysFile.h>
  #include <folly/portability/Unistd.h>
  
  template <class F, class... Args>
  static int wrapPositional(F f, int fd, off_t offset, Args... args) {
    off_t origLoc = lseek(fd, 0, SEEK_CUR);
    if (origLoc == off_t(-1)) {
      return -1;
    }
    if (lseek(fd, offset, SEEK_SET) == off_t(-1)) {
      return -1;
    }
  
    int res = (int)f(fd, args...);
  
    int curErrNo = errno;
    if (lseek(fd, origLoc, SEEK_SET) == off_t(-1)) {
      if (res == -1) {
        errno = curErrNo;
      }
      return -1;
    }
    errno = curErrNo;
  
    return res;
  }
  
  #if !FOLLY_HAVE_PREADV
  extern "C" ssize_t preadv(int fd, const iovec* iov, int count, off_t offset) {
    return wrapPositional(readv, fd, offset, iov, count);
  }
  #endif
  
  #if !FOLLY_HAVE_PWRITEV
  extern "C" ssize_t pwritev(int fd, const iovec* iov, int count, off_t offset) {
    return wrapPositional(writev, fd, offset, iov, count);
  }
  #endif
  
  #ifdef _WIN32
  template <bool isRead>
  static ssize_t doVecOperation(int fd, const iovec* iov, int count) {
    if (!count) {
      return 0;
    }
    if (count < 0 || count > folly::kIovMax) {
      errno = EINVAL;
      return -1;
    }
  
    if (lockf(fd, F_LOCK, 0) == -1) {
      return -1;
    }
    SCOPE_EXIT { lockf(fd, F_ULOCK, 0); };
  
    ssize_t bytesProcessed = 0;
    int curIov = 0;
    void* curBase = iov[0].iov_base;
    size_t curLen = iov[0].iov_len;
    while (curIov < count) {
      int res = 0;
      if (isRead) {
        res = read(fd, curBase, (unsigned int)curLen);
        if (res == 0 && curLen != 0) {
          break; // End of File
        }
      } else {
        res = write(fd, curBase, (unsigned int)curLen);
        // Write of zero bytes is fine.
      }
  
      if (res == -1) {
        return -1;
      }
  
      if (res == curLen) {
        curIov++;
        if (curIov < count) {
          curBase = iov[curIov].iov_base;
          curLen = iov[curIov].iov_len;
        }
      } else {
        curBase = (void*)((char*)curBase + res);
        curLen -= res;
      }
  
      if (bytesProcessed + res < 0) {
        // Overflow
        errno = EINVAL;
        return -1;
      }
      bytesProcessed += res;
    }
  
    return bytesProcessed;
  }
  
  extern "C" ssize_t readv(int fd, const iovec* iov, int count) {
    return doVecOperation<true>(fd, iov, count);
  }
  
  extern "C" ssize_t writev(int fd, const iovec* iov, int count) {
    return doVecOperation<false>(fd, iov, count);
  }
  #endif
  