Function readAtOffset

system/lib/wasmfs/syscalls.cpp:179–248

Internal read function called by __wasi_fd_read and __wasi_fd_pread Receives an open file state offset. Optionally sets open file state offset. TODO: combine this with writeAtOffset because the code is nearly identical.

// Optionally sets open file state offset.
// TODO: combine this with writeAtOffset because the code is nearly identical.
static __wasi_errno_t readAtOffset(OffsetHandling setOffset,
                                   __wasi_fd_t fd,
                                   const __wasi_iovec_t* iovs,
                                   size_t iovs_len,
                                   __wasi_size_t* nread,
                                   __wasi_filesize_t offset = 0) {
  auto openFile = wasmFS.getFileTable().locked().getEntry(fd);
  if (!openFile) {
    return __WASI_ERRNO_BADF;
  }

  auto lockedOpenFile = openFile->locked();

  if (setOffset == OffsetHandling::OpenFileState) {
    offset = lockedOpenFile.getPosition();
  }

  if (iovs_len < 0 || offset < 0) {
    return __WASI_ERRNO_INVAL;
  }

  // TODO: Check open file access mode for read permissions.

  auto file = lockedOpenFile.getFile()->dynCast<DataFile>();

  // If file is nullptr, then the file was not a DataFile.
  if (!file) {
    return __WASI_ERRNO_ISDIR;
  }

  auto lockedFile = file->locked();

  size_t bytesRead = 0;
  for (size_t i = 0; i < iovs_len; i++) {
    uint8_t* buf = iovs[i].buf;
    size_t len = iovs[i].buf_len;

    if (!buf && len > 0) {
      return __WASI_ERRNO_INVAL;
    }

    // TODO: Check for overflow when adding offset + bytesRead.
    auto result = lockedFile.read(buf, len, offset + bytesRead);
    if (result < 0) {
      // This individual read failed. Report the error unless we've already read
      // some bytes, in which case report a successful short read.
      if (bytesRead > 0) {
        break;
      }
      return -result;
    }

    // The read was successful.

    // Backends must only return len or less.
    assert(result <= len);

    bytesRead += result;