void PcapFile::writePackets(const std::vector<PcapPkt>& pkts) {
folly::fbvector<PktHeader> hdrs;
hdrs.reserve(pkts.size());
folly::fbvector<struct iovec> iov;
// Reserve enough space, assuming each packet is in a single IOBuf.
// If some packets are split across IOBuf chains then we will end up
// allocating more space as needed in the loop below.
iov.reserve(pkts.size() * 2);
// Build iovecs for all of the packet headers and data
for (const auto& pkt : pkts) {
hdrs.emplace_back(pkt);
PktHeader* curHdr = &hdrs.back();
iov.push_back({(void*)curHdr, sizeof(PktHeader)});
pkt.buf()->appendToIov(&iov);
}
int ret = writevFull(file_.fd(), iov.data(), iov.size());
folly::checkUnixError(ret, "error writing pcap data");
}
int writeFileAtomicNoThrow(
StringPiece filename,
iovec* iov,
int count,
mode_t permissions) {
// We write the data to a temporary file name first, then atomically rename
// it into place. This ensures that the file contents will always be valid,
// even if we crash or are killed partway through writing out data.
//
// Create a buffer that will contain two things:
// - A nul-terminated version of the filename
// - The temporary file name
std::vector<char> pathBuffer;
// Note that we have to explicitly pass in the size here to make
// sure the nul byte gets included in the data.
constexpr folly::StringPiece suffix(".XXXXXX\0", 8);
pathBuffer.resize((2 * filename.size()) + 1 + suffix.size());
// Copy in the filename and then a nul terminator
memcpy(pathBuffer.data(), filename.data(), filename.size());
pathBuffer[filename.size()] = '\0';
const char* const filenameCStr = pathBuffer.data();
// Now prepare the temporary path template
char* const tempPath = pathBuffer.data() + filename.size() + 1;
memcpy(tempPath, filename.data(), filename.size());
memcpy(tempPath + filename.size(), suffix.data(), suffix.size());
auto tmpFD = mkstemp(tempPath);
if (tmpFD == -1) {
return errno;
}
bool success = false;
SCOPE_EXIT {
if (tmpFD != -1) {
close(tmpFD);
}
if (!success) {
unlink(tempPath);
}
};
auto rc = writevFull(tmpFD, iov, count);
if (rc == -1) {
return errno;
}
rc = fchmod(tmpFD, permissions);
if (rc == -1) {
return errno;
}
// Close the file before renaming to make sure all data has
// been successfully written.
rc = close(tmpFD);
tmpFD = -1;
if (rc == -1) {
return errno;
}
rc = rename(tempPath, filenameCStr);
if (rc == -1) {
return errno;
}
success = true;
return 0;
}
