Dartium build error in Release mode - c++

Question

I was following http://code.google.com/p/dart/wiki/BuildingDartium#Build and attempted to build dartium in Release mode and encountered the following error:
$ ./dartium_tools/build.py --mode=Release
.
.
.
CXX(target) out/Release/obj.target/webrtc_video_coding/third_party/webrtc/modules/video_coding/main/source/rtt_filter.o
CXX(target) out/Release/obj.target/webrtc_video_coding/third_party/webrtc/modules/video_coding/main/source/session_info.o
CXX(target) out/Release/obj.target/webrtc_video_coding/third_party/webrtc/modules/video_coding/main/source/timestamp_extrapolator.o
third_party/webrtc/modules/video_coding/main/source/session_info.cc: In member function ‘int webrtc::VCMSessionInfo::PrepareForDecode(uint8_t*)’:
third_party/webrtc/modules/video_coding/main/source/session_info.cc:590:8: error: variable ‘previous_lost’ set but not used [-Werror=unused-but-set-variable]
cc1plus: all warnings being treated as errors
make: *** [out/Release/obj.target/webrtc_video_coding/third_party/webrtc/modules/video_coding/main/source/session_info.o] Error 1
make: *** Waiting for unfinished jobs....
Traceback (most recent call last):
File "./dartium_tools/build.py", line 67, in <module>
main()
File "./dartium_tools/build.py", line 64, in main
[target for (target, _) in targets])
File "/home/sangeeth/work/g/dartium/src/dartium_tools/utils.py", line 97, in runCommand
raise Exception('Failed to run command. return code=%s' % p.returncode)
Exception: Failed to run command. return code=2
$
But when I looked at the code dartium/src/third_party/webrtc/modules/video_coding/main/source/session_info.cc::VCMSessionInfo::PrepareForDecode(), I saw the following:
int VCMSessionInfo::PrepareForDecode(uint8_t* frame_buffer) {
int length = SessionLength();
int real_data_bytes = 0;
if (length == 0)
return length;
bool previous_lost = false;
PacketIterator it = packets_.begin();
PacketIterator prev_it = it;
for (; it != packets_.end(); ++it) {
bool packet_loss = ((*prev_it).sizeBytes == 0 ||
!InSequence(it, prev_it));
if ((*it).bits) {
if (prev_it != it) { // Not the first packet.
uint8_t* ptr_first_byte =
const_cast<uint8_t*>((*it).dataPtr);
if (packet_loss) {
// It is be better to throw away this packet if we are
// missing the previous packet.
memset(ptr_first_byte, 0, (*it).sizeBytes);
previous_lost = true;
++packets_not_decodable_;
} else if ((*it).sizeBytes > 0) {
// Glue with previous byte.
// Move everything from [this packet start + 1, end of buffer] one
// byte to the left.
uint8_t* ptr_prev_byte =
const_cast<uint8_t*>((*prev_it).dataPtr) +
(*prev_it).sizeBytes - 1;
*ptr_prev_byte = (*ptr_prev_byte) | (*ptr_first_byte);
memmove(const_cast<uint8_t*>((*it).dataPtr),
(*it).dataPtr + 1, (*it).sizeBytes - 1);
ShiftSubsequentPackets(it, -1);
(*it).sizeBytes--;
length--;
previous_lost = false;
real_data_bytes += (*it).sizeBytes;
}
} else {
memset(const_cast<uint8_t*>((*it).dataPtr), 0,
(*it).sizeBytes);
previous_lost = true;
++packets_not_decodable_;
}
} else if (packet_loss &&
(*it).codecSpecificHeader.codec == kRTPVideoH263) {
// Pad H.263 packet losses with 10 zeros to make it easier
// for the decoder.
const int kPaddingLength = 10;
WebRtc_UWord8 padding_data[kPaddingLength] = {0};
// Make a copy of the previous packet.
VCMPacket padding_packet(*it);
++padding_packet.seqNum;
padding_packet.dataPtr = padding_data;
padding_packet.sizeBytes = kPaddingLength;
length += InsertPacket(padding_packet, frame_buffer, false, 0);
previous_lost = true;
} else {
real_data_bytes += (*it).sizeBytes;
previous_lost = false;
}
prev_it = it;
}
if (real_data_bytes == 0) {
// Drop the frame since all it contains are zeros.
for (it = packets_.begin(); it != packets_.end(); ++it)
(*it).sizeBytes = 0;
length = 0;
}
return length;
}
The bool variable previous_lost has been used (set to false and true) at many places.
Any valuable input on how to go about this would be a great help.

Answer 1

Setting a bool to true or false is pointless if you don't then read the value later - in the code you posted, a value for previous_lost is indeed set in several places, but nowhere is the value that's set used to do anything (such as a if (previous_lost) ..., for example). That's what's meant by "used" in this context: a variable is used if its value is read.
Note that this (generally) applies for any type of variable: if I set an int to 42 and don't use it any later calculation, it's not "used" either.
In these cases, the variable is completely redundant - it does nothing useful since it is not used by other code. Generally this kind of warning is meant to indicate that perhaps the code that was supposed to use previous_lost was accidentally omitted (or was removed) and needs to be accounted for. Alternatively, the variable might simply be redundant and can be removed entirely.