Let's say we've got the following piece of code and we've decided to optimise it a bit:
/// BM_NormalString
bool value = false;
std::string str;
str = "orthogonal";
if (str == "orthogonal") {
value = true;
}
So far we've come up with two pretty simple and straightforward strategies:
/// BM_charString
bool value = false;
char *str = new char[11];
std::strcpy(str, "orthogonal");
if (std::strcmp(str, "orthogonal") == 0) {
value = true;
}
delete[] str;
/// BM_charStringMalloc
bool value = false;
char *str = (char *) std::malloc(11);
std::strcpy(str, "orthogonal");
if (std::strcmp(str, "orthogonal") == 0) {
value = true;
}
free(str);
If we try and benchmark our three approaches we, quite surprisingly, won't see much of a difference.
Although bench-marking it locally gives me even more surprisingly disconcerting results:
| Benchmark | Time | CPU | Iterations |
|----------------------|------------------|-----------|---------------|
| BM_charString | 52.1 ns | 51.6 ns | 11200000 |
| BM_charStringMalloc | 47.4 ns | 47.5 ns | 15448276 |
| **BM_NormalString** | 17.1 ns | 16.9 ns | 40727273 |
Would you say then, that for such rather small strings there's almost no point in going 'bare metal' style (by using C-style string functions)?
For small strings, there's no point using dynamic storage. The allocation itself is slower than the comparison. Standard library implementers know this and have optimised std::basic_string to not use dynamic storage with small strings.
Using C-strings is not an "optimisation".
Related
I am looking for something similar to SHM (SHared Memory) SPSC queue setup offered by boost::lockfree::spsc_queue and boost::interprocess but without allocating strings and storing them flat i.e. next to each other for maximum efficiency.
If I understand correctly that setup stores strings offset in the queue and allocates memory for the string somewhere else in the SHM.
Queue design can be:
| size | string 1 | size | string 2 | size | string 3 | ...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SHM segment
in a circular buffer fashion. Idea:
struct Writer {
std::byte *shm;
void write(std::string_view str) {
// write size
const uint32_t sz = str.size();
std::memcpy(shm, &sz, sizeof(sz));
shm += sizeof(sz);
// write string
std::memcpy(shm, str.data(), sz);
shm += sz;
}
};
tl;dr
It is possible, although you'll have to deal with a bunch of extra edge-cases.
Here's a working godbolt of a spsc queue that fulfils your requirements.
(see bottom of this post in case the link goes bad)
Please also check 2. Potential alternatives for viable alternatives.
1. Assumptions
Given the information you provided i'm going to assume you want the following properties for the single-producer, single-consumer (spsc) queue:
It should be backed by a ring buffer (that might be located within a shared memory region)
(this is also the case for boost::lockfree::spsc_queue)
Elements should be returned in FIFO order
(like boost::lockfree::spsc_queue)
The string-elements should be stored entirely within the ring buffer (no additional allocations)
The queue should be wait-free
(like boost::lockfree::spsc_queue, but not like most stuff in boost::interprocess (e.g. boost::interprocess::message_queue utilizes mutexes))
2. Potential alternatives
Depending on your requirements there could be a few alternative options:
Fixed-length strings:
Like #John Zwinck suggested in his answer you could use a fixed-length string buffer.
The trade-off would be that your maximum string length is bounded by this size, and - depending on your expected variation of possible string sizes - might result in a lot of unused buffer space.
If you go this route i'd recommend you to use boost::static_string - it's essentially a std::string with the dynamic allocation stuff removed and solely relying on its internal buffer.
i.e. boost::lockfree::spsc_queue<boost::static_string<N>>, where N is the maximum size for the string values.
Only store pointers in the queue and allocate the strings separately:
If you're already using boost::interprocess you could use a boost::interprocess::basic_string with a boost::interprocess::allocator that allocates the string separately in the same shared memory region.
Here's a answer that contains a full example of this approach (it even uses boost::lockfree::spsc_queue) (direct link to code example)
The trade-off in this case is that the strings will be stored somewhere outside the spsc queue (but still within the same shared memory region).
If your strings are relatively long this might even be faster than storing the strings directly within the queue (the ring buffer can be alot smaller if it only needs to store pointers, and therefore would have a much better cache-locality).
(cache locality won't help in this case - see this excellent comment from #Peter Cordes)
3. Design Considerations
3.1 Wrapped-around writes
A ringbuffer with fixed-size elements essentially splits the raw buffer into element-sized slots that are contiguous within the buffer, e.g.:
/---------------------- buffer ----------------------\
/ \
+----------+----------+----------+----------+----------+
| Object 1 | Object 2 | Object 3 | ... | Object N |
+----------+----------+----------+----------+----------+
This automatically ensures that all objects within the buffer are contiguous. i.e. you never have to deal with a wrapped-around object:
/---------------------- buffer ----------------------\
/ \
+-----+----------+----------+----------+----------+-----+
|ct 1 | | | | | Obje|
+-----+----------+----------+----------+----------+-----+
If the element-size is not fixed however, we do have to handle this case somehow.
Assume for example an empty 8-byte ringbuffer with the read and write pointer on the 7th byte (the buffer is currently empty):
/--------- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| | | | | | | | |
+----+----+----+----+----+----+----+----+
^
\---------- write pointer
If we now attempt to write "bar" into the buffer (prefixed by it's length), we would get a wrapped-around string:
/--------- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| a | r | | | | | 03 | b |
+----+----+----+----+----+----+----+----+
^
\----------------------------- write pointer
There are 2 ways to deal with this:
Let the consumer deal with wrapped-around writes.
Don't allow wrapped-around writes; ensure each string is written in a contiguous block within the buffer.
3.1.1 Making it part of the interface
The 1st option would be the easiest to implement, but it would be quite cumbersome to use for the consumer of the queue, because it needs to deal with 2 separate pointers + sizes that in combination represent the string.
i.e. a potential interface for this could be:
class spsc_queue {
// ...
bool push(const char* str, std::size_t size);
bool read(
const char*& str1, std::size_t& size1,
const char*& str2, std::size_t& size2
);
bool pop();
// ...
};
// Usage example (assuming the state from "bar" example above)
const char* str1;
std::size_t size1;
const char* str2;
std::size_t size2;
if(queue.read(str1, size1, str2, size2)) {
// str1 would point to buffer[7] ("b")
// size1 would be 1
// str2 would point to buffer[0] ("ar")
// size2 would be 2
queue.pop();
}
Having to deal with 2 pointers and 2 sizes all the time for the odd case of a wrapped-around write is not the best solution imho, so i went with option 2:
3.1.2 Prevent wrap-arounds
The alternative option would be to prevent wrap-arounds from occuring in the first place.
A simple way to achieve this is to add a special "wrap-around" marker that tells the reader to immediately jump back to the beginning of the buffer (and therefore skip over all bytes after the wrap-around marker)
Example writing "bar": (WA represents a wrap-around marker)
/--------- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| 03 | b | a | r | | | WA | |
+----+----+----+----+----+----+----+----+
^
\------------------- write pointer
So once the reader tries to read the next element it'll encounter the wrap-around marker.
This instructs it to directly go back to index 0, where the next element is located:
/--------------------------------------- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| 03 | b | a | r | | | WA | |
+----+----+----+----+----+----+----+----+
^
\------------------- write pointer
This technique allows all strings to be stored contiguously within the ring buffer - with the small trade-off that the end of the buffer might not be fully utilized and a couple extra branches in the code.
For this answer i chose the wrap-around marker approach.
3.2 What if there's no space for a wrap-around marker?
Another problem comes up once you want a string-size that's above 255 - at that point the size needs to be larger than 1 byte.
Assume we use a 2 byte-length and write "foo12" (length 5) into the ring buffer:
/--------------------------------------- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| 05 | 00 | f | o | o | 1 | 2 | |
+----+----+----+----+----+----+----+----+
^
\---- write pointer
so far so good, but as soon as the read pointer catches up we have a problem:
there is only a single byte left to write before we need to wrap around, which is not enough to fit a 2-byte length!
So we would need to wrap-around the length on the next write (writing "foo" (length 3) into the ringbuffer):
/---- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| 00 | f | o | o | | | | 03 |
+----+----+----+----+----+----+----+----+
^
\------------------- write pointer
There are three potential ways this could be resolved:
Deal with wrapped-around lengths in the implementation.
The downside to this approach is that it introduces a bunch more branches into the code (a simple memcpy for the size won't suffice anymore), it makes aligning strings more difficult and it goes against the design we chose in 3.1.
Use a single byte wrap-around marker (regardless of the string size)
This would allow us to place a wrap-around marker and prevent wrapped-around string sizes:
/---- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| 03 | 00 | f | o | o | | | WA |
+----+----+----+----+----+----+----+----+
^
\-------------- write pointer
The downside with this approach is that it's rather difficult to differentiate between a wrap-around marker and an actual string-size. We also would need to probe the first byte of each length first to check if it's a wrap-around marker before reading the full length integer.
Automatically advance the write pointer if the remaining size is insufficient for a length entry.
This is the simplest solution i managed to come up with, and the one i ended up implementing. It basically allows the writer to advance the write pointer back to the beginning of the buffer (without writing anything in it), if the remaining size would be less than the size of the length integral type. (The reader recognizes this and jumps back to the beginning as well)
/---- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| 03 | 00 | f | o | o | | | |
+----+----+----+----+----+----+----+----+
^
\-------------- write pointer
3.3 Destructive interference
Depending on how fast your reader is in comparison to your writer you might have a bit of destructive interference within the ring buffer.
If for example your L1 cache line size is 128 bytes, using 1-byte lengths for the strings in the ring-buffer, and only pushing length 1 strings, e.g.:
/--------------------------------------- read pointer
v
+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
+----+----+----+----+----+----+----+----+
| 01 | a | 01 | b | 01 | c | | | ...
+----+----+----+----+----+----+----+----+
^
\--------- write pointer
Then this would result in +/- 64 string entries being stored on the same cache line, which are continously written by the producer while they get read from the consumer => a lot of potential interference.
This can be prevented by padding the strings within the ring buffer to a multiple of your cache line size (in C++ available as std::hardware_destructive_interference_size)
i.e. strings padded to 4-bytes:
/--------------------------------------- read pointer
v
+----+----+----+----+----+----+----+----+----+
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
+----+----+----+----+----+----+----+----+----+
| 01 | a | | | 01 | c | | | | ...
+----+----+----+----+----+----+----+----+----+
^
\--------- write pointer
The trade-off here is of course that this will potentially waste a lot of space within the ring buffer.
So you'll have to profile how much padding you want for your string values.
The padding value you choose should be between those two values:
1 <= N <= std::hardware_destructive_interference_size
Where 1 is basically no padding => best space utilization, worst potential interference
And std::hardware_destructive_interference_size=> worst space utilization, no potential interference
4. Implementation
This is a full implementation of a wait-free, string only, spsc fifo queue - based on the design considerations listed above.
I've only implemented the bare minimum interface required, but you can easily create all the utility functions boost::lockfree::spsc_queue provides around those 3 core functions:
godbolt
template<std::unsigned_integral size_type, std::size_t padding = 1>
class lockfree_spsc_string_queue {
public:
lockfree_spsc_string_queue(void* buffer, std::size_t buffer_size, bool init = false);
// producer:
// tries to push the given string into the queue
// returns true if adding the string was successfull
// (contents will be copied into the ringbuffer)
bool push(std::string_view str);
// consumer:
// reads the next element in the queue.
// returns an empty optional if the queue is empty,
// otherwise a string_view that points to the string
// within the ringbuffer.
// Does NOT remove the element from the queue.
std::optional<std::string_view> front();
// Removes the next element from the queue.
// Returns true if an element has been removed.
bool pop();
};
size_type is the integral type that is used to store the length of the strings within the buffer, i.e. if you use unsigned char each string could be at most 254 bytes in length (with unsigned short (assuming 2 bytes) it would be 65534, etc...) (the maximum value is used as a wrap-around marker)
padding is the alignment that's used for the string values. If it is set to 1 then strings will be packed as tightly as possible into the ring buffer (best space utilization). If you set it to std::hardware_destructive_interference_size then there will be no interference between different string values in the ring buffer, at the cost of space utilization.
Usage example: godbolt
void producer() {
lockfree_spsc_string_queue<unsigned short> queue(BUFFER_POINTER, BUFFER_SIZE, true);
while(true) {
// retry until successful
while(!queue.push("foobar"));
}
}
void consumer() {
lockfree_spsc_string_queue<unsigned short> queue(BUFFER_POINTER, BUFFER_SIZE);
while(true) {
std::optional<std::string_view> result;
// retry until successful
while(!result) result = queue.front();
std::cout << *result << std::endl;
bool pop_result = queue.pop();
assert(pop_result);
}
}
boost::lockfree::spsc_queue::consume_all e.g. could be implemented like this (in terms of the 3 functions provided by this minimal implementation):
template<class Functor>
std::size_t consume_all(Functor&& f) {
std::size_t cnt = 0;
for(auto el = front(); el; el = front()) {
std::forward<Functor>(f)(*el);
bool pop_result = pop();
assert(pop_result);
++cnt;
}
return cnt;
}
Full implementation: godbolt
// wait-free single-producer, single consumer fifo queue
//
// The constructor must be called once with `init = true` for a specific region.
// After the construction of the queue with `init = true` has succeeded additional instances
// can be created for the region by passing `init = false`.
template<
std::unsigned_integral size_type,
std::size_t padding = 1>
class lockfree_spsc_string_queue {
// we use the max value of size_type as a special marker
// to indicate that the writer needed to wrap-around early to accommodate a string value.
// this means the maximum size a string entry can be is `size_type_max - 1`.
static constexpr size_type size_type_max = std::numeric_limits<size_type>::max();
// calculates the padding necessary that is required after
// a T member to align the next member onto the next cache line.
template<class T>
static constexpr std::size_t padding_to_next_cache_line =
std::hardware_destructive_interference_size -
sizeof(T) % std::hardware_destructive_interference_size;
public:
// internal struct that will be placed in the shared memory region
struct spsc_shm_block {
using atomic_size = std::atomic<std::size_t>;
// read head
atomic_size read_offset;
char pad1[padding_to_next_cache_line<atomic_size>];
// write head
atomic_size write_offset;
char pad2[padding_to_next_cache_line<atomic_size>];
std::size_t usable_buffer_size;
char pad3[padding_to_next_cache_line<std::size_t>];
// actual data
std::byte buffer[];
[[nodiscard]] static inline spsc_shm_block* init_shm(void* ptr, std::size_t size) {
spsc_shm_block* block = open_shm(ptr, size);
// atomics *must* be lock-free, otherwise they won't work across process boundaries.
assert(block->read_offset.is_lock_free());
assert(block->write_offset.is_lock_free());
block->read_offset = 0;
block->write_offset = 0;
block->usable_buffer_size = size - offsetof(spsc_shm_block, buffer);
return block;
}
[[nodiscard]] static inline spsc_shm_block* open_shm(void* ptr, std::size_t size) {
// this type must be trivially copyable, otherwise we can't implicitly start its lifetime.
// It also needs to have a standard layout for offsetof.
static_assert(std::is_trivially_copyable_v<spsc_shm_block>);
static_assert(std::is_standard_layout_v<spsc_shm_block>);
// size must be at least as large as the header
assert(size >= sizeof(spsc_shm_block));
// ptr must be properly aligned for the header
assert(reinterpret_cast<std::uintptr_t>(ptr) % alignof(spsc_shm_block) == 0);
// implicitly start lifetime of spsc_shm_block
return std::launder(reinterpret_cast<spsc_shm_block*>(ptr));
}
};
public:
inline lockfree_spsc_string_queue(void* ptr, std::size_t size, bool init = false)
: block(init ? spsc_shm_block::init_shm(ptr, size) : spsc_shm_block::open_shm(ptr, size))
{
// requires a buffer at least 1 byte larger than size_type
assert(block->usable_buffer_size > sizeof(size_type));
}
// prevent copying / moving
lockfree_spsc_string_queue(lockfree_spsc_string_queue const&) = delete;
lockfree_spsc_string_queue(lockfree_spsc_string_queue&&) = delete;
lockfree_spsc_string_queue& operator=(lockfree_spsc_string_queue const&) = delete;
lockfree_spsc_string_queue& operator=(lockfree_spsc_string_queue&&) = delete;
// producer: tries to add `str` to the queue.
// returns true if the string has been added to the queue.
[[nodiscard]] inline bool push(std::string_view str) {
std::size_t write_size = pad_size(sizeof(size_type) + str.size());
// impossible to satisfy write (not enough space / insufficient size_type)
if(write_size > max_possible_write_size() || str.size() >= size_type_max) [[unlikely]] {
assert(write_size < max_possible_write_size());
assert(str.size() < size_type_max);
return false;
}
std::size_t write_off = block->write_offset.load(std::memory_order_relaxed);
std::size_t read_off = block->read_offset.load(std::memory_order_acquire);
std::size_t new_write_off = write_off;
if(try_align_for_push(read_off, new_write_off, write_size)) {
new_write_off = push_element(new_write_off, write_size, str);
block->write_offset.store(new_write_off, std::memory_order_release);
return true;
}
if(new_write_off != write_off) {
block->write_offset.store(new_write_off, std::memory_order_release);
}
return false;
}
// consumer: discards the current element to be read (if there is one)
// returns true if an element has been removed, false otherwise.
[[nodiscard]] inline bool pop() {
std::size_t read_off;
std::size_t str_size;
if(!read_element(read_off, str_size)) {
return false;
}
std::size_t read_size = pad_size(sizeof(size_type) + str_size);
std::size_t new_read_off = advance_offset(read_off, read_size);
block->read_offset.store(new_read_off, std::memory_order_release);
return true;
}
// consumer: returns the current element to be read (if there is one)
// this does not remove the element from the queue.
[[nodiscard]] inline std::optional<std::string_view> front() {
std::size_t read_off;
std::size_t str_size;
if(!read_element(read_off, str_size)) {
return std::nullopt;
}
// return string_view into buffer
return std::string_view{
reinterpret_cast<std::string_view::value_type*>(&block->buffer[read_off + sizeof(size_type)]),
str_size
};
}
private:
// handles implicit and explicit wrap-around for the writer
[[nodiscard]] inline bool try_align_for_push(
std::size_t read_off,
std::size_t& write_off,
std::size_t write_size) {
std::size_t cont_avail = max_avail_contiguous_write_size(write_off, read_off);
// there is enough contiguous space in the buffer to push the string in one go
if(write_size <= cont_avail) {
return true;
}
// not enough contiguous space in the buffer.
// check if the element could fit contiguously into
// the buffer at the current write_offset.
std::size_t write_off_to_end = block->usable_buffer_size - write_off;
if(write_size <= write_off_to_end) {
// element could fit at current position, but the reader would need
// to consume more elements first
// -> do nothing
return false;
}
// element can NOT fit contiguously at current write_offset
// -> we need a wrap-around
std::size_t avail = max_avail_write_size(write_off, read_off);
// not enough space for a wrap-around marker
// -> implicit wrap-around
if(write_off_to_end < sizeof(size_type)) {
// the read marker has advanced far enough
// that we can perform a wrap-around and try again.
if(avail >= write_off_to_end) {
write_off = 0;
return try_align_for_push(read_off, write_off, write_size);
}
// reader must first read more elements
return false;
}
// explicit wrap-around
if(avail >= write_off_to_end) {
std::memcpy(&block->buffer[write_off], &size_type_max, sizeof(size_type));
write_off = 0;
return try_align_for_push(read_off, write_off, write_size);
}
// explicit wrap-around not possible
// (reader must advance first)
return false;
}
// writes the element into the buffer at the provided offset
// and calculates new write_offset
[[nodiscard]] inline std::size_t push_element(
std::size_t write_off,
std::size_t write_size,
std::string_view str) {
// write size + string into buffer
size_type size = static_cast<size_type>(str.size());
std::memcpy(&block->buffer[write_off], &size, sizeof(size_type));
std::memcpy(&block->buffer[write_off + sizeof(size_type)], str.data(), str.size());
// calculate new write_offset
return advance_offset(write_off, write_size);
}
// returns true if there is an element that can be read (and sets read_off & str_size)
// returns false otherwise.
// internally handles implicit and explicit wrap-around.
[[nodiscard]] inline bool read_element(std::size_t& read_off, std::size_t& str_size) {
std::size_t write_off = block->write_offset.load(std::memory_order_acquire);
std::size_t orig_read_off = block->read_offset.load(std::memory_order_relaxed);
read_off = orig_read_off;
str_size = 0;
if(read_off == write_off) {
return false;
}
// remaining space would be insufficient for a size_type
// -> implicit wrap-around
if(block->usable_buffer_size - read_off < sizeof(size_type)) {
read_off = 0;
if(read_off == write_off) {
block->read_offset.store(read_off, std::memory_order_release);
return false;
}
}
size_type size;
std::memcpy(&size, &block->buffer[read_off], sizeof(size_type));
// wrap-around marker
// -> explicit wrap-around
if(size == size_type_max) {
read_off = 0;
if(read_off == write_off) {
block->read_offset.store(read_off, std::memory_order_release);
return false;
}
std::memcpy(&size, &block->buffer[read_off], sizeof(size_type));
}
// modified read_off -> store
if(read_off != orig_read_off) {
block->read_offset.store(read_off, std::memory_order_release);
}
str_size = size;
return true;
}
// the maximum number of contiguous bytes we are currently able
// to fit within the memory block (without wrapping around)
[[nodiscard]] inline std::size_t max_avail_contiguous_write_size(
std::size_t write_off,
std::size_t read_off) {
if(write_off >= read_off) {
std::size_t ret = block->usable_buffer_size - write_off;
ret -= read_off == 0 ? 1 : 0;
return ret;
}
// write_off < read_off
return read_off - write_off - 1;
}
// the maximum number of bytes we are currently able
// to fit within the memory block (might include a wrap-around)
[[nodiscard]] inline std::size_t max_avail_write_size(std::size_t write_off, std::size_t read_off) {
std::size_t avail = read_off - write_off - 1;
if (write_off >= read_off)
avail += block->usable_buffer_size;
return avail;
}
// the largest possible size an element could be and still
// fit within the memory block.
[[nodiscard]] inline std::size_t max_possible_write_size() {
return block->usable_buffer_size - 1;
}
// pads a given size to be a multiple of the template parameter padding
[[nodiscard]] inline std::size_t pad_size(std::size_t size) {
if(size % padding != 0) {
size += padding - size % padding;
}
return size;
}
// advances offset and wraps around if required
[[nodiscard]] inline std::size_t advance_offset(std::size_t offset, std::size_t element_size) {
std::size_t new_offset = offset + element_size;
// wrap-around
if(new_offset >= block->usable_buffer_size) {
new_offset -= block->usable_buffer_size;
}
return new_offset;
}
private:
spsc_shm_block* block;
};
Create your own string type that does what you want:
struct MyString
{
uint8_t size; // how much of data is actually populated
std::array<char, 127> data; // null terminated? up to you
};
Now an spsc_queue<MyString> can store strings without separate allocation.
This code works OK when compiled with g++ on Linux, but when I try to execute them in VS 2015 (both debug and release) I receive runtime error. What's wrong with it?
#include "stdafx.h"
#include <string.h>
#include <iostream>
using namespace std;
struct Stru1
{
int mem;
};
struct Stru2 : public Stru1
{
char szMem1[256];
int dwMem2;
int dwMem3;
};
static void clFun(Stru1* s) {
Stru2* s2 = (Stru2*)s;
cout << s2->szMem1 << endl;//blahblah
cout << s2->dwMem2 << endl;//runtime error
}
class Temp {
public:
void callDispatch() {
simRecv->mem = 2;
Stru2* sro = (Stru2*)simRecv;
strcpy(sro->szMem1, "blahblah");
sro->dwMem2 = 11;
sro->dwMem3 = 77;
//cout << sro->szMem1 << endl;//blahblah
//cout << sro->dwMem2 << endl;//runtime error when uncommented
clFun(simRecv);
}
~Temp() { delete simRecv; }
Stru1* simRecv = new Stru1;
};
int main()
{
Temp tmp;
tmp.callDispatch();
return 0;
}
Error:
Exception thrown at 0x0000000077A0F23C (ntdll.dll) in ConsoleApplication1.exe: 0xC0000005: Access violation reading location 0x00000FB00188C508.
Stru2* sro = (Stru2*)simRecv;
simRecv is a Stru1, so your unsafe cast to Stru2 is invalid in this line.
In this line you create this Stru1,
Stru1* simRecv = new Stru1;
Here Stru1 is assigned the memory needed to create a Stru1, which is smaller than a Stru2.
By doing:
Stru2* sro = (Stru2*)simRecv;
You are just saying: I have this "thing" and treat it as a Stru2. But there hasn't been a new Stru2 created anywhere so the object just isn't there.
It is basically the same as saying
I have a large wall, but I'll treat it as a house and expect a door in it.
The reason that it might work on a different platform can be due to a different memory allocation of the platform.
As for the analogy: you might have reached the end of the wall and hence don't hurt your head, but you are not inside the house and you won't leave your wallet there.
For example, this line will eventually point to somewhere:
sro->dwMem3 = 77;
The question is: is this within valid program space? If it is, no error will occur, but that doesn't mean it is good. You are probably altering a variable, somewhere else, leading to unpredictable results.
An example:
Platform1:
| Stru1 | some variable | some other variable |
| mem | 0 | 11 |
| | | ((Stru2*) simRecv)->dwMem2 |
//no errors, but strange side effects
Platform2:
| Stru1 | some variable | some other program space |
| mem | 0 | ERROR: ACCES VIOLATION |
| | | ((Stru2*) simRecv)->dwMem2 |
//0xC0000005
If you assign a Stru2 (by actually creating it) in the first place, all will be good:
Stru1* simRecv = (Stru1*) new Stru2;
Having said that; these cast are considered unsafe (for now obvious reasons).
The alternative is to use, for example a static_cast. It will make sure you'll get a build error when trying to do something "illegal".
http://www.cplusplus.com/doc/tutorial/typecasting/
additional note about cplusplus see: What's wrong with cplusplus.com?
I used something like this:
struct Node
{
char name[50];
Node *left,*right;
};
int main()
{
char cmd[10];
Node* p=NULL;
scanf("%s",&cmd);
if (p == NULL)
{
// do something
// THIS NEVER GETS EXECUTED
// WHYYYYY????
//THIS IS STRANGE
}
}
So basically, the pointer p changes its value after I read into the cmd variable. I tried to commented out the scanf code, and then everything works. Very strange.
You have a buffer overflow.
The memory looks like this:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
^ ^
cmd[10] p
If scanf reads more than 10 bytes, it will overflow to p. Any non-zero value will fail the NULL check, even if it is not a valid pointer.
sometimes I have the task of finding if some nested getter inside a value returned by getter has some property.
classic c++ would be something like:
for (const auto& label: labels)
for (const auto& artist: label.artists())
if (artist.name = "Arcade Fire")
return true;
return false;
What is the best way to do this with ranges?
I think something like this may work:
labels| transform (&Label::artists) | join | transform(&Artist::name) | join | find_if_fn([](){/*...*/}) ;
But it is quite long, (partially because instead of .member_fn you must write Class:member_fn.
Is there a shorter way to do this?
I think this:
using namespace ranges;
auto rng = labels | view::transform(&Label::artists) | view::join;
return find(rng, "Arcade Fire", &Artist::name) != end(rng);
gets the job done in a fairly straightforward way. A view::filter formulation:
using namespace ranges;
auto rng = labels | view::transform(&Label::artists) | view::join |
view::filter([](const Artist& a){ return a.name() == "Arcade Fire"; });
return !empty(rng);
Is a bit wordier, but probably has similar performance. It's also fairly clear how to generalize it from "Is there a foo?" to "Return all of the foos."
BTW: I found the problem: (See my answer below)
When I build my program at home it works fine, but when I use my universities system is crashing on me. When I go at it with GDB I get this:
(gdb) r t.c-
Starting program: /home/shro8822/p5/c- t.c-
*--Code Gen Function: main
*--in function 'main' variable offsets start at 2
Program received signal SIGSEGV, Segmentation fault.
0x08084410 in ObjectCode::ResolveRef (this=0xbfb3dd20) at CodeOutput.cpp:44
44 p->Resolve(this);
(gdb) list
39 {
40 std::list<Patch*>::iterator pos;
41 for(pos = Patchups.begin(); pos != Patchups.end(); ++pos)
42 {
43 Patch* p = *pos;
44 p->Resolve(this);
45 //delete p;
46 }
47
48 }
(gdb) p p
$1 = (class ObjectCode::Patch *) 0x2064696c
(gdb) p this
$2 = (ObjectCode * const) 0xbfb3dd20
It crashes from a SEG-V on a line with a virtual function call involving 2 variable and neither is NULL. I don't think there is anywhere else that stuff from this list is deleted.
Tossing it a Valgrind gives one error:
==5714== Invalid read of size 4
==5714== at 0x8084410: ObjectCode::ResolveRef() (CodeOutput.cpp:44)
==5714== by 0x8086E00: ObjectCode::Finish() (CodeOutput.cpp:196)
==5714== by 0x807EC97: WalkGlobal::Finish() (CodeGen_G.cpp:211)
==5714== by 0x808D53C: Compile::RunV() (cs445.cpp:120)
==5714== by 0x808D7C2: ProcessFile::Run() (cs445.cpp:49)
==5714== by 0x808CCD9: main (cs445.cpp:234)
==5714== Address 0x2064696C is not stack'd, malloc'd or (recently) free'd
Seg fault
Any idea were to start looking?
BTW: I populate the list using only statements like this: Patchups.push_back(new PatchType());
shro8822 p5 $ grep Patchups *.cpp *.h -n
CodeOutput.cpp:41: for(pos = Patchups.begin(); pos != Patchups.end(); ++pos)
CodeOutput_Slot.cpp:124: { Stream->Patchups.push_back(new FunctionPatch(it,GetSlotBefor(),at)); }
CodeOutput_Slot.cpp:126: { Stream->Patchups.push_back(new GotoPatch(target,GetSlotBefor(),at,"goto")); }
CodeOutput_Slot.cpp:128: { Stream->Patchups.push_back(new GotoPatch(target,GetSlotBefor(),at,c)); }
CodeOutput_Slot.cpp:130: { Stream->Patchups.push_back(new BranchPatch(target,GetSlotBefor(),type,from,at,c)); }
CodeOutput.h:222: std::list Patchups;
Yet more: It happens that the home and school systems are both x86 (RHEL 3 and 5 respectively) so I ran the binary I compiled at home on the system at school and it runs fine.
The value of the pointer is probably the victim of a wild write from somewhere else.
The variable p shown in your debugger output is 0x2064696c. That is probably the string "lid ", depending on your byte ordering. You should look for somewhere in your code where that string (or value) was stored.
One of the pointers in your list is invalid. This could be because it is null (not in your case), uninitialized, initialized via a bad cast or the valid object that it once pointed to has been destroyed.
Because it works in one environment and not in another you are probably seeing the results of some undefined behaviour.
When you push the pointers onto the list, what objects are they pointing to and what happens to those objects at the time you call Finish?
You're dereferencing p on line 44 to an object that doesn't exist.
Either p was never initialized, or *p has already been deleted.
Edit: I'd recommend to start looking where this list is populated, and verify that your list items are initialized to 0, and that you actually assign pointers to your Patch instances to the list. Also you might look for other errors or exceptions that you're ignoring or catching during the initialization process that are allowing pointers to invalid memory like this to make it into the list.
How I found the problem.
First a tip of the hat to janm for identifying what was going wrong even if it didn't help much in finding where.
I added a Test function that is effectively a copy of the function that fails, but with all the side effects stripped out. With it running all over the place I was able to isolate where things break down into a small windows. Under the debugger, I single stepped from that last valid pass to the first invalid pass and got this:
CodeOutput.cpp:224 | ObjectCode::Test();
CodeOutput.cpp:225 | continue;
CodeOutput.cpp:111 | while(at != ops.end())
stl_list.h:598 | { return iterator(&this->_M_impl._M_node); }
stl_list.h:127 | : _M_node(__x) { }
stl_list.h:174 | { return _M_node != __x._M_node; }
CodeOutput.cpp:113 | printf("%s\n", (*at).TypeStr());
stl_list.h:132 | { return static_cast(_M_node)->_M_data; }
CodeOutput_asm.cpp:33 | switch(Type)
CodeOutput_asm.cpp:36 | Case(OpPlaceholder);
CodeOutput.cpp:115 | switch((*at).Type)
stl_list.h:132 | { return static_cast(_M_node)->_M_data; }
CodeOutput.cpp:216 | char* c = (*at).comment;
stl_list.h:132 | { return static_cast(_M_node)->_M_data; }
CodeOutput.cpp:217 | if((*at).head != NULL && (*at).head[0] != '\0')
stl_list.h:132 | { return static_cast(_M_node)->_M_data; }
stl_list.h:132 | { return static_cast(_M_node)->_M_data; }
CodeOutput.cpp:222 | ++at;// = ops.erase(at);
stl_list.h:141 | _M_node = _M_node->_M_next;
stl_list.h:142 | return *this;
CodeOutput.cpp:223 | (*at).head = c;
stl_list.h:132 | { return static_cast(_M_node)->_M_data; }
CodeOutput.cpp:224 | ObjectCode::Test();
Formatted for clarity, the memory corruption must be caused by one of these lines:
-- last valid test
CodeOutput.cpp:224 | ObjectCode::Test();
CodeOutput.cpp:225 | continue;
-- falls into loop ('at' is list::iterator)
CodeOutput.cpp:111 | while(at != ops.end())
CodeOutput.cpp:113 | printf("%s\n", (*at).TypeStr());
CodeOutput.cpp:115 | switch((*at).Type)
-- OpPlaceholder case
CodeOutput.cpp:216 | char* c = (*at).comment;
-- if gets false ('head' is char*)
CodeOutput.cpp:217 | if((*at).head != NULL && (*at).head[0] != '\0')
CodeOutput.cpp:222 | ++at;
CodeOutput.cpp:223 | (*at).head = c;
-- first invalid test
CodeOutput.cpp:224 | ObjectCode::Test();
-- called from CodeOutput.cpp:113
CodeOutput_asm.cpp:33 | switch(Type)
CodeOutput_asm.cpp:36 | case OpPlaceholder; return "OpPlaceholder";
Because that not to long a list, I just added even more logging till I found that these line causes the problem:
++at;
(*at).head = c;
Now that I know exactl where to look it easy to see the problem, and by switching to:
++at;
if(at != ops.end()) (*at).head = c;
the problem goes away.
The only questions I still have are 1) why did it work at all on my old system? and 2) why didn't that manifest as a seg-v right on the second line? I would think that having *(list.end()) result in a reference to NULL would be a good thing.
You should use < in your conditional statement.
When you increment a pointer using ++, it increases it by the size of whatever it points to. Since you're using !=, it's possible that you're not hitting Patchups.end() exactly, and so you're walking off then end into invalid memory.
Or it might be something else. There might be invalid memory somewhere between begin() and end(), for example.