I have a binary file with multiple Capnp messages which I want to read. Reading sequentially works well, but I have the use-case, that I want to jump to a previously known position.
The data sequential images with metadata including there timestamp. I would like to have the possibility to jump back and forth (like in a video player).
This is what I have tried:
int fd = open(filePath.c_str(), O_RDONLY);
kj::FdInputStream fdStream(fd);
kj::BufferedInputStreamWrapper bufferedStream(fdStream);
for (;;) {
kj::ArrayPtr<const kj::byte> framePtr = bufferedStream.tryGetReadBuffer();
if (framePtr != nullptr) {
capnp::PackedMessageReader message(bufferedStream);
// This should reset the buffer to the last read message?
bufferedStream.read((void*)framePtr.begin(), framePtr.size());
// ...
}
else {
// reset to beginning
}
}
But I get this error:
capnp/serialize.c++:186: failed: expected segmentCount < 512; Message has too many segments
I was assuming that tryGetReadBuffer() returns the position and size of the next packed message. But then again, how is the BufferedInputStream supposed to know what "a message" is.
Question: How can I get position and size of messages and read these messages later on from the BufferedInputStreamWrapper?
Alternative: Reading the whole file once, take ownership of the data and save it to a vector. Such as described here (https://groups.google.com/forum/#!topic/capnproto/Kg_Su1NnPOY). Better solution all along?
BufferedInputStream is not seekable. In order to seek backwards, you will need to destroy bufferedStream and then seek the underlying file descriptor, e.g. with lseek(), then create a new buffered stream.
Note that reading the current position (in order to pass to lseek() later to go back) is also tricky if a buffered stream is present, since the buffered stream will have read past the position in order to fill the buffer. You could calculate it by subtracting off the buffer size, e.g.:
// Determine current file position, so that we can seek to it later.
off_t messageStartPos = lseek(fd, 0, SEEK_CUR) -
bufferedStream.tryGetReadBuffer().size();
// Read a message
{
capnp::PackedMessageReader message(bufferedStream);
// ... do stuff with `message` ...
// Note that `message` is destroyed at this }. It's important that this
// happens before querying the buffered stream again, because
// PackedMesasgeReader updates the buffer position in its destructor.
}
// Determine the end position of the message (if you need it?).
off_t messageEndPos = lseek(fd, 0, SEEK_CUR) -
bufferedStream.tryGetReadBuffer().size();
bufferedStream.read((void*)framePtr.begin(), framePtr.size());
FWIW, the effect of this line is "advance past the current buffer an on to the next one". You don't want to do this when using PackedMessageReader, as it will already have advanced the stream itself. In fact, because PackedMessageReader might have already advanced past the current buffer, framePtr may now be invalid, and this line might segfault.
Alternative: Reading the whole file once, take ownership of the data and save it to a vector. Such as described here (https://groups.google.com/forum/#!topic/capnproto/Kg_Su1NnPOY). Better solution all along?
If the file fits comfortably in RAM, then reading it upfront is usually fine, and probably a good idea if you expect to be seeking back and forth a lot.
Another option is to mmap() it. This makes it appear as if the file is in RAM, but the operating system will actually read in the contents on-demand when you access them.
However, I don't think this will actually simplify the code much. Now you'll be dealing with an ArrayInputStream (a subclass of BufferedInputStream). To "seek" you would create a new ArrayInputStream based on a slice of the buffer starting at the point where you want to start.
What is the best way to cut the end off of a fstream file in C++ 11
I am writing a data persistence class to store audio for my audio editor. I have chosen to use fstream (possibly a bad idea) to create a random access binary read write file.
Each time I record a little sound into my file I simply tack it onto the end of this file. Another internal data structure / file, contains pointers into the audio file and keeps track of edits.
When I undo a recording action and then do something else the last bit of the audio file becomes irrelevant. It is not referenced in the current state of the document and you cannot redo yourself back to a state where you can ever see it again. So I want to chop this part of the file off and start recording at the new end. I don’t need to cut out bitts in the middle, just off the end.
When the user quits this file will remain and be reloaded when they open the project up again.
In my application I expect the user to do this all the time and being able to do this might save me as much as 30% of the file size. This file will be long, potentially very, very long, so rewriting it to another file every time this happens is not a viable option.
Rewriting it when the user saves could be an option but it is still not that attractive.
I could stick a value at the start that says how long the file is supposed to be and then overwrite the end to recycle the space but in the mean time. If I wanted to continually update the data store file in case of crash this would mean I would be rewriting the start over and over again. I worry that this might be bad for flash drives. I could also recomputed the end of the useful part of the file on load, by analyzing the pointer file but in the mean time I would be wasting all that space potentially, and that is complicated.
Is there a simple call for this in the fstream API?
Am I using the wrong library? Note I want to stick to something generic STL I preferred, so I can keep the code as cross platform as possible.
I can’t seem to find it in the documentation and have looked for many hours. It is not the end of the earth but would make this a little simpler and potentially more efficient. Maybe I am just missing it somehow.
Thanks for your help
Andre’
Is there a simple call for this in the fstream API?
If you have C++17 compiler then use std::filesystem::resize_file. In previous standards there was no such thing in standard library.
With older compilers ... on Windows you can use SetFilePointer or SetFilePointerEx to set the current position to the size you want, then call SetEndOfFile. On Unixes you can use truncate or ftruncate. If you want portable code then you can use Boost.Filesystem. From it is simplest to migrate to std::filesystem in the future because the std::filesystem was mostly specified based on it.
If you have variable, that contains your current position in the file, you could seek back for the length of your "unnedeed chunk", and just continue to write from there.
// Somewhere in the begining of your code:
std::ofstream *file = new std::ofstream();
file->open("/home/user/my-audio/my-file.dat");
// ...... long story of writing data .......
// Lets say, we are on a one millin byte now (in the file)
int current_file_pos = 1000000;
// Your last chunk size:
int last_chunk_size = 12345;
// Your chunk, that you are saving
char *last_chunk = get_audio_chunk_to_save();
// Writing chunk
file->write(last_chunk, last_chunk_size);
// Moving pointer:
current_file_pos += last_chunk_size;
// Lets undo it now!
current_file_pos -= last_chunk_size;
file->seekp(current_file_pos);
// Now you can write new chunks from the place, where you were before writing and unding the last one!
// .....
// When you want to finally write file to disk, you just close it
file->close();
// And when, truncate it to the size of current_file_pos
truncate("/home/user/my-audio/my-file.dat", current_file_pos);
Unfortunatelly, you'll have to write a crossplatform function truncate, that would call SetEndOfFile in windows, and truncate in linux. It's easy enough with using preprocessor macros.
I have a program I'm trying to write that constantly monitors a log file and outputs specific items into a new file.
I'm using essentially.
for (int i = 1; i < y; i ++)
getline(read, line); // skips to the last known end
while (getline(read, line))
{
cout << line;
}
read.clear();
I also keep track of the line I'm on just using the increment operator on a variable. At the end of the file I clear the eof bit and seek to the last line I was on. From using the debugger it seems that it works. I retrieve the next line in the file as its being written but when I call back to my while (getline(read,line)); it skips going through the while loop, why is that?
program reads the last line in the file.
Sleeps for 5 minutes.
The input file has had new lines added to it from a third party.
After the sleep it wakes up and goes back to the while loop.
It successfully retrieves the new lines from the input. But fails to
enter the while loop again
When using std::getline() at the end of a file both std::iostate::eofbit and std::iostate::failbit are set. In fact, it is std::iostate::failbit which causes the loop to exit. You'll need to clear both of these flags prior to any seek.
For a system which needs to use IOStreams I would actually not bother reading the leading lines but merely wait a bit, clear the flags, and try again. The main issue is detecting whether a complete line is read which could be done by simply reading individual characters, e.g., using std::istreambuf_iterator<char>.
Otherwise I'd look for a system API which provide some sort of indication that new data is available on a file. Older system generally don't provide such facilities but newer system generally have some event-based interface which can be used to get hold of newly available data. The advantage is normally that the processes doesn't poll for new data but idly waits until it gets notified about new data. I haven't used it myself but it seems libuv does this sort of operations in a somewhat platform-independent form.
I have some parsing code that allows for escape sequences to be entered into a string of text:
// In a file or large, multi-line string ...
my_parameter="A setting for the parameter\nthat contains \"escape sequence\" characters"
When I parse it, I handle the backslashes and add the appropriate character to the string that I am building using a std::ostringstream instance. Line feeds, quotes, backslashes and such all work fine. However, I was contemplating whether or not to allow the \b sequence, and went looking to see if I can "unput" the last character from my ostringstream like you can "unget" from any std::istream. Can you do such a thing? If the function does not exist, is there a simple way to push the write position back one character and simply have the next character read overwrite it?
This is not mission-critical or anything like that, but I was curious if anyone else has come across this before.
Streams are an awful lot like the mail. each message sent on a stream is like a letter, and messages can be queued into buffers, which are like mailboxes.
If you were responsible for both putting messages into and taking messages out of a mail-box, then you could certainly know that a letter you just put there is still there for you to take back. Of course, you probably wouldn't go to the trouble of putting it in a mailbox at all, since you own both ends.
If instead, you are putting a letter in your girlfriends mailbox, you don't really have much control of when she will check her mailbox and take out all of the letters. it could be that she's sitting by the door and will snatch the letter right up and read it as soon as it passes through the slot.
More likely, you're actually delivering the letter to a box owned by the post-office (the operating system). Although many such receptacles are just bins, and mail carrier checks it once per day, It could be that the slot is connected directly to a sorting machine, and the letter gets delivered the instant you drop it.
In a streaming interface with concurrency, there is no general way to retake ownership of a write once written. If you need that, you should place an intermediate buffer between you and the stream, and flush it out to the stream only when you know for sure that you are ready.
You may use seekp to set the position of cursor in the stream (see: http://cplusplus.com/reference/iostream/ostream/seekp/).
If you may want to take back a character, don't send it until you're sure you won't want to take it back. You could implement your own "allow takebacks" logic thus:
int pending_ch = -1;
void output_char(int ch)
{
if (pending_ch >= 0)
putch(F, ch);
pending_ch = ch;
}
void unput_char(void)
{
pending_ch = -1;
}
void force_put_char(void)
{
output_char(-1);
}
A bit clunky, but that general approach can be useful for delaying output to a stream.
You could simply input a backspace character yourself. I've done it in the Windows Console, at least. This should ensure that the correct behaviour is observed regardless of the destination.
I need to read the number of lines in a file before doing some operations on that file. When I try to read the file and increment the line_count variable at each iteration until I reach EOF. It was not that fast in my case. I used both ifstream and fgets. They were both slow. Is there a hacky way to do this, which is also used by, for instance BSD, Linux kernel or berkeley db (may be by using bitwise operations).
The number of lines is in the millions in that file and it keeps getting larger, each line is about 40 or 50 characters. I'm using Linux.
Note:
I'm sure there will be people who might say use a DB idiot. But briefly in my case I can't use a db.
The only way to find the line count is to read the whole file and count the number of line-end characters. The fastest way to do this is probably to read the whole file into a large buffer with one read operation and then go through the buffer counting the '\n' characters.
As your current file size appears to be about 60Mb, this is not an attractive option. You can get some of the speed by not reading the whole file, but reading it in chunks, say of size 1Mb. You also say that a database is out of the question, but it really does look to be the best long-term solution.
Edit: I just ran a small benchmark on this and using the buffered approach (buffer size 1024K) seems to be a bit more than twice as fast as reading a line at a time with getline(). Here's the code - my tests were done with g++ using -O2 optimisation level:
#include <iostream>
#include <fstream>
#include <vector>
#include <ctime>
using namespace std;
unsigned int FileRead( istream & is, vector <char> & buff ) {
is.read( &buff[0], buff.size() );
return is.gcount();
}
unsigned int CountLines( const vector <char> & buff, int sz ) {
int newlines = 0;
const char * p = &buff[0];
for ( int i = 0; i < sz; i++ ) {
if ( p[i] == '\n' ) {
newlines++;
}
}
return newlines;
}
int main( int argc, char * argv[] ) {
time_t now = time(0);
if ( argc == 1 ) {
cout << "lines\n";
ifstream ifs( "lines.dat" );
int n = 0;
string s;
while( getline( ifs, s ) ) {
n++;
}
cout << n << endl;
}
else {
cout << "buffer\n";
const int SZ = 1024 * 1024;
std::vector <char> buff( SZ );
ifstream ifs( "lines.dat" );
int n = 0;
while( int cc = FileRead( ifs, buff ) ) {
n += CountLines( buff, cc );
}
cout << n << endl;
}
cout << time(0) - now << endl;
}
Don't use C++ stl strings and getline ( or C's fgets), just C style raw pointers and either block read in page-size chunks or mmap the file.
Then scan the block at the native word size of your system ( ie either uint32_t or uint64_t) using one of the magic algorithms 'SIMD Within A Register (SWAR) Operations' for testing the bytes within the word. An example is here; the loop with the 0x0a0a0a0a0a0a0a0aLL in it scans for line breaks. ( that code gets to around 5 cycles per input byte matching a regex on each line of a file )
If the file is only a few tens or a hundred or so megabytes, and it keeps growing (ie something keeps writing to it), then there's a good likelihood that linux has it cached in memory, so it won't be disk IO limited, but memory bandwidth limited.
If the file is only ever being appended to, you could also remember the number of lines
and previous length, and start from there.
It has been pointed out that you could use mmap with C++ stl algorithms, and create a functor to pass to std::foreach. I suggested that you shouldn't do it not because you can't do it that way, but there is no gain in writing the extra code to do so. Or you can use boost's mmapped iterator, which handles it all for you; but for the problem the code I linked to was written for this was much, much slower, and the question was about speed not style.
You wrote that it keeps getting larger.
This sounds like it is a log file or something similar where new lines are appended but existing lines are not changed. If this is the case you could try an incremental approach:
Parse to the end of file.
Remember the line count and the offset of EOF.
When the file grows fseek to the offset, parse to EOF and update the line count and the offset.
There's a difference between counting lines and counting line separators. Some common gotchas to watch out for if getting an exact line count is important:
What's the file encoding? The byte-by-byte solutions will work for ASCII and UTF-8, but watch out if you have UTF-16 or some multibyte encoding that doesn't guarantee that a byte with the value of a line feed necessarily encodes a line feed.
Many text files don't have a line separator at the end of the last line. So if your file says "Hello, World!", you could end up with a count of 0 instead of 1. Rather than just counting the line separators, you'll need a simple state machine to keep track.
Some very obscure files use Unicode U+2028 LINE SEPARATOR (or even U+2029 PARAGRAPH SEPARATOR) as line separators instead of the more common carriage return and/or line feed. You might also want to watch out for U+0085 NEXT LINE (NEL).
You'll have to consider whether you want to count some other control characters as line breakers. For example, should a U+000C FORM FEED or U+000B LINE TABULATION (a.k.a. vertical tab) be considered going to a new line?
Text files from older versions of Mac OS (before OS X) use carriage returns (U+000D) rather than line feeds (U+000A) to separate lines. If you're reading the raw bytes into a buffer (e.g., with your stream in binary mode) and scanning them, you'll come up with a count of 0 on these files. You can't count both carriage returns and line feeds, because PC files generally end a line with both. Again, you'll need a simple state machine. (Alternatively, you can read the file in text mode rather than binary mode. The text interfaces will normalize line separators to '\n' for files that conform to the convention used on your platform. If you're reading files from other platforms, you'll be back to binary mode with a state machine.)
If you ever have a super long line in the file, the getline() approach can throw an exception causing your simple line counter to fail on a small number of files. (This is particularly true if you're reading an old Mac file on a non-Mac platform, causing getline() to see the entire file as one gigantic line.) By reading chunks into a fixed-size buffer and using a state machine, you can make it bullet proof.
The code in the accepted answer suffers from most of these traps. Make it right before you make it fast.
Remember that all fstreams are buffered. So they in-effect do actually reads in chunks so you do not have to recreate this functionality. So all you need to do is scan the buffer. Don't use getline() though as this will force you to size a string. So I would just use the STL std::count and stream iterators.
#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
struct TestEOL
{
bool operator()(char c)
{
last = c;
return last == '\n';
}
char last;
};
int main()
{
std::fstream file("Plop.txt");
TestEOL test;
std::size_t count = std::count_if(std::istreambuf_iterator<char>(file),
std::istreambuf_iterator<char>(),
test);
if (test.last != '\n') // If the last character checked is not '\n'
{ // then the last line in the file has not been
++count; // counted. So increement the count so we count
} // the last line even if it is not '\n' terminated.
}
It isn't slow because of your algorithm , It is slow because IO operations are slow. I suppose you are using a simple O(n) algorithm that is simply going over the file sequentially. In that case , there is no faster algorithm that can optimize your program.
However , I said there is no faster algorithm , but there is a faster mechanism which called "Memory Mapped file " , There are some drawback for mapped files and it might not be appropiate for you case , So you'll have to read about it and figure out by yourself.
Memory mapped files won't let you implement an algorithm better then O(n) but it may will reduce IO access time.
You can only get a definitive answer by scanning the entire file looking for newline characters. There's no way around that.
However, there are a couple of possibilities which you may want to consider.
1/ If you're using a simplistic loop, reading one character at a time checking for newlines, don't. Even though the I/O may be buffered, function calls themselves are expensive, time-wise.
A better option is to read large chunks of the file (say 5M) into memory with a single I/O operation, then process that. You probably don't need to worry too much about special assembly instruction since the C runtime library will be optimized anyway - a simple strchr() should do it.
2/ If you're saying that the general line length is about 40-50 characters and you don't need an exact line count, just grab the file size and divide by 45 (or whatever average you deem to use).
3/ If this is something like a log file and you don't have to keep it in one file (may require rework on other parts of the system), consider splitting the file periodically.
For example, when it gets to 5M, move it (e.g., x.log) to a dated file name (e.g., x_20090101_1022.log) and work out how many lines there are at that point (storing it in x_20090101_1022.count, then start a new x.log log file. Characteristics of log files mean that this dated section that was created will never change so you will never have to recalculate the number of lines.
To process the log "file", you'd just cat x_*.log through some process pipe rather than cat x.log. To get the line count of the "file", do a wc -l on the current x.log (relatively fast) and add it to the sum of all the values in the x_*.count files.
The thing that takes time is loading 40+ MB into memory. The fastest way to do that is to either memorymap it, or load it in one go into a big buffer. Once you have it in memory, one way or another, a loop traversing the data looking for \n characters is almost instantaneous, no matter how it is implemented.
So really, the most important trick is to load the file into memory as fast as possible. And the fastest way to do that is to do it as a single operation.
Otherwise, plenty of tricks may exist to speed up the algorithm. If lines are only added, never modified or removed, and if you're reading the file repeatedly, you can cache the lines read previously, and the next time you have to read the file, only read the newly added lines.
Or perhaps you can maintain a separate index file showing the location of known '\n' characters, so those parts of the file can be skipped over.
Reading large amounts of data from the harddrive is slow. There's no way around that.
If your file only grows, then Ludwig Weinzierl is the best solution if you do not have control of the writers. Otherwise, you can make it even faster: increment the counter by one each time a line is written to the file. If multiple writers may try to write to the file simultaneously, then make sure to use a lock. Locking your existing file is enough. The counter can be 4 or 8 bytes written in binary in a file written under /run/<your-prog-name>/counter (which is RAM so dead fast).
Ludwig Algorithm
Initialize offset to 0
Read file from offset to EOF counting '\n' (as mentioned by others, make sure to use buffered I/O and count the '\n' inside that buffer)
Update offset with position at EOF
Save counter & offset to a file or in a variable if you only need it in your software
Repeat from "Read file ..." on a change
This is actually how various software processing log files function (i.e. fail2ban comes to mind).
The first time, it has to process a huge file. Afterward, it is very small and thus goes very fast.
Proactive Algorithm
When creating the files, reset counter to 0.
Then each time you receive a new line to add to the file:
Lock file
Write one line
Load counter
Add one to counter
Save counter
Unlock file
This is very close to what database systems do so a SELECT COUNT(*) FROM table on a table with millions of rows return instantly. Databases also do that per index. So if you add a WHERE clause which matches a specific index, you also get the total instantly. Same principle as above.
Personal note: I see a huge number of Internet software which are backward. A watchdog makes sense for various things in a software environment. However, in most cases, when something of importance happens, you should send a message at the time it happens. Not use a backward concept of checking logs to detect that something bad just happened.
For example, you detect that a user tried to access a website and entered the wrong password 5 times in a row. You want to send a instant message to the admin to make sure there wasn't a 6th time which was successful and the hacker can now see all your user's data... If you use logs, the "instant message" is going to be late by seconds if not minutes.
Don't do processing backward.