C/C++ best way to send a number of bytes to stdout

Question

Profiling my program and the function print is taking a lot of time to perform. How can I send "raw" byte output directly to stdout instead of using fwrite, and making it faster (need to send all 9bytes in the print() at the same time to the stdout) ?

void print(){
    unsigned char temp[9];

    temp[0] = matrix[0][0];
    temp[1] = matrix[0][1];
    temp[2] = matrix[0][2];
    temp[3] = matrix[1][0];
    temp[4] = matrix[1][1];
    temp[5] = matrix[1][2];
    temp[6] = matrix[2][0];
    temp[7] = matrix[2][1];
    temp[8] = matrix[2][2];

    fwrite(temp,1,9,stdout);

}

Matrix is defined globally to be a unsigned char matrix[3][3];

Answer 1

All printing is fairly slow, although iostreams are really slow for printing.

Your best bet would be to use printf, something along the lines of:

printf("%c%c%c%c%c%c%c%c%c\n", matrix[0][0], matrix[0][1], matrix[0][2], matrix[1][0],
  matrix[1][1], matrix[1][2], matrix[2][0], matrix[2][1], matrix[2][2]);

Answer 2

IO is not an inexpensive operation. It is, in fact, a blocking operation, meaning that the OS can preempt your process when you call write to allow more CPU-bound processes to run, before the IO device you're writing to completes the operation.

The only lower level function you can use (if you're developing on a *nix machine), is to use the raw write function, but even then your performance will not be that much faster than it is now. Simply put: IO is expensive.

Answer 3

What's wrong with:

fwrite(matrix,1,9,stdout);

both the one and the two dimensional arrays take up the same memory.

Answer 4

You can simply:

std::cout << temp;

printf is more C-Style.

Yet, IO operations are costly, so use them wisely.

Answer 5

The rawest form of output you can do is the probable the write system call, like this

write (1, matrix, 9);

1 is the file descriptor for standard out (0 is standard in, and 2 is standard error). Your standard out will only write as fast as the one reading it at the other end (i.e. the terminal, or the program you're pipeing into) which might be rather slow.

I'm not 100% sure, but you could try setting non-blocking IO on fd 1 (using fcntl) and hope the OS will buffer it for you until it can be consumed by the other end. It's been a while, but I think it works like this

fcntl (1, F_SETFL, O_NONBLOCK);

YMMV though. Please correct me if I'm wrong on the syntax, as I said, it's been a while.

Answer 6

Perhaps your problem is not that fwrite() is slow, but that it is buffered. Try calling fflush(stdout) after the fwrite().

This all really depends on your definition of slow in this context.

Answer 7

Try running the program twice. Once with output and once without. You will notice that overall, the one without the io is the fastest. Also, you could fork the process (or create a thread), one writing to a file(stdout), and one doing the operations.

Answer 8

As everyone has pointed out IO in tight inner loop is expensive. I have normally ended up doing conditional cout of Matrix based on some criteria when required to debug it.

If your app is console app then try redirecting it to a file, it will be lot faster than doing console refreshes. e.g app.exe > matrixDump.txt