I can just speak a little English so I hope you can understand what I said.
I fork a child process , then I do ADD in child process. EX: 56+48=104
If the value lower than 255 , I can use "wexitstatus(status)" to get the answer.
But if the value higher than 256, it would be wrong !
How can I do?
If the program returns an exit code > 255, the program is simply wrong and needs to be fixed. That's against Unix standard. If you're not using standard Unix, you're probably going to need specialist help from within your organisation contacts.
From manpage for wait():
WEXITSTATUS(stat_val)
If the value of WIFEXITED(stat_val) is non-zero, this macro evaluates to the low-order 8 bits of the status argument that the child process passed to _exit() or exit(), or the value the child process returned from main().
It's limited to 8-bits, which means 1 byte, which means the int from WEXITSTATUS can only range from 0-255. In fact, any Unix program will only ever return a max of 255.
Additionally, many OS's/programs reserve > 127 for system designated codes, so you shouldn't even use anything above that.
If you need more return codes than 126 (since 0 is no error), consider writing it to STDOUT and hooking that.
Related
I was reviewing some code from a library for Arduino and saw the following if statement in the main loop:
draw_state++;
if ( draw_state >= 14*8 )
draw_state = 0;
draw_state is a uint8_t.
Why is 14*8 written here instead of 112? I initially thought this was done to save space, as 14 and 8 can both be represented by a single byte, but then so can 112.
I can't see why a compiler wouldn't optimize this to 112, since otherwise it would mean a multiplication has to be done every iteration instead of the lookup of a value. This looks to me like there is some form of memory and processing tradeoff.
Does anyone have a suggestion as to why this was done?
Note: I had a hard time coming up with a clear title, so suggestions are welcome.
Probably to explicitly show where the number 112 came from. For example, it could be number of bits in 14 bytes (but of course I don't know the context of the code, so I could be wrong). It would then be more obvious to humans where the value came from, than wiriting just 112.
And as you pointed out, the compiler will probably optimize it, so there will be no multiplication in the machine code.
I am still having a little bit of trouble understanding the sensitivity list and it activating a process.
most textbooks say that a process is activated every time an event occurs on a signal inside the sensitivity list.
process(in)
begin
x <= in;
end process;
Now looking at this example, "in" is an input declared in the entity. Now if "in" starts off at 0 and changes to 1 then the process would activate and the value of x would take in value "in". Now suppose after in changed from 0 to 1 that it now stays at constant value of 1. Does this mean the process will not get activated? Will x still give output of '1'? I want to say that it wont get activated and will only activate once in changes back from 1 back to 0. Can someone please confirm?
Within the sensitivity list (I assume this is hardware language, VHDL has the same exact syntax and format), whenever there is some type of signal change (L-> H, 0 ->1, 1-> 0... any change in the variable you listed within the sensitivity list), it will activate the process and the process will execute until completion, which then the process will end. When the process end, signals/outputs (depends on how you interpret them) will be stored on a driver, which will update given signals after some propagation delay.
So from your second statement, yes. If it changes to 0 -> 1, the process activates, if its 1 -> 0, the process activates, and if in remains 1, the process will not be activated. So x's value remains.
According to the original specification '98, Ben Olmstead Malbolge VM fill empty memory cells using crazy op on two previous cells. "Cells which are not initialized are set by performing op on the previous two cells repetitively." I.e.
[m] = crz [m-2], [m-1]
For the sake of sanity what should I do if the program contains only 1 instruction?
Or should I assume the last character always to be EOF?
Judging by the implementation and language-lawyering, there are two options -
If we consider the definition of "two previous cells" as, literally, the two previous cells, then a single-char or empty malbolge program is illegal in the language, because it can not be executed according to the specs.
If we consider the definition of [m] = crz [m-2], [m-1], it gets interesting. The main implementation (alongside probably most of the rest) uses unsigned short (or int) for the memory pointer. When you try subtracting 2 from 1 (m-2) it results in 0xffff, decimal 65535 (see this answer for details), which is just a bit over malbolge's 59049 memory limit. That glitch runs (almost) perfectly on a normal machine, using the 0xffff cell for crazy-op computing (without even harming the out-of-environment memory!), but will fail on a limited-memory or virtual machine.
You might end up with 0xffffffff instead of 0xffff, depending on the way you use the pointer.
In short,
If you run it by hand, assume it fails.
If you run it on a virtual machine, it fails.
If you run it on a simulator, it will probably work, but will fail the point of running itself, since 0xffff is a random-valued memory cell, leading to random values along the environment memory. On the other hand, what can you expect from a single-byte malbolge program?
I'm looking at the stratum protocol and I'm having a problem with the nbits value of the mining.notify method. I have trouble calculating it, I assume it's the currency difficulty.
I pull a notify from a dogecoin pool and it returned 1b3cc366 and at the time the difficulty was 1078.52975077.
I'm assuming here that 1b3cc366 should give me 1078.52975077 when converted. But I can't seem to do the conversion right.
I've looked here, here and also tried the .NET function BitConverter.Int64BitsToDouble.
Can someone help me understand what the nbits value signify?
You are right, nbits is current network difficulty.
Difficulty encoding is throughly described here.
Hexadecimal representation like 0x1b3cc366 consists of two parts:
0x1b -- number of bytes in a target
0x3cc366 -- target prefix
This means that valid hash should be less than 0x3cc366000000000000000000000000000000000000000000000000 (it is exactly 0x1b = 27 bytes long).
Floating point representation of difficulty shows how much current target is harder than the one used in the genesis block.
Satoshi decided to use 0x1d00ffff as a difficulty for the genesis block, so the target was
0x00ffff0000000000000000000000000000000000000000000000000000.
And 1078.52975077 is how much current target is greater than the initial one:
$ echo 'ibase=16;FFFF0000000000000000000000000000000000000000000000000000 / 3CC366000000000000000000000000000000000000000000000000' | bc -l
1078.52975077482646448605
So, this should be an easy question for anyone who has used FORTH before, but I am a newbie trying to learn how to code this language (and this is a lot different than C++).
Anyways, I'm just trying to create a variable in FORTH called "Height" and I want a user to be able to input a value for "Height" whenever a certain word "setHeight" is called. However, everything I try seems to be failing because I don't know how to set up the variable nor how to grab user input and put it in the variable.
VARIABLE Height 5 ALLOT
: setHeight 5 ACCEPT ATOI CR ;
I hope this is an easy problem to fix, and any help would be greatly appreciated.
Thank you in advance.
Take a look at Rosettacode input/output examples for string or number input in FORTH:
String Input
: INPUT$ ( n -- addr n )
PAD SWAP ACCEPT
PAD SWAP ;
Number Input
: INPUT# ( -- u true | false )
0. 16 INPUT$ DUP >R
>NUMBER NIP NIP
R> <> DUP 0= IF NIP THEN ;
A big point to remember for your self-edification -- C++ is heavily typecasted, Forth is the complete opposite. Do you want Height to be a string, an integer, or a float, and is it signed or unsigned? Each has its own use cases. Whatever you choose, you must interact with the Height variable with the type you choose kept in mind. Think about what your bits mean every time.
Judging by your ATOI call, I assume you want the value of Height as an integer. A 5 byte integer is unusual, though, so I'm still not certain. But here goes with that assumption:
VARIABLE Height 1 CELLS ALLOT
VARIABLE StrBuffer 7 ALLOT
: setHeight ( -- )
StrBuffer 8 ACCEPT
DECIMAL ATOI Height ! ;
The CELLS call makes sure you're creating a variable with the number of bits your CPU prefers. The DECIMAL call makes sure you didn't change to HEX somewhere along the way before your ATOI.
Creating the StrBuffer variable is one of numerous ways to get a scratch space for strings. Assuming your CELL is 16-bit, you will need a maximum of 7 characters for a zero-terminated 16-bit signed integer -- for example, "-32767\0". Some implementations have PAD, which could be used instead of creating your own buffer. Another common word is SCRATCH, but I don't think it works the way we want.
If you stick with creating your own string buffer space, which I personally like because you know exactly how much space you got, then consider creating one large buffer for all your words' string handling needs. For example:
VARIABLE StrBuffer 201 ALLOT
This also keeps you from having to make the 16-bit CELL assumption, as 200 characters easily accommodates a 64-bit signed integer, in case that's your implementation's CELL size now or some day down the road.