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What is a safest way to set all bits of a variable to true in C?

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Re: What is a safest way to set all bits of a variable to true in C?

#21

Post by Natsu »

sqrt17 wrote: post #14
points wrote: post #6

How many 'ones complement' machines exist?

The most well-known are 30-40 years old by now and predate ANSI C (=c89), some predate even K&R C. They also have 18- or 36-bit words and other oddities. Compare that to non-ASCII systems (e.g. AS/400), which are still much in use now and probably have a sizable bit of C/C++ programs running on them (besides COBOL and Java). If you're programming for one of the more common platforms (i.e., x86, x64, ARM, PowerPC, 68k…

It's been quite a while since I heard VAX called one of the "more common" platforms. I'm too young. The only one I ever saw was in an ASU computer lab and we wrote our assembly language assignments on it (86HC11 assembly via some external test board... I don't even know what was in the VAX).

Re: What is a safest way to set all bits of a variable to true in C?

#22

Post by loup-vaillant »

But but but, this doesn't answer the question!? It explicitly acknowledges that -1 will not always set all bits to one, yet it recommends it!

That makes me very surprised by (1) the number of up-votes, and (2) the green "check" mark of approval.

Re: What is a safest way to set all bits of a variable to true in C?

#23

Post by protomyth »

sfk wrote: post #15
senki wrote: post #12

The "limits.h" (ISO C99 Standard) defines UINT_MAX as the maximum value for a variable of type unsigned int which is 4294967295 (0xffffffff).

Sigh : "The contents of the header are given below, in alphabetical order. The minimum magnitudes shown shall be replaced by implementation-defined magnitudes with the same sign." #define UINT_MAX 65535 Key here is implementation-defined , with a minimum of 65535.

I always though that was set by the vendor correctly. Oh well..

I always liked the definition of true in Forth (all bits set to 1). It really made it a lot easier.

Re: What is a safest way to set all bits of a variable to true in C?

#24

Post by BrandonM »

loup-vaillant wrote: post #22

But but but, this doesn't answer the question!? It explicitly acknowledges that -1 will not always set all bits to one, yet it recommends it! That makes me very surprised by (1) the number of up-votes, and (2) the green "check" mark of approval.

But but but, you apparently didn't understand the answer. It doesn't matter what the representation of -1 is. The C standard defines the cast of a negative number to an unsigned int as the (UINT_MAX + 1) modulo of the number. By definition,

  unsigned int foo = -1;
will set foo to 0xFFFF..., automatically setting all bits to 1 regardless of the number of bits in int types and without respect to the representation of negative numbers.

Re: What is a safest way to set all bits of a variable to true in C?

#25

Post by loup-vaillant »

BrandonM wrote: post #24
loup-vaillant wrote: post #22

But but but, this doesn't answer the question!? It explicitly acknowledges that -1 will not always set all bits to one, yet it recommends it! That makes me very surprised by (1) the number of up-votes, and (2) the green "check" mark of approval.

But but but, you apparently didn't understand the answer. It doesn't matter what the representation of -1 is. The C standard defines the cast of a negative number to an unsigned int as the (UINT_MAX + 1) modulo of the number. By definition , unsigned int foo = -1; will set foo to 0xFFFF..., automatically setting all bits to 1 regardless of the number of bits in int types and without respect to the representation of n…

EDIT : OK, just got it: I got the logic backwards: first, -1 is converted to uint. Second, -1 uint means UINT_MAX. Third, the binary representation of UINT_MAX is all 1s. The way I previously understood it, the -1 would be a signed integer which has some binary representation, and that binary representation would become the uint.

Weird bit of arcana. Below is my mistaken comment. (Notice that I pretended that UINT_MAX is not all 1s, which is silly. I suppose I made that mistake because I "couldn't be wrong" or something.)

As far as I know, your definition can't be inferred from the C standard. The answer itself acknowledges that -1 doesn't yield 0xFFFF… on every platform. The only guarantee is that it will yield UINT_MAX, which is not what was asked.

Otherwise, that would mean that C basically mandates a two's complement representation. Does it?

Re: What is a safest way to set all bits of a variable to true in C?

#26

Post by philwelch »

edanm wrote: post #17
jrockway wrote: post #2

The next question: Why? (To clarify: I mean, "why do you care what the bits are set to".)

I'm not sure what you mean. If you meant "why worry about bits, you should be dealing with values", then there are plenty of cases where that isn't true. Lots of programs (e.g. embedded programs) have to deal with actual bits, not with the values themselves. Just as an example, flags. (I don't know if this is what you meant, so apologies if I misunderstood your question.)

Also bitmaps. There are a lot of bitmaps at the system level denoting, f.ex., used and unused inumbers and blocks. (Though it is easier--and more common--to initialize bitmaps to 0 than to initialize bitmaps to 1).

Re: What is a safest way to set all bits of a variable to true in C?

#27

Post by tedunangst »

senki wrote: post #12

The "limits.h" (ISO C99 Standard) defines UINT_MAX as the maximum value for a variable of type unsigned int which is 4294967295 (0xffffffff).

The standard doesn't require that an int use all of the bits of storage it takes. A hypothetical 33-bit machine may present a C environment where ints are 32-bits, with the extra bit unused (and unset).

Re: What is a safest way to set all bits of a variable to true in C?

#28

Post by alnayyir »

edanm wrote: post #7

Personally I'd use: unsigned int a = 0xFFFF, not unsigned int a = -1. This is a classic case of a readability issue. 0xFFFF is, in my mind, much clearer on your intention than -1. The only problem is that you're assuming a specific int size, but really, if you're working with bits, chances are good that you're working on a platform where you know the architecture size (at least on embedded platforms).

Title said safest. use -1 and leave a very short comment.

Re: What is a safest way to set all bits of a variable to true in C?

#29

Post by Someone »

In C99, integer types can have padding bits that may not be writable, and writing all ones can be 'a trap representation' (except for the cas of unsigned char). So, I would guess that the portable way to do this requires taking the address of the variable, casting to (char unsigned *), and writing sizeof(var) all-ones unsigned char patterns (however that has to be done). I am not a C expert, though, so feel free to correct me.

Re: What is a safest way to set all bits of a variable to true in C?

#30

Post by saurik »

edanm wrote: post #7

Personally I'd use: unsigned int a = 0xFFFF, not unsigned int a = -1. This is a classic case of a readability issue. 0xFFFF is, in my mind, much clearer on your intention than -1. The only problem is that you're assuming a specific int size, but really, if you're working with bits, chances are good that you're working on a platform where you know the architecture size (at least on embedded platforms).

I actually work with bits a lot without knowing what the type is at all: enter templates, exit simplicity. My answer is to use C++ ~static_cast(0) or C ~((Type_) 0).
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