5 ms·

Why do you need a "ptrdiff_t" type, why isn't "size_t" sufficient?

by 0x0 11y ago

deleted 11y ago
[deleted]
colomon 11y ago
Also, why use the signed ptrdiff_t for a loop index that is never negative?
- benmmurphy 11y ago
  I thought using signed indexes in loops allow GCC to perform some optimizations because signed overflow is undefined.
- pmelendez 11y ago
  "size_t can store a pointer, it is better to use another unsinged integer type uintptr_t for that purpose " So he is actually recommending the unsigned version. Although there are some people (like the Google guidelines for C++) that recommend not using unsigned types because of overflows (which is arguable but it is a point)
- aCthrowa 11y ago
  ptrdiff_t is the wrong type for exactly this reason. OP's article is... not good.
nhaehnle 11y ago
ptrdiff_t is a signed type - after all, the diff stands for difference, which implies that it could be negative. size_t is an unsigned type - it is used to represent the sizes of objects in memory, which can never be negative.
- 0x0 11y ago
  But couldn't you just get along with unsigned types for both (i.e. just size_t)? I mean, two's complement means adding or subtracting is the same for unsigned and signed ints? Is it just for the convenience of being able to compare along the lines of (somePtrDiff < 0)? But then again you could just as well compare (somePtr > someOtherPtr)? Another funny thing I just discovered: You can't declare an array like "char somearray[SIZE_MAX]"; clang gives "error: array is too large (18446744073709551615 elements)". But you can use SIZE_MAX as an array index (although it gives a warning "array index -1 is past the end of the array"). Also, trying to declare an array like "char somearray[SIZE_MAX/8]" inside a simple main() just sends clang into 100% cpu...
  - ben0x539 11y ago
    Sometimes you actually want to do math beyond adding and ubstracting with ptrdiff_t. Multiply a potentially negative offset with something? Compare two offsets, one of them negative, with each other?
    - 0x0 11y ago
      For multiplication, I think the math works out the same for unsigned and signed, within mod bit-size and you throw away the "overflow"? I.e. (for 16bit pointers), 0xff_ff * 2 == 0x1_ff_fe (-1*2 == -2). For greater-than/smaller-than comparisons, I see the utility of working in the signed space though. I guess it all depends on expectations. If you have a pointer to the very last byte of a 2^32 address space (0xFFFF_FFFF), and subtract a pointer to the second byte from the start (0x0000_0001), you'll end up with a diff that should be "SIZE_MAX-2", but will be parsed as "-2" in the signed interpretation.
      - mikeash 11y ago
        Pointer subtraction is only valid for pointers within the same object. To overflow a signed ptrdiff_t, you'd need to allocate an array of char with more than 2^31 elements. This can be done on some systems, but it's really rare.
      - ben0x539 11y ago
        > For multiplication, I think the math works out the same for unsigned and signed, within mod bit-size and you throw away the "overflow"? Yeah, true. I recalled there being separate instructions and then messed up my napkin math to check if it made a difference within the relevant range.
seiji 11y ago
ptrdiff_t is signed (because subtraction). size_t is unsigned (because length). This page is a much shorter summary than the article: http://www.gnu.org/software/libc/manual/html_node/Important-Data-Types.html http://www.gnu.org/software/libc/manual/html_node/Important-... Also an important note: if you use special types like ptrdiff_t and size_t, you must not use the underlying native type printf specifiers when printing them. e.g. never print a ptrdiff_t using %ld, use the ptrdiff_t format specifier %td. (because some systems may say ptrdiff_t is long while others may say long long, and all your printf strings will then print compile time warnings.) Also see https://en.wikipedia.org/wiki/Printf_format_string#Length_field https://en.wikipedia.org/wiki/Printf_format_string#Length_fi... Plus, the medium article is a complete copy/paste of this 2009 article: http://www.viva64.com/en/a/0050/ http://www.viva64.com/en/a/0050/ — looks like the account https://news.ycombinator.com/user?id=PVS-Studio https://news.ycombinator.com/user?id=PVS-Studio is copying from viva64 and posting to medium for a lot of old articles.
- w0000t 11y ago
  Article republished by the author’s permission. I guess that is one way to write articles.
  - gjm11 11y ago
    The author of the original article is one Andrey Karpov. The Medium article is posted by someone called Sergey Vasiliev Marina Makarova and Andrey Karpov recommend". I guess it's the same Andrey Karpov in both cases, especially as hovering over the name Andrey Karpov on the Medium article indicates that his username is "Code_Analysis" and the original article was posted on the blog of a static-code-analysis company. Seems legit unless someone's actually impersonating someone else. I don't quite understand why, if Karpov wrote the original article and is a Medium contributor, Vasiliev rather than Karpov should be doing the reposting. (You could imagine something like this: K is a senior technical guy at PVS-Studio, V is a more junior technical guy or is in the marketing department; in 2009 they thought it better to post things under K's name to get it out there, but now K wants only the more impressive articles under his name and lets V post the others. But this doesn't feel terribly plausible.)
hamburglar 11y ago
A diff can be negative, whereas a size can't. However, my inclination was to ask the opposite: doesn't ptrdiff_t need to actually be larger than size_t if it must hold signed values and still cover the entire size_t range of inputs?
- phkahler 11y ago
  >> doesn't ptrdiff_t need to actually be larger than size_t if it must hold signed values and still cover the entire size_t range of inputs? Yes, but if you're just storing the delta between two pointers and then adding/subtracting this diff to get from one to the other it will still work. Of course it will also work using size_t.
  - hamburglar 11y ago
    >> if you're just storing the delta between two pointers and then adding/subtracting this diff to get from one to the other This would require keeping track of which one is bigger somewhere else unless you had enough room to store a sign. Seems like a pretty goofy way to go about it if you're going to the trouble of making a type specifically to hold a delta.
    - aCthrowa 11y ago
      Yes, this limits the utility of ptrdiff_t. But I don't believe you're allowed to subtract arbitrary pointers in C anyway. So you might need a >63-bit char array in order to legally hit this limit, and you probably don't have 263 of RAM. Practically, amd64 is already limited to more like 48 bits (4 levels of 9 bit pagetables describing 12 bit pages) of memory.
      - 0x0 11y ago
        I guess it could happen for applications targeting win32 with the /3GB switch
    - phkahler 11y ago
      >> This would require keeping track of which one is bigger somewhere else unless you had enough room to store a sign. Nope. You assume the math is being done mod 2^32 or 2^64 and it just works weather you use signed or unsigned integers. Pro-tip: the hardware does the exact same bit operations for signed and unsigned addition and subtraction (except for carry/borrow which aren't part of the C standard anyway). integer overflow is undefined, but this behavior is AFAICT universal anyway. If it didn't behave this way then a difference would in fact need the extra bit.
AndreyKarpov 11y ago
Why need a "ptrdiff_t" type: 1) http://www.viva64.com/en/l/0013/ http://www.viva64.com/en/l/0013/ 2) http://www.viva64.com/en/b/0039/ http://www.viva64.com/en/b/0039/

Why do you need a "ptrdiff_t" type, why isn't "size_t" sufficient?

by 0x0 11y ago

Why do you need a "ptrdiff_t" type, why isn't "size_t" sufficient?

deleted 11y ago

[deleted]

colomon 11y ago

Also, why use the signed ptrdiff_t for a loop index that is never negative?

benmmurphy 11y ago
I thought using signed indexes in loops allow GCC to perform some optimizations because signed overflow is undefined.
pmelendez 11y ago
"size_t can store a pointer, it is better to use another unsinged integer type uintptr_t for that purpose " So he is actually recommending the unsigned version. Although there are some people (like the Google guidelines for C++) that recommend not using unsigned types because of overflows (which is arguable but it is a point)
aCthrowa 11y ago
ptrdiff_t is the wrong type for exactly this reason. OP's article is... not good.

nhaehnle 11y ago

ptrdiff_t is a signed type - after all, the diff stands for difference, which implies that it could be negative. size_t is an unsigned type - it is used to represent the sizes of objects in memory, which can never be negative.

0x0 11y ago
But couldn't you just get along with unsigned types for both (i.e. just size_t)? I mean, two's complement means adding or subtracting is the same for unsigned and signed ints? Is it just for the convenience of being able to compare along the lines of (somePtrDiff < 0)? But then again you could just as well compare (somePtr > someOtherPtr)? Another funny thing I just discovered: You can't declare an array like "char somearray[SIZE_MAX]"; clang gives "error: array is too large (18446744073709551615 elements)". But you can use SIZE_MAX as an array index (although it gives a warning "array index -1 is past the end of the array"). Also, trying to declare an array like "char somearray[SIZE_MAX/8]" inside a simple main() just sends clang into 100% cpu...
- ben0x539 11y ago
  Sometimes you actually want to do math beyond adding and ubstracting with ptrdiff_t. Multiply a potentially negative offset with something? Compare two offsets, one of them negative, with each other?
  - 0x0 11y ago
    For multiplication, I think the math works out the same for unsigned and signed, within mod bit-size and you throw away the "overflow"? I.e. (for 16bit pointers), 0xff_ff * 2 == 0x1_ff_fe (-1*2 == -2). For greater-than/smaller-than comparisons, I see the utility of working in the signed space though. I guess it all depends on expectations. If you have a pointer to the very last byte of a 2^32 address space (0xFFFF_FFFF), and subtract a pointer to the second byte from the start (0x0000_0001), you'll end up with a diff that should be "SIZE_MAX-2", but will be parsed as "-2" in the signed interpretation.
    - mikeash 11y ago
      Pointer subtraction is only valid for pointers within the same object. To overflow a signed ptrdiff_t, you'd need to allocate an array of char with more than 2^31 elements. This can be done on some systems, but it's really rare.
    - ben0x539 11y ago
      > For multiplication, I think the math works out the same for unsigned and signed, within mod bit-size and you throw away the "overflow"? Yeah, true. I recalled there being separate instructions and then messed up my napkin math to check if it made a difference within the relevant range.

seiji 11y ago

ptrdiff_t is signed (because subtraction). size_t is unsigned (because length). This page is a much shorter summary than the article: http://www.gnu.org/software/libc/manual/html_node/Important-Data-Types.html http://www.gnu.org/software/libc/manual/html_node/Important-... Also an important note: if you use special types like ptrdiff_t and size_t, you must not use the underlying native type printf specifiers when printing them. e.g. never print a ptrdiff_t using %ld, use the ptrdiff_t format specifier %td. (because some systems may say ptrdiff_t is long while others may say long long, and all your printf strings will then print compile time warnings.) Also see https://en.wikipedia.org/wiki/Printf_format_string#Length_field https://en.wikipedia.org/wiki/Printf_format_string#Length_fi... Plus, the medium article is a complete copy/paste of this 2009 article: http://www.viva64.com/en/a/0050/ http://www.viva64.com/en/a/0050/ — looks like the account https://news.ycombinator.com/user?id=PVS-Studio https://news.ycombinator.com/user?id=PVS-Studio is copying from viva64 and posting to medium for a lot of old articles.

w0000t 11y ago
Article republished by the author’s permission. I guess that is one way to write articles.
- gjm11 11y ago
  The author of the original article is one Andrey Karpov. The Medium article is posted by someone called Sergey Vasiliev Marina Makarova and Andrey Karpov recommend". I guess it's the same Andrey Karpov in both cases, especially as hovering over the name Andrey Karpov on the Medium article indicates that his username is "Code_Analysis" and the original article was posted on the blog of a static-code-analysis company. Seems legit unless someone's actually impersonating someone else. I don't quite understand why, if Karpov wrote the original article and is a Medium contributor, Vasiliev rather than Karpov should be doing the reposting. (You could imagine something like this: K is a senior technical guy at PVS-Studio, V is a more junior technical guy or is in the marketing department; in 2009 they thought it better to post things under K's name to get it out there, but now K wants only the more impressive articles under his name and lets V post the others. But this doesn't feel terribly plausible.)

hamburglar 11y ago

A diff can be negative, whereas a size can't. However, my inclination was to ask the opposite: doesn't ptrdiff_t need to actually be larger than size_t if it must hold signed values and still cover the entire size_t range of inputs?

phkahler 11y ago
>> doesn't ptrdiff_t need to actually be larger than size_t if it must hold signed values and still cover the entire size_t range of inputs? Yes, but if you're just storing the delta between two pointers and then adding/subtracting this diff to get from one to the other it will still work. Of course it will also work using size_t.
- hamburglar 11y ago
  >> if you're just storing the delta between two pointers and then adding/subtracting this diff to get from one to the other This would require keeping track of which one is bigger somewhere else unless you had enough room to store a sign. Seems like a pretty goofy way to go about it if you're going to the trouble of making a type specifically to hold a delta.
  - aCthrowa 11y ago
    Yes, this limits the utility of ptrdiff_t. But I don't believe you're allowed to subtract arbitrary pointers in C anyway. So you might need a >63-bit char array in order to legally hit this limit, and you probably don't have 263 of RAM. Practically, amd64 is already limited to more like 48 bits (4 levels of 9 bit pagetables describing 12 bit pages) of memory.
    - 0x0 11y ago
      I guess it could happen for applications targeting win32 with the /3GB switch
  - phkahler 11y ago
    >> This would require keeping track of which one is bigger somewhere else unless you had enough room to store a sign. Nope. You assume the math is being done mod 2^32 or 2^64 and it just works weather you use signed or unsigned integers. Pro-tip: the hardware does the exact same bit operations for signed and unsigned addition and subtraction (except for carry/borrow which aren't part of the C standard anyway). integer overflow is undefined, but this behavior is AFAICT universal anyway. If it didn't behave this way then a difference would in fact need the extra bit.

AndreyKarpov 11y ago

Why need a "ptrdiff_t" type: 1) http://www.viva64.com/en/l/0013/ http://www.viva64.com/en/l/0013/ 2) http://www.viva64.com/en/b/0039/ http://www.viva64.com/en/b/0039/