Fixes a technicality regarding the size of C's char type
#89114
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Specifically, ISO/IEC 9899:2018 — better known as "C18" — (and at least
C11, C99 and C89) do not specify the size of `byte` in bits.
Section 3.6 defines "byte" as "addressable unit of data storage" while
section 6.2.5 ("Types") only defines "char" as "large enough to store
any member of the basic execution set" giving it a lower bound of 7 bit
(since there are 96 characters in the basic execution set).
With section 6.5.3.4 paragraph 4 "When sizeof is applied to an operant
that has type char […] the result is 1" you could read this as the size
of `char` in bits being defined as exactly the same as the number of
bits in a byte but it's also valid to read that as an exception.
In general implementations take `char` as the smallest unit of
addressable memory, which for modern byte-addressed architectures is
overwhelmingly 8 bits to the point of this convention being completely
cemented into just about all of our software.
So is any of this actually relevant at all? I hope not. I sincerely hope
that this never, ever comes up.
But if for some reason a poor rustacean is having to interface with C
code running on a Cray X1 that in 2003 is still doing word-addressed
memory with 64-bit words and they trust the docs here blindly it will
blow up in her face. And I'll be truly sorry for her to have to deal
with … all of that.
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Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @yaahc (or someone else) soon. Please see the contribution instructions for more information. |
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@bors r+ Thank you for the clarification ^_^ |
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📌 Commit 23c608f has been approved by |
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Rollup of 12 pull requests Successful merges: - rust-lang#88795 (Print a note if a character literal contains a variation selector) - rust-lang#89015 (core::ascii::escape_default: reduce struct size) - rust-lang#89078 (Cleanup: Remove needless reference in ParentHirIterator) - rust-lang#89086 (Stabilize `Iterator::map_while`) - rust-lang#89096 ([bootstrap] Improve the error message when `ninja` is not found to link to installation instructions) - rust-lang#89113 (dont `.ensure()` the `thir_abstract_const` query call in `mir_build`) - rust-lang#89114 (Fixes a technicality regarding the size of C's `char` type) - rust-lang#89115 (:arrow_up: rust-analyzer) - rust-lang#89126 (Fix ICE when `indirect_structural_match` is allowed) - rust-lang#89141 (Impl `Error` for `FromSecsError` without foreign type) - rust-lang#89142 (Fix match for placeholder region) - rust-lang#89147 (add case for checking const refs in check_const_value_eq) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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Sep 28, 2021
Rollup of 12 pull requests Successful merges: - rust-lang#88795 (Print a note if a character literal contains a variation selector) - rust-lang#89015 (core::ascii::escape_default: reduce struct size) - rust-lang#89078 (Cleanup: Remove needless reference in ParentHirIterator) - rust-lang#89086 (Stabilize `Iterator::map_while`) - rust-lang#89096 ([bootstrap] Improve the error message when `ninja` is not found to link to installation instructions) - rust-lang#89113 (dont `.ensure()` the `thir_abstract_const` query call in `mir_build`) - rust-lang#89114 (Fixes a technicality regarding the size of C's `char` type) - rust-lang#89115 (:arrow_up: rust-analyzer) - rust-lang#89126 (Fix ICE when `indirect_structural_match` is allowed) - rust-lang#89141 (Impl `Error` for `FromSecsError` without foreign type) - rust-lang#89142 (Fix match for placeholder region) - rust-lang#89147 (add case for checking const refs in check_const_value_eq) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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Specifically, ISO/IEC 9899:2018 — better known as "C18" — (and at least
C11, C99 and C89) do not specify the size of
bytein bits.Section 3.6 defines "byte" as "addressable unit of data storage" while
section 6.2.5 ("Types") only defines "char" as "large enough to store
any member of the basic execution set" giving it a lower bound of 7 bit
(since there are 96 characters in the basic execution set).
With section 6.5.3.4 paragraph 4 "When sizeof is applied to an operant
that has type char […] the result is 1" you could read this as the size
of
charin bits being defined as exactly the same as the number ofbits in a byte but it's also valid to read that as an exception.
In general implementations take
charas the smallest unit ofaddressable memory, which for modern byte-addressed architectures is
overwhelmingly 8 bits to the point of this convention being completely
cemented into just about all of our software.
So is any of this actually relevant at all? I hope not. I sincerely hope
that this never, ever comes up.
But if for some reason a poor rustacean is having to interface with C
code running on a Cray X1 that in 2003 is still doing word-addressed
memory with 64-bit chars and they trust the docs here blindly it will
blow up in her face. And I'll be truly sorry for her to have to deal
with … all of that.