5 ms·

char is 1 byte exactly.

by jwilk 10y ago

John23832 10y ago
A char is 1 byte on most systems, but the actual standard defines it as having CHAR_BITS number of bits. Modern C defines CHAR_BITS as CHAR_BITS >= 8. Some older machines have 7 bits. Theres nothing stopping machines from changing what CHAR_BITS is. <Limits.h>
- kps 10y ago
  People are talking past each other. In the C standard ‘byte’ means minimum addressable unit — the unit of sizeof — which need not be exactly 8 bits (but in ANSI C must be at least 8 bits). In C, 'char' is one byte, always and exactly. It's also possible for 'long' to be one byte, provided CHAR_BIT≥32.
  - John23832 10y ago
    > In the C standard ‘byte’ means minimum addressable unit — the unit of sizeof Do you have a reference? I've always seen a byte defined as 8 bits. No matter what. (Unless you're talking to a hard drive manufacturer, that's a different bucket of worms though.)
    - kps 10y ago
      Orginally? See §7.4 of Planning a Computer System: Project Stretch, edited by Werner Buchholz who coined the term. http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/Buchholz_102636426.pdf http://archive.computerhistory.org/resources/text/IBM/Stretc... For C, see C11 §3.6: “byte — addressable unit of data storage large enough to hold any member of the basic character set of the execution environment” http://www.iso-9899.info/n1570.html#3.6 http://www.iso-9899.info/n1570.html#3.6
      - John23832 10y ago
        Makes sense now. Thanks
    - jwilk 10y ago
      (From the C99 standard:) § 6.5.3.4 The sizeof operator […] Semantics 2 The sizeof operator yields the size (in bytes) of its operand, […] 3 When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1.
    - Someone 10y ago
      The draft ISO/IEC 9899:201x PDF of "Programming Languages — C" says: "3.5 bit unit of data storage in the execution environment large enough to hold an object that may have one of two values NOTE It need not be possible to express the address of each individual bit of an object. 3.6 byte addressable unit of data storage large enough to hold any member of the basic character set of the execution environment NOTE 1 It is possible to express the address of each individual byte of an object uniquely. NOTE 2 A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined. The least significant bit is called the low-order bit; the most significant bit is called the high-order bit." I can't find any indication that a byte is the minimum addressable unit. I guess that is because of 4-bit CPUs and/or CPUs that can address individual bits. It also uses "byte" very sparingly, most of the time in the context of multibyte (not hyphenated) to single-byte (hyphenated) conversions, but also in section 5.2.1: "The representation of each member of the source and execution basic character sets shall fit in a byte." And there also is a guarantee that a unsigned char is as large as a byte in a footnote to section 6.2.6.1: "A byte contains CHAR_BIT bits, and the values of type unsigned char range from 0 to 2^CHAR_BIT − 1" I doubt that char and unsinged char can be of different size, but didn't check that.
Koshkin 10y ago
More accurately, sizeof(char) is always 1. This is simply because sizeof returns the size of the object measured in terms of the equivalent number of chars (rather than 'bytes').
- John23832 10y ago
  That's not true. sizeof(char) returns 1 because the size of a char in that implementation is 1 byte and C doesn't directly address bits.
  - Koshkin 10y ago
    Well, according to the Wikipedia article about sizeof, In the programming languages C and C++, the unary operator sizeof generates the size of a variable or datatype, measured in the number of char size storage units required for the type. As such, the construct sizeof (char) is guaranteed to be 1.
    - John23832 10y ago
      I stand corrected.
      - Koshkin 10y ago
        You were correct after all. The C99 standard does indeed talk about sizeof in terms of bytes!
        John23832 10y ago
        I guess this is one of those few times that we can say we were both correct.
- jwilk 10y ago
  Perhaps your notion of what a byte is different, but according to the C99 standard: - sizeof returns size in bytes; - sizeof (char) == 1; - one byte has CHAR_BIT bits; - CHAR_BIT >= 8.
iainmerrick 10y ago
I was about to jump in and say "no it's not!" but I see several people have pointed out that the C Standard of all things says bytes don't necessarily have 8 bits. Wow! What I'd like to know is, how common are non-8-bit-byte CPUs these days, for new devices? I worked with Texas Instruments C54x and C55x DSPs back in the early 2000s (https://en.wikipedia.org/wiki/Texas_Instruments_TMS320#C5000_series https://en.wikipedia.org/wiki/Texas_Instruments_TMS320#C5000...). They had 16-bit chars and that was incredibly annoying -- very few low-level third-party libraries would even compile properly. Are those still used? It seems like everything has an ARM chip on the side these days, so maybe (hopefully) there's less need to run application code on bizarre DSPs. Except for those unsung heroes maintaining old set-top boxes and the like.

char is 1 byte exactly.

by jwilk 10y ago

char is 1 byte exactly.

A char is 1 byte on most systems, but the actual standard defines it as having CHAR_BITS number of bits. Modern C defines CHAR_BITS as CHAR_BITS >= 8. Some older machines have 7 bits. Theres nothing stopping machines from changing what CHAR_BITS is. <Limits.h>

kps 10y ago
People are talking past each other. In the C standard ‘byte’ means minimum addressable unit — the unit of sizeof — which need not be exactly 8 bits (but in ANSI C must be at least 8 bits). In C, 'char' is one byte, always and exactly. It's also possible for 'long' to be one byte, provided CHAR_BIT≥32.
- John23832 10y ago
  > In the C standard ‘byte’ means minimum addressable unit — the unit of sizeof Do you have a reference? I've always seen a byte defined as 8 bits. No matter what. (Unless you're talking to a hard drive manufacturer, that's a different bucket of worms though.)
  - kps 10y ago
    Orginally? See §7.4 of Planning a Computer System: Project Stretch, edited by Werner Buchholz who coined the term. http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/Buchholz_102636426.pdf http://archive.computerhistory.org/resources/text/IBM/Stretc... For C, see C11 §3.6: “byte — addressable unit of data storage large enough to hold any member of the basic character set of the execution environment” http://www.iso-9899.info/n1570.html#3.6 http://www.iso-9899.info/n1570.html#3.6
    - John23832 10y ago
      Makes sense now. Thanks
  - jwilk 10y ago
    (From the C99 standard:) § 6.5.3.4 The sizeof operator […] Semantics 2 The sizeof operator yields the size (in bytes) of its operand, […] 3 When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1.
  - Someone 10y ago
    The draft ISO/IEC 9899:201x PDF of "Programming Languages — C" says: "3.5 bit unit of data storage in the execution environment large enough to hold an object that may have one of two values NOTE It need not be possible to express the address of each individual bit of an object. 3.6 byte addressable unit of data storage large enough to hold any member of the basic character set of the execution environment NOTE 1 It is possible to express the address of each individual byte of an object uniquely. NOTE 2 A byte is composed of a contiguous sequence of bits, the number of which is implementation-defined. The least significant bit is called the low-order bit; the most significant bit is called the high-order bit." I can't find any indication that a byte is the minimum addressable unit. I guess that is because of 4-bit CPUs and/or CPUs that can address individual bits. It also uses "byte" very sparingly, most of the time in the context of multibyte (not hyphenated) to single-byte (hyphenated) conversions, but also in section 5.2.1: "The representation of each member of the source and execution basic character sets shall fit in a byte." And there also is a guarantee that a unsigned char is as large as a byte in a footnote to section 6.2.6.1: "A byte contains CHAR_BIT bits, and the values of type unsigned char range from 0 to 2^CHAR_BIT − 1" I doubt that char and unsinged char can be of different size, but didn't check that.

Koshkin 10y ago

More accurately, sizeof(char) is always 1. This is simply because sizeof returns the size of the object measured in terms of the equivalent number of chars (rather than 'bytes').

John23832 10y ago
That's not true. sizeof(char) returns 1 because the size of a char in that implementation is 1 byte and C doesn't directly address bits.
- Koshkin 10y ago
  Well, according to the Wikipedia article about sizeof, In the programming languages C and C++, the unary operator sizeof generates the size of a variable or datatype, measured in the number of char size storage units required for the type. As such, the construct sizeof (char) is guaranteed to be 1.
  - John23832 10y ago
    I stand corrected.
    - Koshkin 10y ago
      You were correct after all. The C99 standard does indeed talk about sizeof in terms of bytes!
      - John23832 10y ago
        I guess this is one of those few times that we can say we were both correct.
jwilk 10y ago
Perhaps your notion of what a byte is different, but according to the C99 standard: - sizeof returns size in bytes; - sizeof (char) == 1; - one byte has CHAR_BIT bits; - CHAR_BIT >= 8.

iainmerrick 10y ago

I was about to jump in and say "no it's not!" but I see several people have pointed out that the C Standard of all things says bytes don't necessarily have 8 bits. Wow! What I'd like to know is, how common are non-8-bit-byte CPUs these days, for new devices? I worked with Texas Instruments C54x and C55x DSPs back in the early 2000s (https://en.wikipedia.org/wiki/Texas_Instruments_TMS320#C5000_series https://en.wikipedia.org/wiki/Texas_Instruments_TMS320#C5000...). They had 16-bit chars and that was incredibly annoying -- very few low-level third-party libraries would even compile properly. Are those still used? It seems like everything has an ARM chip on the side these days, so maybe (hopefully) there's less need to run application code on bizarre DSPs. Except for those unsung heroes maintaining old set-top boxes and the like.