What is an unsigned char

Question

In C C    what an unsigned char is used for  How is it different from a regular char

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In terms of direct values a regular char is used when the values are known to be between CHAR_MIN and CHAR_MAX while an unsigned char provides double the range on the positive end. For example, if CHAR_BIT is 8, the range of regular char is only guaranteed to be [0, 127] (because it can be signed or unsigned) while unsigned char will be [0, 255] and signed char will be [-127, 127].

In terms of what it's used for, the standards allow objects of POD (plain old data) to be directly converted to an array of unsigned char. This allows you to examine the representation and bit patterns of the object. The same guarantee of safe type punning doesn't exist for char or signed char.

User · Answer

In C    there are three distinct character types    char signed char unsigned char   If you are using character types for text  use the unqualified char    it is the type of character literals like  a  or  0   it is the type that makes up C strings like  abcde    It also works out as a number value  but it is unspecified whether that value is treated as signed or unsigned  Beware character comparisons through inequalities - although if you limit yourself to ASCII  0-127  you re just about safe   If you are using character types as numbers  use    signed char  which gives you at least the -127 to 127 range   -128 to 127 is common  unsigned char  which gives you at least the 0 to 255 range     At least   because the C   standard only gives the minimum range of values that each numeric type is required to cover  sizeof  char  is required to be 1  i e  one byte   but a byte could in theory be for example 32 bits  sizeof would still be report its size as 1 - meaning that you could have sizeof  char     sizeof  long     1

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An unsigned char uses the bit that is reserved for the sign of a regular char as another number  This changes the range to  0 - 255  as opposed to  -128 - 127     Generally unsigned chars are used when you don t want a sign  This will make a difference when doing things like shifting bits  shift extends the sign  and other things when dealing with a char as a byte rather than using it as a number

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signed char has range -128 to 127  unsigned char has range 0 to 255   char will be equivalent to either signed char or unsigned char  depending on the compiler  but is a distinct type   If you re using C-style strings  just use char  If you need to use chars for arithmetic  pretty rare   specify signed or unsigned explicitly for portability

User · Answer

This is implementation dependent  as the C standard does NOT define the signed-ness of char   Depending on the platform  char may be signed or unsigned  so you need to explicitly ask for signed char or unsigned char if your implementation depends on it   Just use char if you intend to represent characters from strings  as this will match what your platform puts in the string   The difference between signed char and unsigned char is as you d expect   On most platforms  signed char will be an 8-bit two s complement number ranging from -128 to 127  and unsigned char will be an 8-bit unsigned integer  0 to 255    Note the standard does NOT require that char types have 8 bits  only that sizeof char  return 1   You can get at the number of bits in a char with CHAR BIT in limits h   There are few if any platforms today where this will be something other than 8  though   There is a nice summary of this issue here   As others have mentioned since I posted this  you re better off using int8 t and uint8 t if you really want to represent small integers

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If you want to use a character as a small integer  the safest way to do it is with the int8 tand uint8 t types

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quoted frome  the c programming laugage  book   The qualifier signed or unsigned may be applied to char or any integer  unsigned numbers are always positive or zero  and obey the laws of arithmetic modulo 2 n  where n is the number of bits in the type  So  for instance  if chars are 8 bits  unsigned char variables have values between 0 and 255  while signed chars have values between -128 and 127  in a two  s complement machine   Whether plain chars are signed or unsigned is machine-dependent  but printable characters are always positive

User · Answer

An unsigned char uses the bit that is reserved for the sign of a regular char as another number  This changes the range to  0 - 255  as opposed to  -128 - 127     Generally unsigned chars are used when you don t want a sign  This will make a difference when doing things like shifting bits  shift extends the sign  and other things when dealing with a char as a byte rather than using it as a number

User · Answer

unsigned char takes only positive values  0 to 255 while signed char takes positive and negative values  -128 to  127

User · Answer

If you like using various types of specific length and signedness  you re probably better off with uint8 t  int8 t  uint16 t  etc simply because they do exactly what they say

User · Answer

signed char has range -128 to 127  unsigned char has range 0 to 255   char will be equivalent to either signed char or unsigned char  depending on the compiler  but is a distinct type   If you re using C-style strings  just use char  If you need to use chars for arithmetic  pretty rare   specify signed or unsigned explicitly for portability

User · Answer

An unsigned char is an unsigned byte value  0 to 255    You may be thinking of char in terms of being a  character  but it is really a numerical value   The regular char is signed  so you have 128 values  and these values map to characters using ASCII encoding   But in either case  what you are storing in memory is a byte value

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char and unsigned char aren t guaranteed to be 8-bit types on all platforms mdash they are guaranteed to be 8-bit or larger  Some platforms have 9-bit  32-bit  or 64-bit bytes  However  the most common platforms today  Windows  Mac  Linux x86  etc   have 8-bit bytes

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unsigned char is the heart of all bit trickery  In almost ALL compiler for ALL platform an unsigned char is simply a byte and an unsigned integer of  usually  8 bits that can be treated as a small integer or a pack of bits    In addiction  as someone else has said  the standard doesn t define the sign of a char  so you have 3 distinct char types  char  signed char  unsigned char

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quoted frome  the c programming laugage  book   The qualifier signed or unsigned may be applied to char or any integer  unsigned numbers are always positive or zero  and obey the laws of arithmetic modulo 2 n  where n is the number of bits in the type  So  for instance  if chars are 8 bits  unsigned char variables have values between 0 and 255  while signed chars have values between -128 and 127  in a two  s complement machine   Whether plain chars are signed or unsigned is machine-dependent  but printable characters are always positive

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Because i feel it s really called for  i just want to state some rules of C and C    they are the same in this regard   First  all bits of unsigned char participate in determining the value if any unsigned char object  Second  unsigned char is explicitly stated unsigned   Now  i had a discussion with someone about what happens when you convert the value -1 of type int to unsigned char  He refused the idea that the resulting unsigned char has all its bits set to 1  because he was worried about sign representation  But he don t have to  It s immediately following out of this rule that the conversion does what is intended      If the new type is unsigned  the value is converted by repeatedly adding or   subtracting one more than the maximum value that can be represented in the new type   until the value is in the range of the new type   6 3 1 3p2 in a C99 draft    That s a mathematical description  C   describes it in terms of modulo calculus  which yields to the same rule  Anyway  what is not guaranteed is that all bits in the integer -1 are one before the conversion  So  what do we have so we can claim that the resulting unsigned char has all its CHAR BIT bits turned to 1    All bits participate in determining its value - that is  no padding bits occur in the object   Adding only one time UCHAR MAX 1 to -1 will yield a value in range  namely UCHAR MAX   That s enough  actually  So whenever you want to have an unsigned char having all its bits one  you do  unsigned char c    unsigned char -1    It also follows that a conversion is not just truncating higher order bits  The fortunate event for two s complement is that it is just a truncation there  but the same isn t necessarily true for other sign representations

User · Answer

In C    there are three distinct character types    char signed char unsigned char   If you are using character types for text  use the unqualified char    it is the type of character literals like  a  or  0   it is the type that makes up C strings like  abcde    It also works out as a number value  but it is unspecified whether that value is treated as signed or unsigned  Beware character comparisons through inequalities - although if you limit yourself to ASCII  0-127  you re just about safe   If you are using character types as numbers  use    signed char  which gives you at least the -127 to 127 range   -128 to 127 is common  unsigned char  which gives you at least the 0 to 255 range     At least   because the C   standard only gives the minimum range of values that each numeric type is required to cover  sizeof  char  is required to be 1  i e  one byte   but a byte could in theory be for example 32 bits  sizeof would still be report its size as 1 - meaning that you could have sizeof  char     sizeof  long     1

User · Answer

If you like using various types of specific length and signedness  you re probably better off with uint8 t  int8 t  uint16 t  etc simply because they do exactly what they say

User · Answer

unsigned char is the heart of all bit trickery  In almost ALL compiler for ALL platform an unsigned char is simply a byte and an unsigned integer of  usually  8 bits that can be treated as a small integer or a pack of bits    In addiction  as someone else has said  the standard doesn t define the sign of a char  so you have 3 distinct char types  char  signed char  unsigned char

User · Answer

If you want to use a character as a small integer  the safest way to do it is with the int8 tand uint8 t types

User · Answer

unsigned char takes only positive values  0 to 255 while signed char takes positive and negative values  -128 to  127

User · Answer

signed char and unsigned char both represent 1byte  but they have different ranges      Type               range ------------------------------- signed char       -128 to  127 unsigned char        0 to 255   In signed char if we consider char letter    A    A  is represent binary of 65 in ASCII Unicode  If 65 can be stored  -65 also can be stored  There are no negative binary values in ASCII Unicode there for no need to worry about negative values   Example   include  lt stdio h gt   int main         signed char char1   255      signed char char2   -128      unsigned char char3   255      unsigned char char4   -128       printf  Signed char 255     d n  char1       printf  Unsigned char 255     d n  char3        printf   nSigned char -128     d n  char2       printf  Unsigned char -128     d n  char4        return 0      Output -   Signed char 255    -1 Unsigned char 255    255  Signed char -128    -128 Unsigned char -128    128

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This is implementation dependent  as the C standard does NOT define the signed-ness of char   Depending on the platform  char may be signed or unsigned  so you need to explicitly ask for signed char or unsigned char if your implementation depends on it   Just use char if you intend to represent characters from strings  as this will match what your platform puts in the string   The difference between signed char and unsigned char is as you d expect   On most platforms  signed char will be an 8-bit two s complement number ranging from -128 to 127  and unsigned char will be an 8-bit unsigned integer  0 to 255    Note the standard does NOT require that char types have 8 bits  only that sizeof char  return 1   You can get at the number of bits in a char with CHAR BIT in limits h   There are few if any platforms today where this will be something other than 8  though   There is a nice summary of this issue here   As others have mentioned since I posted this  you re better off using int8 t and uint8 t if you really want to represent small integers

User · Answer

unsigned char takes only positive values    like 0 to 255  where as  signed char takes both positive and negative values    like -128 to  127

User · Answer

If you want to use a character as a small integer  the safest way to do it is with the int8 tand uint8 t types

User · Answer

char and unsigned char aren t guaranteed to be 8-bit types on all platforms mdash they are guaranteed to be 8-bit or larger  Some platforms have 9-bit  32-bit  or 64-bit bytes  However  the most common platforms today  Windows  Mac  Linux x86  etc   have 8-bit bytes

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Some googling found this  where people had a discussion about this   An unsigned char is basically a single byte   So  you would use this if you need one byte of data  for example  maybe you want to use it to set flags on and off to be passed to a function  as is often done in the Windows API

User · Answer

In terms of direct values a regular char is used when the values are known to be between CHAR_MIN and CHAR_MAX while an unsigned char provides double the range on the positive end. For example, if CHAR_BIT is 8, the range of regular char is only guaranteed to be [0, 127] (because it can be signed or unsigned) while unsigned char will be [0, 255] and signed char will be [-127, 127].

In terms of what it's used for, the standards allow objects of POD (plain old data) to be directly converted to an array of unsigned char. This allows you to examine the representation and bit patterns of the object. The same guarantee of safe type punning doesn't exist for char or signed char.

User · Answer

Because i feel it s really called for  i just want to state some rules of C and C    they are the same in this regard   First  all bits of unsigned char participate in determining the value if any unsigned char object  Second  unsigned char is explicitly stated unsigned   Now  i had a discussion with someone about what happens when you convert the value -1 of type int to unsigned char  He refused the idea that the resulting unsigned char has all its bits set to 1  because he was worried about sign representation  But he don t have to  It s immediately following out of this rule that the conversion does what is intended      If the new type is unsigned  the value is converted by repeatedly adding or   subtracting one more than the maximum value that can be represented in the new type   until the value is in the range of the new type   6 3 1 3p2 in a C99 draft    That s a mathematical description  C   describes it in terms of modulo calculus  which yields to the same rule  Anyway  what is not guaranteed is that all bits in the integer -1 are one before the conversion  So  what do we have so we can claim that the resulting unsigned char has all its CHAR BIT bits turned to 1    All bits participate in determining its value - that is  no padding bits occur in the object   Adding only one time UCHAR MAX 1 to -1 will yield a value in range  namely UCHAR MAX   That s enough  actually  So whenever you want to have an unsigned char having all its bits one  you do  unsigned char c    unsigned char -1    It also follows that a conversion is not just truncating higher order bits  The fortunate event for two s complement is that it is just a truncation there  but the same isn t necessarily true for other sign representations

User · Answer

unsigned char is the heart of all bit trickery  In almost ALL compiler for ALL platform an unsigned char is simply a byte and an unsigned integer of  usually  8 bits that can be treated as a small integer or a pack of bits    In addiction  as someone else has said  the standard doesn t define the sign of a char  so you have 3 distinct char types  char  signed char  unsigned char

User · Answer

As for example usages of unsigned char   unsigned char is often used in computer graphics  which very often  though not always  assigns a single byte to each colour component  It is common to see an RGB  or RGBA  colour represented as 24  or 32  bits  each an unsigned char  Since unsigned char values fall in the range  0 255   the values are typically interpreted as     0 meaning a total lack of a given colour component  255 meaning 100  of a given colour pigment    So you would end up with RGB red as  255 0 0  -   100  red  0  green  0  blue    Why not use a signed char  Arithmetic and bit shifting becomes problematic  As explained already  a signed char s range is essentially shifted by -128  A very simple and naive  mostly unused  method for converting RGB to grayscale is to average all three colour components  but this runs into problems when the values of the colour components are negative  Red  255  0  0  averages to  85  85  85  when using unsigned char arithmetic  However  if the values were signed chars  127 -128 -128   we would end up with  -99  -99  -99   which would be  29  29  29  in our unsigned char space  which is incorrect

User · Answer

Because i feel it s really called for  i just want to state some rules of C and C    they are the same in this regard   First  all bits of unsigned char participate in determining the value if any unsigned char object  Second  unsigned char is explicitly stated unsigned   Now  i had a discussion with someone about what happens when you convert the value -1 of type int to unsigned char  He refused the idea that the resulting unsigned char has all its bits set to 1  because he was worried about sign representation  But he don t have to  It s immediately following out of this rule that the conversion does what is intended      If the new type is unsigned  the value is converted by repeatedly adding or   subtracting one more than the maximum value that can be represented in the new type   until the value is in the range of the new type   6 3 1 3p2 in a C99 draft    That s a mathematical description  C   describes it in terms of modulo calculus  which yields to the same rule  Anyway  what is not guaranteed is that all bits in the integer -1 are one before the conversion  So  what do we have so we can claim that the resulting unsigned char has all its CHAR BIT bits turned to 1    All bits participate in determining its value - that is  no padding bits occur in the object   Adding only one time UCHAR MAX 1 to -1 will yield a value in range  namely UCHAR MAX   That s enough  actually  So whenever you want to have an unsigned char having all its bits one  you do  unsigned char c    unsigned char -1    It also follows that a conversion is not just truncating higher order bits  The fortunate event for two s complement is that it is just a truncation there  but the same isn t necessarily true for other sign representations

User · Answer

If you like using various types of specific length and signedness  you re probably better off with uint8 t  int8 t  uint16 t  etc simply because they do exactly what they say

User · Answer

signed char has range -128 to 127  unsigned char has range 0 to 255   char will be equivalent to either signed char or unsigned char  depending on the compiler  but is a distinct type   If you re using C-style strings  just use char  If you need to use chars for arithmetic  pretty rare   specify signed or unsigned explicitly for portability

User · Answer

Some googling found this  where people had a discussion about this   An unsigned char is basically a single byte   So  you would use this if you need one byte of data  for example  maybe you want to use it to set flags on and off to be passed to a function  as is often done in the Windows API

User · Answer

This is implementation dependent  as the C standard does NOT define the signed-ness of char   Depending on the platform  char may be signed or unsigned  so you need to explicitly ask for signed char or unsigned char if your implementation depends on it   Just use char if you intend to represent characters from strings  as this will match what your platform puts in the string   The difference between signed char and unsigned char is as you d expect   On most platforms  signed char will be an 8-bit two s complement number ranging from -128 to 127  and unsigned char will be an 8-bit unsigned integer  0 to 255    Note the standard does NOT require that char types have 8 bits  only that sizeof char  return 1   You can get at the number of bits in a char with CHAR BIT in limits h   There are few if any platforms today where this will be something other than 8  though   There is a nice summary of this issue here   As others have mentioned since I posted this  you re better off using int8 t and uint8 t if you really want to represent small integers

User · Answer

An unsigned char uses the bit that is reserved for the sign of a regular char as another number  This changes the range to  0 - 255  as opposed to  -128 - 127     Generally unsigned chars are used when you don t want a sign  This will make a difference when doing things like shifting bits  shift extends the sign  and other things when dealing with a char as a byte rather than using it as a number

User · Answer

An unsigned char uses the bit that is reserved for the sign of a regular char as another number  This changes the range to  0 - 255  as opposed to  -128 - 127     Generally unsigned chars are used when you don t want a sign  This will make a difference when doing things like shifting bits  shift extends the sign  and other things when dealing with a char as a byte rather than using it as a number

User · Answer

char and unsigned char aren t guaranteed to be 8-bit types on all platforms mdash they are guaranteed to be 8-bit or larger  Some platforms have 9-bit  32-bit  or 64-bit bytes  However  the most common platforms today  Windows  Mac  Linux x86  etc   have 8-bit bytes

User · Answer

unsigned char is the heart of all bit trickery  In almost ALL compiler for ALL platform an unsigned char is simply a byte and an unsigned integer of  usually  8 bits that can be treated as a small integer or a pack of bits    In addiction  as someone else has said  the standard doesn t define the sign of a char  so you have 3 distinct char types  char  signed char  unsigned char

User · Answer

unsigned char takes only positive values    like 0 to 255  where as  signed char takes both positive and negative values    like -128 to  127

User · Answer

signed char has range -128 to 127  unsigned char has range 0 to 255   char will be equivalent to either signed char or unsigned char  depending on the compiler  but is a distinct type   If you re using C-style strings  just use char  If you need to use chars for arithmetic  pretty rare   specify signed or unsigned explicitly for portability

User · Answer

char and unsigned char aren t guaranteed to be 8-bit types on all platforms mdash they are guaranteed to be 8-bit or larger  Some platforms have 9-bit  32-bit  or 64-bit bytes  However  the most common platforms today  Windows  Mac  Linux x86  etc   have 8-bit bytes

User · Answer

If you want to use a character as a small integer  the safest way to do it is with the int8 tand uint8 t types

User · Answer

signed char and unsigned char both represent 1byte  but they have different ranges      Type               range ------------------------------- signed char       -128 to  127 unsigned char        0 to 255   In signed char if we consider char letter    A    A  is represent binary of 65 in ASCII Unicode  If 65 can be stored  -65 also can be stored  There are no negative binary values in ASCII Unicode there for no need to worry about negative values   Example   include  lt stdio h gt   int main         signed char char1   255      signed char char2   -128      unsigned char char3   255      unsigned char char4   -128       printf  Signed char 255     d n  char1       printf  Unsigned char 255     d n  char3        printf   nSigned char -128     d n  char2       printf  Unsigned char -128     d n  char4        return 0      Output -   Signed char 255    -1 Unsigned char 255    255  Signed char -128    -128 Unsigned char -128    128

User · Answer

An unsigned char is an unsigned byte value  0 to 255    You may be thinking of char in terms of being a  character  but it is really a numerical value   The regular char is signed  so you have 128 values  and these values map to characters using ASCII encoding   But in either case  what you are storing in memory is a byte value

User · Answer

Because i feel it s really called for  i just want to state some rules of C and C    they are the same in this regard   First  all bits of unsigned char participate in determining the value if any unsigned char object  Second  unsigned char is explicitly stated unsigned   Now  i had a discussion with someone about what happens when you convert the value -1 of type int to unsigned char  He refused the idea that the resulting unsigned char has all its bits set to 1  because he was worried about sign representation  But he don t have to  It s immediately following out of this rule that the conversion does what is intended      If the new type is unsigned  the value is converted by repeatedly adding or   subtracting one more than the maximum value that can be represented in the new type   until the value is in the range of the new type   6 3 1 3p2 in a C99 draft    That s a mathematical description  C   describes it in terms of modulo calculus  which yields to the same rule  Anyway  what is not guaranteed is that all bits in the integer -1 are one before the conversion  So  what do we have so we can claim that the resulting unsigned char has all its CHAR BIT bits turned to 1    All bits participate in determining its value - that is  no padding bits occur in the object   Adding only one time UCHAR MAX 1 to -1 will yield a value in range  namely UCHAR MAX   That s enough  actually  So whenever you want to have an unsigned char having all its bits one  you do  unsigned char c    unsigned char -1    It also follows that a conversion is not just truncating higher order bits  The fortunate event for two s complement is that it is just a truncation there  but the same isn t necessarily true for other sign representations

User · Answer

An unsigned char is an unsigned byte value  0 to 255    You may be thinking of char in terms of being a  character  but it is really a numerical value   The regular char is signed  so you have 128 values  and these values map to characters using ASCII encoding   But in either case  what you are storing in memory is a byte value

User · Answer

In terms of direct values a regular char is used when the values are known to be between CHAR_MIN and CHAR_MAX while an unsigned char provides double the range on the positive end. For example, if CHAR_BIT is 8, the range of regular char is only guaranteed to be [0, 127] (because it can be signed or unsigned) while unsigned char will be [0, 255] and signed char will be [-127, 127].

In terms of what it's used for, the standards allow objects of POD (plain old data) to be directly converted to an array of unsigned char. This allows you to examine the representation and bit patterns of the object. The same guarantee of safe type punning doesn't exist for char or signed char.

User · Answer

As for example usages of unsigned char   unsigned char is often used in computer graphics  which very often  though not always  assigns a single byte to each colour component  It is common to see an RGB  or RGBA  colour represented as 24  or 32  bits  each an unsigned char  Since unsigned char values fall in the range  0 255   the values are typically interpreted as     0 meaning a total lack of a given colour component  255 meaning 100  of a given colour pigment    So you would end up with RGB red as  255 0 0  -   100  red  0  green  0  blue    Why not use a signed char  Arithmetic and bit shifting becomes problematic  As explained already  a signed char s range is essentially shifted by -128  A very simple and naive  mostly unused  method for converting RGB to grayscale is to average all three colour components  but this runs into problems when the values of the colour components are negative  Red  255  0  0  averages to  85  85  85  when using unsigned char arithmetic  However  if the values were signed chars  127 -128 -128   we would end up with  -99  -99  -99   which would be  29  29  29  in our unsigned char space  which is incorrect

User · Answer

In C    there are three distinct character types    char signed char unsigned char   If you are using character types for text  use the unqualified char    it is the type of character literals like  a  or  0   it is the type that makes up C strings like  abcde    It also works out as a number value  but it is unspecified whether that value is treated as signed or unsigned  Beware character comparisons through inequalities - although if you limit yourself to ASCII  0-127  you re just about safe   If you are using character types as numbers  use    signed char  which gives you at least the -127 to 127 range   -128 to 127 is common  unsigned char  which gives you at least the 0 to 255 range     At least   because the C   standard only gives the minimum range of values that each numeric type is required to cover  sizeof  char  is required to be 1  i e  one byte   but a byte could in theory be for example 32 bits  sizeof would still be report its size as 1 - meaning that you could have sizeof  char     sizeof  long     1

User · Answer

An unsigned char is an unsigned byte value  0 to 255    You may be thinking of char in terms of being a  character  but it is really a numerical value   The regular char is signed  so you have 128 values  and these values map to characters using ASCII encoding   But in either case  what you are storing in memory is a byte value

User · Answer

Some googling found this  where people had a discussion about this   An unsigned char is basically a single byte   So  you would use this if you need one byte of data  for example  maybe you want to use it to set flags on and off to be passed to a function  as is often done in the Windows API

User · Answer

If you like using various types of specific length and signedness  you re probably better off with uint8 t  int8 t  uint16 t  etc simply because they do exactly what they say

User · Answer

In C    there are three distinct character types    char signed char unsigned char   If you are using character types for text  use the unqualified char    it is the type of character literals like  a  or  0   it is the type that makes up C strings like  abcde    It also works out as a number value  but it is unspecified whether that value is treated as signed or unsigned  Beware character comparisons through inequalities - although if you limit yourself to ASCII  0-127  you re just about safe   If you are using character types as numbers  use    signed char  which gives you at least the -127 to 127 range   -128 to 127 is common  unsigned char  which gives you at least the 0 to 255 range     At least   because the C   standard only gives the minimum range of values that each numeric type is required to cover  sizeof  char  is required to be 1  i e  one byte   but a byte could in theory be for example 32 bits  sizeof would still be report its size as 1 - meaning that you could have sizeof  char     sizeof  long     1

User · Answer

As for example usages of unsigned char   unsigned char is often used in computer graphics  which very often  though not always  assigns a single byte to each colour component  It is common to see an RGB  or RGBA  colour represented as 24  or 32  bits  each an unsigned char  Since unsigned char values fall in the range  0 255   the values are typically interpreted as     0 meaning a total lack of a given colour component  255 meaning 100  of a given colour pigment    So you would end up with RGB red as  255 0 0  -   100  red  0  green  0  blue    Why not use a signed char  Arithmetic and bit shifting becomes problematic  As explained already  a signed char s range is essentially shifted by -128  A very simple and naive  mostly unused  method for converting RGB to grayscale is to average all three colour components  but this runs into problems when the values of the colour components are negative  Red  255  0  0  averages to  85  85  85  when using unsigned char arithmetic  However  if the values were signed chars  127 -128 -128   we would end up with  -99  -99  -99   which would be  29  29  29  in our unsigned char space  which is incorrect

User · Answer

In terms of direct values a regular char is used when the values are known to be between CHAR_MIN and CHAR_MAX while an unsigned char provides double the range on the positive end. For example, if CHAR_BIT is 8, the range of regular char is only guaranteed to be [0, 127] (because it can be signed or unsigned) while unsigned char will be [0, 255] and signed char will be [-127, 127].

In terms of what it's used for, the standards allow objects of POD (plain old data) to be directly converted to an array of unsigned char. This allows you to examine the representation and bit patterns of the object. The same guarantee of safe type punning doesn't exist for char or signed char.

User · Answer

Some googling found this  where people had a discussion about this   An unsigned char is basically a single byte   So  you would use this if you need one byte of data  for example  maybe you want to use it to set flags on and off to be passed to a function  as is often done in the Windows API

User · Answer

This is implementation dependent  as the C standard does NOT define the signed-ness of char   Depending on the platform  char may be signed or unsigned  so you need to explicitly ask for signed char or unsigned char if your implementation depends on it   Just use char if you intend to represent characters from strings  as this will match what your platform puts in the string   The difference between signed char and unsigned char is as you d expect   On most platforms  signed char will be an 8-bit two s complement number ranging from -128 to 127  and unsigned char will be an 8-bit unsigned integer  0 to 255    Note the standard does NOT require that char types have 8 bits  only that sizeof char  return 1   You can get at the number of bits in a char with CHAR BIT in limits h   There are few if any platforms today where this will be something other than 8  though   There is a nice summary of this issue here   As others have mentioned since I posted this  you re better off using int8 t and uint8 t if you really want to represent small integers

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As for example usages of unsigned char   unsigned char is often used in computer graphics  which very often  though not always  assigns a single byte to each colour component  It is common to see an RGB  or RGBA  colour represented as 24  or 32  bits  each an unsigned char  Since unsigned char values fall in the range  0 255   the values are typically interpreted as     0 meaning a total lack of a given colour component  255 meaning 100  of a given colour pigment    So you would end up with RGB red as  255 0 0  -   100  red  0  green  0  blue    Why not use a signed char  Arithmetic and bit shifting becomes problematic  As explained already  a signed char s range is essentially shifted by -128  A very simple and naive  mostly unused  method for converting RGB to grayscale is to average all three colour components  but this runs into problems when the values of the colour components are negative  Red  255  0  0  averages to  85  85  85  when using unsigned char arithmetic  However  if the values were signed chars  127 -128 -128   we would end up with  -99  -99  -99   which would be  29  29  29  in our unsigned char space  which is incorrect

[c++] What is an unsigned char?

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