What is size t in C

Question

I am getting confused with size t in C  I know that it is returned by the sizeof operator  But what exactly is it  Is it a data type   Let s say I have a for loop   for i   0  i  lt  some size  i      Should I use int i  or size t i

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From my understanding  size t is an unsigned integer whose bit size is large enough to hold a pointer of the native architecture   So   sizeof size t   gt   sizeof void

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To go into why size t needed to exist and how we got here   In pragmatic terms  size t and ptrdiff t are guaranteed to be 64 bits wide on a 64-bit implementation  32 bits wide on a 32-bit implementation  and so on   They could not force any existing type to mean that  on every compiler  without breaking legacy code   A size t or ptrdiff t is not necessarily the same as an intptr t or uintptr t   They were different on certain architectures that were still in use when size t and ptrdiff t were added to the Standard in the late    80s  and becoming obsolete when C99 added many new types but not gone yet  such as 16-bit Windows   The x86 in 16-bit protected mode had a segmented memory where the largest possible array or structure could be only 65 536 bytes in size  but a far pointer needed to be 32 bits wide  wider than the registers   On those  intptr t would have been 32 bits wide but size t and ptrdiff t could be 16 bits wide and fit in a register  And who knew what kind of operating system might be written in the future   In theory  the i386 architecture offers a 32-bit segmentation model with 48-bit pointers that no operating system has ever actually used   The type of a memory offset could not be long because far too much legacy code assumes that long is exactly 32 bits wide   This assumption was even built into the UNIX and Windows APIs   Unfortunately  a lot of other legacy code also assumed that a long is wide enough to hold a pointer  a file offset  the number of seconds that have elapsed since 1970  and so on   POSIX now provides a standardized way to force the latter assumption to be true instead of the former  but neither is a portable assumption to make   It couldn   t be int because only a tiny handful of compilers in the    90s made int 64 bits wide   Then they really got weird by keeping long 32 bits wide   The next revision of the Standard declared it illegal for int to be wider than long  but int is still 32 bits wide on most 64-bit systems   It couldn   t be long long int  which anyway was added later  since that was created to be at least 64 bits wide even on 32-bit systems   So  a new type was needed   Even if it weren   t  all those other types meant something other than an offset within an array or object   And if there was one lesson from the fiasco of 32-to-64-bit migration  it was to be specific about what properties a type needed to have  and not use one that meant different things in different programs

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In general  if you are starting at 0 and going upward  always use an unsigned type to avoid an overflow taking you into a negative value situation  This is critically important  because if your array bounds happens to be less than the max of your loop  but your loop max happens to be greater than the max of your type  you will wrap around negative and you may experience a segmentation fault  SIGSEGV   So  in general  never use int for a loop starting at 0 and going upwards  Use an unsigned

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From Wikipedia      According to the 1999 ISO C standard    C99   size t is an unsigned integer   type of at least 16 bit  see sections   7 17 and 7 18 3        size tis an unsigned data type   defined by several C C   standards    e g  the C99 ISO IEC 9899 standard    that is defined in stddef h 1 It can   be further imported by inclusion of   stdlib h as this file internally sub   includes stddef h       This type is used to represent the   size of an object  Library functions   that take or return sizes expect them   to be of type or have the return type   of size t  Further  the most   frequently used compiler-based   operator sizeof should evaluate to a   constant value that is compatible with   size t    As an implication  size t is a type guaranteed to hold any array index

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size t and int are not interchangeable  For instance on 64-bit Linux size t is 64-bit in size  i e  sizeof void    but int is 32-bit   Also note that size t is unsigned  If you need signed version then there is ssize t on some platforms and it would be more relevant to your example   As a general rule I would suggest using int for most general cases and only use size t ssize t when there is a specific need for it  with mmap   for example

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size t is an unsigned type   So  it cannot represent any negative values  lt 0    You use it when you are counting something  and are sure that it cannot be negative   For example  strlen   returns a size t because the length of a string has to be at least 0   In your example  if your loop index is going to be always greater than 0  it might make sense to use size t  or any other unsigned data type   When you use a size t object  you have to make sure that in all the contexts it is used  including arithmetic  you want non-negative values   For example  let s say you have   size t s1   strlen str1   size t s2   strlen str2     and you want to find the difference of the lengths of str2 and str1   You cannot do   int diff   s2 - s1     bad      This is because the value assigned to diff is always going to be a positive number  even when s2  lt  s1  because the calculation is done with unsigned types   In this case  depending upon what your use case is  you might be better off using int  or long long  for s1 and s2   There are some functions in C POSIX that could should use size t  but don t because of historical reasons   For example  the second parameter to fgets should ideally be size t  but is int

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size t is an unsigned integer data type which can assign only 0 and greater than 0 integer values  It measure bytes of any object s size and returned by sizeof operator  const is the syntax representation of size t  but without const you can run the programm   const size t number    size t regularly used for array indexing and loop counting  If the compiler is 32-bit it would work on unsigned int  If the compiler is 64-bit it would work on unsigned long long int also  There for maximum size of size t depending on compiler type   size t already define on  lt stdio h gt  header file  but It can also define by   lt stddef h gt    lt stdlib h gt    lt string h gt    lt time h gt    lt wchar h gt  headers     Example  with const     include  lt stdio h gt   int main         const size t value   200      size t i      int arr value        for  i   0   i  lt  value     i                arr i    i             size t size   sizeof arr       printf  size    zu n   size       Output -  size   800     Example  without const     include  lt stdio h gt   int main         size t value   200      size t i      int arr value        for  i   0   i  lt  value     i                arr i    i             size t size   sizeof arr       printf  size    zu n   size       Output -  size   800

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size t or any unsigned type might be seen used as loop variable as loop variables are typically greater than or equal to 0   When we use a size t object  we have to make sure that in all the contexts it is used  including arithmetic  we want only non-negative values  For instance  following program would definitely give the unexpected result      C program to demonstrate that size t or    any unsigned int type should be used     carefully when used in a loop   include lt stdio h gt  int main     const size t N   10  int a N       This is fine for  size t n   0  n  lt  N    n  a n    n      But reverse cycles are tricky for unsigned     types as can lead to infinite loop for  size t n   N-1  n  gt   0  --n  printf   d    a n       Output Infinite loop and then segmentation fault

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Since nobody has yet mentioned it  the primary linguistic significance of size t is that the sizeof operator returns a value of that type   Likewise  the primary significance of ptrdiff t is that subtracting one pointer from another will yield a value of that type   Library functions that accept it do so because it will allow such functions to work with objects whose size exceeds UINT MAX on systems where such objects could exist  without forcing callers to waste code passing a value larger than  unsigned int  on systems where the larger type would suffice for all possible objects

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This is a platform-specific typedef  For example  on a particular machine  it might be unsigned int or unsigned long  You should use this definition for more portability of your code

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size t is unsigned integer data type  On systems using the GNU C Library  this will be  unsigned  int  or  unsigned  long  int  size t  is commonly used for array indexing and loop counting

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size t is a type that can hold any array index   Depending on the implementation  it can be any of   unsigned char  unsigned short  unsigned int  unsigned long  unsigned long long  Here s how size t is defined in stddef h of my machine   typedef unsigned long size t

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The manpage for types h says      size t shall be an unsigned integer type

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If you are the empirical type   echo   gcc -E -xc -include  stddef h  -   grep size t   Output for Ubuntu 14 04 64-bit GCC 4 8   typedef long unsigned int size t    Note that stddef h is provided by GCC and not glibc under src gcc ginclude stddef h in GCC 4 2   Interesting C99 appearances   malloc takes size t as an argument  so it determines the maximum size that may be allocated   And since it is also returned by sizeof  I think it limits the maximum size of any array   See also  What is the maximum size of an array in C

[c] What is size_t in C?

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