What s sizeof size t on 32-bit vs the various 64-bit data models

Question

On a 64-bit system  sizeof unsigned long  depends on the data model implemented by the system  for example  it is 4 bytes on LLP64  Windows   8 bytes on LP64  Linux  etc     What s sizeof size t  supposed to be   Does it vary with data model like sizeof long  does   If so  how     References   64-bit data models on Wikipedia

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size t is 64 bit normally on 64 bit machine

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size t is defined by the C standard to be the unsigned integer return type of the sizeof operator  C99 6 3 5 4 4   and the argument of malloc and friends  C99 7 20 3 3 etc   The actual range is set such that the maximum  SIZE MAX  is at least 65535  C99 7 18 3 2    However  this doesn t let us determine sizeof size t   The implementation is free to use any representation it likes for size t - so there is no upper bound on size - and the implementation is also free to define a byte as 16-bits  in which case size t can be equivalent to unsigned char   Putting that aside  however  in general you ll have 32-bit size t on 32-bit programs  and 64-bit on 64-bit programs  regardless of the data model  Generally the data model only affects static data  for example  in GCC    -mcmodel small       Generate code for the small code model  the program and its      symbols must be linked in the lower 2 GB of the address space       Pointers are 64 bits   Programs can be statically or dynamically      linked   This is the default code model    -mcmodel kernel       Generate code for the kernel code model   The kernel runs in the      negative 2 GB of the address space   This model has to be used for      Linux kernel code    -mcmodel medium       Generate code for the medium model  The program is linked in the      lower 2 GB of the address space but symbols can be located      anywhere in the address space   Programs can be statically or      dynamically linked  but building of shared libraries are not      supported with the medium model    -mcmodel large       Generate code for the large model  This model makes no assumptions      about addresses and sizes of sections    You ll note that pointers are 64-bit in all cases  and there s little point to having 64-bit pointers but not 64-bit sizes  after all

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it should vary with the architecture because it represents the size of any object  So on a 32-bit system size t will likely be at least 32-bits wide  On a 64-bit system it will likely be at least 64-bit wide

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EDIT  Thanks for the comments - I looked it up in the C99 standard  which says in section 6 5 3 4      The value of the result is   implementation-defined  and its type    an unsigned integer type  is size t    defined in  lt stddef h gt   and other   headers    So  the size of size t is not specified  only that it has to be an unsigned integer type  However  an interesting specification can be found in chapter 7 18 3 of the standard      limit of size t      SIZE MAX 65535   Which basically means that  irrespective of the size of size t  the allowed value range is from 0-65535  the rest is implementation dependent

[c] What's sizeof(size_t) on 32-bit vs the various 64-bit data models?

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