Maximum size of size_t

Question

I know in C return type of sizeof operator is size_t being unsigned integer type defined in <stdint.h>. Which means max size of it should be 65535 as stated in C99 standard 7.18.3:

limit of size_t
  SIZE_MAX             65535

However in gcc-4.8.2 header file stdint.h has defined its size much greater than 65535 contradicting to which is stated in C99 standard as below shown,

/* Limit of `size_t' type.  */
# if __WORDSIZE == 64
#  define SIZE_MAX              (18446744073709551615UL)
# else
#  define SIZE_MAX              (4294967295U)
# endif

Kindly help me in understanding why there is a difference or reason behind my misinterpretation.

Solution

The standard says that SIZE_MAX must be at least 65535.

It specifies no upper bound, and gcc's implementation is perfectly valid.

Quoting the reference you cited (emphasis added):

Its implementation-defined value shall be equal to or greater in magnitude (absolute value) than the corresponding value given below, with the same sign.

Comments

[phuclv]

How to get SIZE_MAX in C89

[Sunil Bojanapally]

Now it makes sense. SIZE_MAX 65535 is minimum limit, which I got confused with limit of size_t thinking it as maximum limit. Thanks

[Keith Thompson]

@SunEric: Right, 65535 is the minimum maximum.

[chux - Reinstate Monica]

Suspect SIZE_MAX upper bound is UINTMAX_MAX.

[Keith Thompson]

@chux: SIZE_MAX can't be bigger than UINTMAX_MAX, but there's no specified upper bound on UINTMAX_MAX. (It's only slightly less useful to say that the upper bound of SIZE_MAX is SIZE_MAX.)

[chux - Reinstate Monica]

Various limits in C often have a relationship such as SIZE_MAX <= UINTMAX_MAX. (likely all unsigned are so limited - yet many do not know this - hence my comment.) But some limits are not well ordered like SIZE_MAX compared to LONG_MAX. These 2 comes into interaction using fseek() (which returns long) to allocate a block to match a file using malloc() which takes a size_t.