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?

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Answer 1

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?