Where is ssize_t defined in linux?

问题:

OS: Debian 9 (Linux 4.9)

Compiler: GCC 8.2

Currently I am including <stddef.h> (where size_t is defined) and <stdint.h> (where most integral types are defined), but I still don't have ssize_t.

Where is it defined?

回答1:

ssize_t is defined in sys/types.h.

Per the POSIX documentation:

NAME

sys/types.h - data types

SYNOPSIS

#include <sys/types.h>

DESCRIPTION

The header shall define at least the following types:

...

ssize_t

    Used for a count of bytes or an error indication.

回答2:

(https://stackoverflow.com/a/29984840/6872717):

The division of the POSIX and C header in fine grained files probably comes from the old days when compilation might take a long time, and adding unnecesary header files made the time longer.

If you only need the OS types, say for the prototypes of your functions, then just #include <sys/types.h>. However if you need the function definitions, then you #include <unistd.h> or any of the other system headers, as needed.

Either #include <unistd.h> for many POSIX functions, or just #include <sys/types.h> for a small header that contains the type.