Where is ssize_t defined in Linux?

Since version 5.9, the Linux man-pages document system data types, so that you can find this information easily in a centralized manner.

Just type man ssize_t:

ssize_t(3type)      Linux Programmer’s Manual     ssize_t(3type)

NAME
       ssize_t - count of bytes or an error indication

LIBRARY
       Standard C library (libc)

SYNOPSIS
       #include <sys/types.h>

       typedef /* ... */ ssize_t;

DESCRIPTION
       Used  for  a  count of bytes or an error indication.  Ac‐
       cording to POSIX, it shall be a signed integer type capa‐
       ble  of  storing  values  at  least  in  the  range  [-1,
       SSIZE_MAX],  and  the implementation shall support one or
       more programming environments where the width of  ssize_t
       is no greater than the width of the type long.

       Glibc  and  most  other  implementations provide a length
       modifier for ssize_t for the printf(3) and  the  scanf(3)
       families  of functions, which is z; resulting commonly in
       %zd or %zi for printing ssize_t values.  Although z works
       for ssize_t on most implementations, portable POSIX  pro‐
       grams  should  avoid  using it—for example, by converting
       the value to intmax_t and using its length modifier (j).

VERSIONS
       <aio.h>, <mqueue.h>,  and  <sys/socket.h>  define  ssize_t
       since POSIX.1‐2008.

CONFORMING TO
       POSIX.1‐2001 and later.

NOTES
       The  following  headers  also provide this type: <aio.h>,
       <monetary.h>,   <mqueue.h>,    <stdio.h>,    <sys/msg.h>,
       <sys/socket.h>, <sys/uio.h>, and <unistd.h>.

SEE ALSO
       read(2),   readlink(2),   readv(2),   recv(2),   send(2),
       write(2), ptrdiff_t(3type), size_t(3type)

Linux                      2022‐06‐17             ssize_t(3type)

If you just want ssize_t, you should include <sys/types.h>, which is its canonical header, and probably the lightest one that provides ssize_t. However, it is provided by any of the headers documented, so if you happen to also need a definition in one of those other headers, you can include that other header only.

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.