size_t(3type)

Usage

#include <stddef.h>

typedef /* ... */ size_t;

#include <sys/types.h>

typedef /* ... */ ssize_t;

Standard C library (libc)

size_t: Used for a count of bytes. It is the result of the sizeof() operator. It is an unsigned integer type capable of storing values in the range [0, SIZE_MAX].

ssize_t: Used for a count of bytes or an error indication. It is a signed integer type capable of storing values at least in the range [-1, SSIZE_MAX].

size_t: The length modifier for size_t for the printf(3) and the scanf(3) families of functions is z, resulting commonly in %zu or %zx for printing size_t values.

ssize_t: 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 programs should avoid using it—for example, by converting the value to intmax_t and using its length modifier (j).

size_t: The following headers also provide size_t: <aio.h>, <glob.h>, <grp.h>, <iconv.h>, <monetary.h>, <mqueue.h>, <ndbm.h>, <pwd.h>, <regex.h>, <search.h>, <signal.h>, <stdio.h>, <stdlib.h>, <string.h>, <strings.h>, <sys/mman.h>, <sys/msg.h>, <sys/sem.h>, <sys/shm.h>, <sys/socket.h>, <sys/types.h>, <sys/uio.h>, <time.h>, <unistd.h>, <wchar.h>, and <wordexp.h>.

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

3 releases since Debian 12 (Bookworm); the text changed 2 times.

`size_t`	C11, POSIX.1-2024.
`ssize_t`	POSIX.1-2024.

`size_t`	C89, POSIX.1-1990.
`ssize_t`	POSIX.1-1990.