Advanced Programming in the UNIX® Environment

ptg10805159

40 UNIX Standardization and Implementations Chapter 2

These minimum values do not change from one system to another.They specify the

most restrictive values for these features. A conforming POSIX.1 implementation must

provide values that are at least this large. This is why they arecalled minimums,

although their names all containMAX.Also, to ensureportability,astrictly conforming

application must not requirealarger value. We describe what each of these constants

refers to as we proceed through the text.

Astrictly conforming POSIX application is different from an application that is merely POSIX

conforming. A POSIX-conforming application uses only interfaces defined in IEEE Standard

1003.1-2008. A strictly conforming POSIX application must meet further restrictions, such as

not relying on any undefined behavior,not using any obsolescent interfaces, and not requiring

values of constants larger than the minimums shown in Figure2.8.

Name Description Minimum acceptable value

ARG_MAX maximum length of arguments toexec _POSIX_ARG_MAX

functions

ATEXIT_MAX maximum number of functions that can be 32

registered with theatexitfunction

CHILD_MAX maximum number of child processes per real _POSIX_CHILD_MAX

user ID

DELAYTIMER_MAX maximum number of timer expiration overruns _POSIX_DELAYTIMER_MAX

HOST_NAME_MAX maximum length of a host name as returned by _POSIX_HOST_NAME_MAX

gethostname

LOGIN_NAME_MAX maximum length of a login name _POSIX_LOGIN_NAME_MAX

OPEN_MAX one morethan the maximum value assigned to _POSIX_OPEN_MAX

anewly created file descriptor

PAGESIZE system memory page size, in bytes 1

RTSIG_MAX maximum number of real-time signals reserved _POSIX_RTSIG_MAX

for application use

SEM_NSEMS_MAX maximum number of semaphores a process can _POSIX_SEM_NSEMS_MAX

use

SEM_VALUE_MAX maximum value of a semaphore _POSIX_SEM_VALUE_MAX

SIGQUEUE_MAX maximum number of signals that can be _POSIX_SIGQUEUE_MAX

queued for a process

STREAM_MAX maximum number of standardI/O streams a _POSIX_STREAM_MAX

process can have open at once

SYMLOOP_MAX number of symbolic links that can be traversed _POSIX_SYMLOOP_MAX

during pathname resolution

TIMER_MAX maximum number of timers per process _POSIX_TIMER_MAX

TTY_NAME_MAX length of a terminal device name, including the _POSIX_TTY_NAME_MAX

terminating null

TZNAME_MAX number of bytes for the name of a time zone _POSIX_TZNAME_MAX

Figure 2.9POSIX.1 runtime invariant values from<limits.h>

Unfortunately,some of these invariant minimum values aretoo small to be of

practical use. For example, most UNIX systems today provide far morethan 20 open

files per process. Also, the minimum limit of 256 for_POSIX_PATH_MAXis too small.

Pathnames can exceed this limit. This means that we can’t use the two constants

_POSIX_OPEN_MAXand_POSIX_PATH_MAXas array sizes at compile time.