Advanced Programming in the UNIX® Environment

ptg10805159

Section 2.5 Limits 41

Each of the 25 invariant minimum values in Figure2.8 has an associated

implementation value whose name is formed by removing the_POSIX_prefix from the

name in Figure2.8. The names without the leading_POSIX_wereintended to be the

actual values that a given implementation supports. (These 25 implementation values

arefromitems 1, 4, 5, and 7 from our list earlier in this section: 2 of the runtime

increasable values, 15 of the runtime invariant values, and 7 of the pathname variable

values, along withSSIZE_MAXfrom the numeric values.) The problem is that not all of

the 25 implementation values areguaranteed to be defined in the<limits.h>header.

For example, a particular value may not be included in the header if its actual value

for a given process depends on the amount of memory on the system. If the values are

not defined in the header, we can’t use them as array bounds at compile time. To

determine the actual implementation value at runtime, POSIX.1 decided to provide

three functions for us to call —sysconf,pathconf,andfpathconf.Thereisstill a

problem, however,because some of the values aredefined by POSIX.1 as being possibly

‘‘indeterminate’’(logically infinite). This means that the value has no practical upper

bound. On Solaris, for example, the number of functions you can register withatexit

to be run when a process ends is limited only by the amount of memory on the system.

ThusATEXIT_MAXis considered indeterminate on Solaris. We’ll return to this problem

of indeterminate runtime limits in Section 2.5.5.

2.5.3 XSI Limits

The XSI option also defines constants representing implementation limits. They

include:

1. Minimum values: the five constants in Figure2.10


2. Runtime invariant values, possibly indeterminate:IOV_MAXandPAGE_SIZE

The minimum values arelisted in Figure2.10. The last two illustrate the situation in

which the POSIX.1 minimums weretoo small—presumably to allow for embedded

POSIX.1 implementations—sosymbols with larger minimum values wereadded for

XSI-conforming systems.

Minimum

Name Description acceptable valueTypical value

NL_LANGMAX maximum number of bytes inLANG 14 14

environment variable

NZERO default process priority 20 20

_XOPEN_IOV_MAX maximum number ofiovecstructures that 16 16

can be used withreadvorwritev

_XOPEN_NAME_MAXnumber of bytes in a filename 255 255

_XOPEN_PATH_MAXnumber of bytes in a pathname 1,024 1,024

Figure 2.10 XSI minimum values from<limits.h>