### 31.1 General Capacity Limits

The POSIX.1 and POSIX.2 standards specify a number of parameters that describe capacity limitations of the system. These limits can be fixed constants for a given operating system, or they can vary from machine to machine. For example, some limit values may be configurable by the system administrator, either at run time or by rebuilding the kernel, and this should not require recompiling application programs.

Each of the following limit parameters has a macro that is defined in `limits.h` only if the system has a fixed, uniform limit for the parameter in question. If the system allows different file systems or files to have different limits, then the macro is undefined; use `sysconf` to find out the limit that applies at a particular time on a particular machine. See Sysconf.

Each of these parameters also has another macro, with a name starting with ‘`_POSIX`’, which gives the lowest value that the limit is allowed to have on any POSIX system. See Minimums.

— Macro: int ARG_MAX

> If defined, the unvarying maximum combined length of the argv and environ arguments that can be passed to the `exec` functions.

— Macro: int CHILD_MAX

> If defined, the unvarying maximum number of processes that can exist with the same real user ID at any one time. In BSD and GNU, this is controlled by the `RLIMIT_NPROC` resource limit; see Limits on Resources.

— Macro: int OPEN_MAX

> If defined, the unvarying maximum number of files that a single process can have open simultaneously. In BSD and GNU, this is controlled by the `RLIMIT_NOFILE` resource limit; see Limits on Resources.

— Macro: int STREAM_MAX

> If defined, the unvarying maximum number of streams that a single process can have open simultaneously. See Opening Streams.

— Macro: int TZNAME_MAX

> If defined, the unvarying maximum length of a time zone name. See Time Zone Functions.

These limit macros are always defined in `limits.h`.

— Macro: int NGROUPS_MAX

> The maximum number of supplementary group IDs that one process can have.

> The value of this macro is actually a lower bound for the maximum. That is, you can count on being able to have that many supplementary group IDs, but a particular machine might let you have even more. You can use `sysconf` to see whether a particular machine will let you have more (see Sysconf).

— Macro: ssize_t SSIZE_MAX

> The largest value that can fit in an object of type `ssize_t`. Effectively, this is the limit on the number of bytes that can be read or written in a single operation.

> This macro is defined in all POSIX systems because this limit is never configurable.

— Macro: int RE_DUP_MAX

> The largest number of repetitions you are guaranteed is allowed in the construct ‘`\{min,max\}`’ in a regular expression.

> The value of this macro is actually a lower bound for the maximum. That is, you can count on being able to have that many repetitions, but a particular machine might let you have even more. You can use `sysconf` to see whether a particular machine will let you have more (see Sysconf). And even the value that `sysconf` tells you is just a lower bound—larger values might work.

> This macro is defined in all POSIX.2 systems, because POSIX.2 says it should always be defined even if there is no specific imposed limit.