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| <h3 class="section">31.1 General Capacity Limits</h3> |
| |
| <p><a name="index-POSIX-capacity-limits-3495"></a><a name="index-limits_002c-POSIX-3496"></a><a name="index-capacity-limits_002c-POSIX-3497"></a> |
| 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. |
| |
| <p><a name="index-limits_002eh-3498"></a>Each of the following limit parameters has a macro that is defined in |
| <samp><span class="file">limits.h</span></samp> 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 |
| <code>sysconf</code> to find out the limit that applies at a particular time |
| on a particular machine. See <a href="Sysconf.html#Sysconf">Sysconf</a>. |
| |
| <p>Each of these parameters also has another macro, with a name starting |
| with ‘<samp><span class="samp">_POSIX</span></samp>’, which gives the lowest value that the limit is |
| allowed to have on <em>any</em> POSIX system. See <a href="Minimums.html#Minimums">Minimums</a>. |
| |
| <p><a name="index-limits_002c-program-argument-size-3499"></a><!-- limits.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Macro: int <b>ARG_MAX</b><var><a name="index-ARG_005fMAX-3500"></a></var><br> |
| <blockquote><p>If defined, the unvarying maximum combined length of the <var>argv</var> and |
| <var>environ</var> arguments that can be passed to the <code>exec</code> functions. |
| </p></blockquote></div> |
| |
| <p><a name="index-limits_002c-number-of-processes-3501"></a><!-- limits.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Macro: int <b>CHILD_MAX</b><var><a name="index-CHILD_005fMAX-3502"></a></var><br> |
| <blockquote><p>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 <code>RLIMIT_NPROC</code> resource limit; see <a href="Limits-on-Resources.html#Limits-on-Resources">Limits on Resources</a>. |
| </p></blockquote></div> |
| |
| <p><a name="index-limits_002c-number-of-open-files-3503"></a><!-- limits.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Macro: int <b>OPEN_MAX</b><var><a name="index-OPEN_005fMAX-3504"></a></var><br> |
| <blockquote><p>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 <code>RLIMIT_NOFILE</code> resource limit; see <a href="Limits-on-Resources.html#Limits-on-Resources">Limits on Resources</a>. |
| </p></blockquote></div> |
| |
| <!-- limits.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>STREAM_MAX</b><var><a name="index-STREAM_005fMAX-3505"></a></var><br> |
| <blockquote><p>If defined, the unvarying maximum number of streams that a single |
| process can have open simultaneously. See <a href="Opening-Streams.html#Opening-Streams">Opening Streams</a>. |
| </p></blockquote></div> |
| |
| <p><a name="index-limits_002c-time-zone-name-length-3506"></a><!-- limits.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Macro: int <b>TZNAME_MAX</b><var><a name="index-TZNAME_005fMAX-3507"></a></var><br> |
| <blockquote><p>If defined, the unvarying maximum length of a time zone name. |
| See <a href="Time-Zone-Functions.html#Time-Zone-Functions">Time Zone Functions</a>. |
| </p></blockquote></div> |
| |
| <p>These limit macros are always defined in <samp><span class="file">limits.h</span></samp>. |
| |
| <p><a name="index-limits_002c-number-of-supplementary-group-IDs-3508"></a><!-- limits.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Macro: int <b>NGROUPS_MAX</b><var><a name="index-NGROUPS_005fMAX-3509"></a></var><br> |
| <blockquote><p>The maximum number of supplementary group IDs that one process can have. |
| |
| <p>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 |
| <code>sysconf</code> to see whether a particular machine will let you have |
| more (see <a href="Sysconf.html#Sysconf">Sysconf</a>). |
| </p></blockquote></div> |
| |
| <!-- limits.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Macro: int <b>SSIZE_MAX</b><var><a name="index-SSIZE_005fMAX-3510"></a></var><br> |
| <blockquote><p>The largest value that can fit in an object of type <code>ssize_t</code>. |
| Effectively, this is the limit on the number of bytes that can be read |
| or written in a single operation. |
| |
| <p>This macro is defined in all POSIX systems because this limit is never |
| configurable. |
| </p></blockquote></div> |
| |
| <!-- limits.h --> |
| <!-- POSIX.2 --> |
| <div class="defun"> |
| — Macro: int <b>RE_DUP_MAX</b><var><a name="index-RE_005fDUP_005fMAX-3511"></a></var><br> |
| <blockquote><p>The largest number of repetitions you are guaranteed is allowed in the |
| construct ‘<samp><span class="samp">\{</span><var>min</var><span class="samp">,</span><var>max</var><span class="samp">\}</span></samp>’ in a regular expression. |
| |
| <p>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 |
| <code>sysconf</code> to see whether a particular machine will let you have |
| more (see <a href="Sysconf.html#Sysconf">Sysconf</a>). And even the value that <code>sysconf</code> tells |
| you is just a lower bound—larger values might work. |
| |
| <p>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. |
| </p></blockquote></div> |
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