PHP中的 max_execution_time 是通过setitimer来实现的：
文件： Zend/zend_execute_API.c
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/* This one doesn't exists on QNX */ #ifndef SIGPROF #define SIGPROF 27 #endif void zend_set_timeout(long seconds, int reset_signals) /* {{{ */ { TSRMLS_FETCH(); EG(timeout_seconds) = seconds; #ifdef ZEND_WIN32 if(!seconds) { return; } if (timeout_thread_initialized == 0 && InterlockedIncrement(&timeout_thread_initialized) == 1) { /* We start up this process-wide thread here and not in zend_startup(), because if Zend * is initialized inside a DllMain(), you're not supposed to start threads from it. */ zend_init_timeout_thread(); } PostThreadMessage(timeout_thread_id, WM_REGISTER_ZEND_TIMEOUT, (WPARAM) GetCurrentThreadId(), (LPARAM) seconds); #else # ifdef HAVE_SETITIMER { struct itimerval t_r; /* timeout requested */ sigset_t sigset; if(seconds) { t_r.it_value.tv_sec = seconds; t_r.it_value.tv_usec = t_r.it_interval.tv_sec = t_r.it_interval.tv_usec = 0; # ifdef __CYGWIN__ setitimer(ITIMER_REAL, &t_r, NULL); } if(reset_signals) { signal(SIGALRM, zend_timeout); sigemptyset(&sigset); sigaddset(&sigset, SIGALRM); } # else setitimer(ITIMER_PROF, &t_r, NULL); } if(reset_signals) { signal(SIGPROF, zend_timeout); sigemptyset(&sigset); sigaddset(&sigset, SIGPROF); } # endif if(reset_signals) { sigprocmask(SIG_UNBLOCK, &sigset, NULL); } } # endif #endif } |
说明： 当程序执行到最大时间时，将会收到一个SIGPROF的信号，PHP处理了这个信号，至于哪些时间计算到最大执行时间中，且看setitimer的文档：
SYNOPSIS
#include <sys/time.h>
int getitimer(int which, struct itimerval *value);
int setitimer(int which, const struct itimerval *restrict value,
struct itimerval *restrict ovalue);
DESCRIPTION
The getitimer() function shall store the current value of the timer specified by which into the structure pointed to by value. The setitimer() function shall set the timer specified by which to the value specified in the structure pointed to by value, and if ovalue is not a null pointer, store the previous value of the timer in the structure pointed to by ovalue.
A timer value is defined by the itimerval structure, specified in <sys/time.h>. If it_value is non-zero, it shall indicate
the time to the next timer expiration. If it_interval is non-zero, it shall specify a value to be used in reloading
it_value when the timer expires. Setting it_value to 0 shall disable a timer, regardless of the value of it_interval. Setting it_interval to 0 shall disable a timer after its next expiration (assuming it_value is nonzero).
Implementations may place limitations on the granularity of timer values. For each interval timer, if the requested timer value requires a finer granularity than the implementation supports, the actual timer value shall be rounded up to the next supported value.
An XSI-conforming implementation provides each process with at least three interval timers, which are indicated by the which argument:
ITIMER_REAL
Decrements in real time. A SIGALRM signal is delivered when this timer expires.
ITIMER_VIRTUAL
Decrements in process virtual time. It runs only when the process is executing. A SIGVTALRM signal is delivered when it expires.
ITIMER_PROF
Decrements both in process virtual time and when the system is running on behalf of the process. It is designed to be used by interpreters in statistically profiling the execution of interpreted programs. Each time the ITIMER_PROF
timer expires, the SIGPROF signal is delivered.
The interaction between setitimer() and any of alarm(), sleep(), or usleep() is unspecified.
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疑惑： 既然PHP的执行时间限制是使用setitimer来实现的，那么为什么官方文档还有那么多莫名其妙的解释呢？
如：http://us1.php.net/manual/zh/function.set-time-limit.php
Note:
set_time_limit()函数和配置指令max_execution_time只影响脚本本身执行的时间。任何发生在诸如使用 system()的系统调用，流操作，数据库操作等的脚本执行的最大时间不包括其中，当该脚本已运行。在测量时间是实值的Windows中，情况就不是如此了。