Perl semget 函数

描述

该函数使用系统函数 semget() 返回与 KEY 关联的信号量 ID。查找与 KEY 关联的信号量。

句法

以下是该函数的简单语法 -

semget KEY, NSEMS, FLAGS

返回值

该函数在失败时返回 undef，在成功时返回 0，但返回 true。

例子

以下是示例代码，显示其基本用法，创建信号量并递增其值 -

#!/usr/bin/perl -w

# Assume this file name is left.pl

use IPC::SysV;

#use these next two lines if the previous use fails.
eval 'sub IPC_CREAT {0001000}' unless defined &IPC_CREAT;
eval 'sub IPC_EXCL {0002000}'  unless defined &IPC_EXCL;
eval 'sub IPC_RMID {0}'        unless defined &IPC_RMID;

$key = 1066;

$| = 1;
$num = 0;
$flag = 0;

# Create the semaphor
$id = semget ( $key, 1, &IPC_EXCL|&IPC_CREAT|0777 ) or 
	die "Can't semget: $!";
foreach( 1..5) {
   $op  = 0;
   $operation = pack( "s*", $num, $op, $flags );
   semop( $id, $operation ) or die "Can't semop: $! ";
   print "Left....\n";
   sleep 1;
   $op = 2;
   $operation = pack( "s*", $num, $op, $flags );
   # add 2 to the semaphore ( now 2 )
   semop( $id, $operation ) or die "Can't semop $! ";
}
semctl (  $id, 0, &IPC_RMID, 0 );

使用 $left.pl& 在后台运行上述程序并编写另一个程序。这里 Left 将信号量设置为 2，Right 打印 Right 并将信号量重置为 0。这种情况会一直持续到 Left 完成循环，然后使用 semctl() 销毁信号量

#!/usr/bin/perl -w

# Assume this file name is right.pl

$key = 1066;

$| = 1;
$num = 0;
$flags = 0;

# Identify the semaphore created by left.
$id = semget( $key, 1, 0 ) or die ("Can't semgt : $!" );

foreach( 1..5) {
   $op = -1;
   $operation =  pack( "s*", $num, $op, $flags );
   # Add -1 to the semaphore (now 1)
   semop( $id, $operation ) or die " Can't semop $!";
   print "Right....\n";
   sleep 1;
   $operation = pack( "s*", $num, $op, $flags );
   # Add -1 to the semaphore (now  0)
   semop( $id, $operation ) or die "Can't semop $! ";
}

当执行上面的代码时，它会产生以下结果。现在运行 right.pl ，它将产生以下结果 -

Right....
Left....
Right....
Left....
Right....
Left....
Right....
Left....
Right....
Left....