193_4ECH.PDFIn Depth In this chapter, I’m going to

文件名称: 193_4ECH.PDF

所属分类: Perl

开发工具:

文件大小: 504kb

下载次数: 0

上传时间: 2019-08-18

提供者: drji*****

下载 (504kb)

不能下载？报告错误

详细说明：In Depth In this chapter, I’m going to take a look at the support Perl offers for interprocess communication (IPC). As far as an operating system is concerned, a process is a running job that has its own execution resources, including share of CPU time. Your process can start or interact with other In Depth Youll also find more IPC material in one other place in this book, Chapter 20, on the Internet and socket programming. Most people think of sockets as an Internet phenomenon that you use to connect to other machines across the Internet, but in fact, you can use sockets to connect processes on the same machine. And. that's it it's time to turn to the immediate solutions now 1934eC1P5 8/3001.3:14PM cHapter 1 Built-In Functions: Interprocess Communication i Immediate Solutions Catching Signals The Novice Programmer is complaining. "Users are shutting down my script us ing Ctrl+C, the NP says, which doesnt give me any chance to do any cleanup catching the INT signal. "Oh? "the NP asks. Tell me more/ "y, you could try and close my databasc file as it should be closed. "well n Unix, processes can communicate with each other using signals, and Perl uses an easy mechanism for catching signals sent to your process: You connect a sig nal handling subroutine to the signals entry in the predefined %SIG hash. (See the topic %SIG Signal Handlers"in Chapter 10 for more information. In this first example, I catch the INT signal, which is sent when the user presses Ctrl+C. To catch that signal, you connect a signal handler, which I'll call sig_handler, to the INT signal this way sSIGiINT-\&sig. handler; Note that I'n storing a hard reference to sig_ handler in the %sIg hash here. You can also store symbolic references if you like, but i prefer hard references be cause they dont cause any confusion if you use this code in a module The sig_ handler signal handler is just a subroutine. That subroutine gets the name of the signal passed to it as an argument, so you can display the name of the signal and quit, like this sub sig handler f die" i got signal signal"; Now, you can add code for the rest of the program, and when the user presses Ctrl+C, the code in sig_ handler will be called (note that you can see the c in the output) sub sig handler e4 1934eC1P5 8/3001,3:14PM Immediate Solutions my $signal= shift die i got signal signe SSIGlInt -l&sig handle While(>)t print nCI got signal INT at interruptme. p! iine 3 Using a signal handler like this gives you the chance to perform a little cleanup when the user presses Ctrl+C or you've sent yourself an alarm signal to handle a timeout(see the topic alarm Send an Alarm Signal"in Chapter 12 ). But, you should know one thing: Most Unix system library functions (unlike Windows func tions and even Perl at the deepest level are not re-entrant. When afunction is re entrant, you can interrupt processing while inside the function and call it again without any trouble because the original state of the data in the function is stored on the stack and is restored when the second call to the function exits If a function is not re-entrant, and you interrupt it and call it again, the original data will be overwritten when the second call exits the original call to the fund tion is left in a shambles(and the return address of the original caller is usually lost as wel, with the result that the process will probably hang TIP: / once had the pleasure of having to write a Windows program entirely in assembly language, which also meant making use of assembly language to make it re-entrant so that it could handle multiple concurrent calls. /aon't recommend doing so unless you have as much patience as time-and a ot of both You therefore should keep the code in your signal handlers as short as possible The temptation might be great to add a lot of code after the user has pressed Ctrl+C, for example, but in fact, you have to keep it minimal Note that you can make signal handlers anonymous subroutines, like this SSIGINT- sub my Signal shift die "i got signal signal hile(>)i print: Cl got signa: IN at interruptme. p! line 3. 1934eC1P5 8/3001,3:14PM cHapter 1 Built-In Functions: Interprocess Communication You can also assign strings such as 'ignore and deFault'to signal handlers See the topic %SIG Signal Ilandlers'"in Chapter 10 for more information. You also should note that some signals cant be trapped (orignored), such as the KILL and stoP signals Finally, note that you can make a signal handler temporary, either by reassigning a new value to it (such as 'DEFAULT or by using local like this: sub do_not_interrupt_me local sSIgIINTI ignORE store( gdata) When the code in the subroutine goes out of scope, the int signal handler is restored to the value it had before the subroutine was called. (Because Im using local and not, my here, the samc signal handler will be active for any subroutines that are called from the current subroutine. See the topic "Whats the Difference between my and local? "in Chapter 7.) See also the topic Sending a signal to another Process"in this chapter and the topic"What Signals Are Available? coming up next What Signals Are Available? Okay, the Novice programmer says, I understand that i can catch signals sent to my process in Unix by using the %SIG hash, but what signals are available? i need to know that infornation before i can write any signal handlers " That you say," varies bv system.”“ I knew you' d say that,” the np says. To see what signals are supported on a particular Unix system, you can use the kill-l command at the command prompt %ki11-1 HUP NT IT I TRAP ABR FMT FDF KILL BUS SEGV SYS PPF ARM TERM URG STOP TSTO CONT CHID TTIN TOU 9 XCPU XFS TAI RM PROF WINCH CST USRI USR2 In Perl code, you can use the Config module and take a look at S Config(sig_name) like this use Config print SConfig[sig_name]": 66 1934eC1P5 8/3001,3:14PM Immediate Solutions HUP INT QUIT ILL TRAP ABRT EMT FPE BUS SEGV SYS PIPE ALRM TERM URG STOP TSTP CONT CHLD TTIN TTCU I0 XCPU XFSZ VTALRM PROF WINCH LOST USRI SR2 This is all very well, but those names are pretty terse. What do they stand for? You'll find a list of the standard Unix signals in Table el.l, along with what they do See also the topics "Catching Signals"and"Sending a Signal to Another process ater in this chapter. TE Table e11 Standard unix signals Signal Signal Number Means HUP Hangup nterrupt QUIT Quit legal instruction TRAP Trace trap ABRT 6 Abort EMT Emulator trar FPE Athme c exception KILL Kill(cannot be caught, blocked, or ignored) BUS 10 Bus error SEGV mentation volati SYS Bac argument to system cal PIPE ac wr te to pipe or socke ALRM 14 Aarm clock TERM Sottware termination signal URG Urgent condition present on socket STOP Stop(cannot be caught, blocked, or ignored TSTP Stop s gna generated from keyboard CONT Continue after sto CHLD 20 Child status has changed ITIN Background read trom control termnal TTOU Background write to control terminal /0 is possible on a descr pto XCPU 24 CPU time limit exceeded XFSZ File size limit exceeded (continued) 1934eC1P5 7 8/3001.3:14PM cHapter 1 Built-In Functions: Interprocess Communication Table e11 Standard Unix signals(continued) signal Signal Number Means VTALRM 26 Virual time alarm PROF Prof ling timer alarm WINCH Window change LOST Resource lost USRT User-defined signal USR2 31 User-defined sig nal 2 Using Backticks to Pass Commands to the System I have good news and bad news, the Novice Programmer says. What's the good news? you ask. Users love iny database program so much they never want to leave it, the NP says. And, the bad news? you ask. Theyre demanding a way to execute system commands from inside my program so that they can do thin like check directories to find files, the nP says. Thats no problem, you say. ' Usc the backticks opcrator to cxccutc a command from insidc your codc You can use the backticks operator--that is, the backward-leaning single quote Cto cause Perl to pass a command to the underlying operating systen For example, in code, you can execute the Cnix uptime command (which shows how long the computer has been up)this way: Uptime uptime rint uptime 4: 29oil up 18 adys, 21: 22, 13 users, odd average: 0.30, 0.39,0.42 The backticks operator works the same way in MS-DOS. No uptime command is available in ms-dos, but i can execute a command such as dir this way sdirlist = dir print sdirlist Directory cr C: \per!book\temp 10-07-994:02p 10-07-994:02p IEMP 3,53510-0/- eB 1934eC1P5 8/3001.3:15PM Immediate Solutions Note that i assigned the return value of the backticks operator, which is the out- put of the command, to a scalar. You should assign the value in normal circum- stances because backticks are designed expressly to let you capture a programs output. (If you just want to run a program, see the topic system Fork and run Another Program"later in this chapter. Using backticks without assigning a re turn value to anything is called using backticks in void conteat, and a lot of styl ists frown on that, approach TE In addition, you should check the error returned by the backticks operator, if one occurs. Errors are stored in the predefined variable $? suti print uptime: rint s? if $ TIP: Although you probabiy should check $ many programmers just content themse/ves with checking the return value from the backticks operator, making sure it's not empty like this: unless(command ) print"Error ,T Let me add one final note: Passing unchecked commands that the user types to the backtic ks operator is a very bad idea for security rcasons exec Execute a Program The big boss appears and says, We have so many products now that the public is getting confused. We nccd to have onc unified interfacc that, will launch our othcr programs as appropriate. "You ask, You want me to launch other programs from mv code?”“ Thats right;” the BB says.“Okay," you say,“ Ill use the exec function.” The exec function executes a program or system command exec LIST exec PRUGRAM LISt This function executes a program or systcm command and never returns. (If you want the command to return, use the system function. )That is, this function replaces your program with another one The exec function fails and returns false only if the program you call does not exist. Because exec never returns unless an error occurs, Perl will give you warn ings if you use the -w switch when there are statements after exec that are not die. warn or exit statements 1934eC1P5 8/3001,3:15PM cHapter 1 Built-In Functions: Interprocess Communication Note that system commands can differ by operating system. The following (rare) exec call example will work under both windows and unix exec 'echo Hello Hejia The way exec treats its parameters is complex. It works like this in Unix: IfLIST contains more than one argument (or if lisT is an array with more than one value), exec calls the system function execvp with that list, treating the first clement in the list. as the namc of the program to run. If you pass a single scalar argument (or an array with just one element), Perl checks for shell mctacharactcrs, which you usc to perforin operations such as creating pipes. The Unix shell metacharacters are as follows 8;\"|*?~<>^()[]}$nr If exec finds any metacharacters, the argument is passed to the systems com- mand shell. If on the other hand. perl finds no shell metacharacters the argu- ment is split and passed to execvp In this example, I use the exec function to delete a file using the system unlink comman a=( unlink if(exec(a))die exec call failed: $ 1 In this case, I'm passing the arguments to this function in an array to make sure the function explicitly realizes that two items appear in the list and cant treat unlink delete me. txt as a namc of a program and so fail. Passing arguments in an explicit array like this is often safest when you're using the exec function Bcar in mind that, as with the bac ks operator;, it's a bad idca for sccurity rca. sons to pass strings the user types in to the exec function without checking them system Fork and Run Another Program Hey, the Novice Programmer says, "Tvc becn using the exec function to cxccutc another program, and nothing ever came back. How can i get on with the rest of my code? Easy, you say, dont use exec, which is designed to replace your program with another onc, usc the system function instcad. Oh, the NP says e10 1934eC1P5 8/3001,3:15PM

(系统自动生成,下载前可以参看下载内容)