- 本文地址: https://www.laruence.com/2012/02/01/2503.html
- 转载请注明出处
今天有人在微博上问了一个问题, 使用:
string exec ( string $command [, array &$output [, int &$return_var ]] )
调用一个程序, 程序退出-1, 但是PHP得到的为什么是255?
这个问题简单的说, 是因为exit或者main函数中的return, 只能使用0~255之间的值. -1 的unsigned值就是255.
那么复杂点的说呢?
我们知道, 在Shell中, 运行一个命令, 一个程序, 都是fork一个子进程(然后exec)来执行的, 而这个程序的退出码, 被Shell(父进程), 通过wait来收集而后报告给我们的.
pid_t wait(int *statloc);
而对于wait来说, 历史上原因, 他将通过statloc返回一个16bit的interge(现在也有用32位表示的, 但是会兼容已有的设计). 这16bits的interge中, 高8位就是程序退出的值(exit, 或者return), 而低八位表示导致这个程序退出的信号(其中一位表示是否有Core文件产生), 如果程序是正常退出, 那么低八位为0[1].
所以, 如果我们返回-1, 并且因为我们是正常退出, 所以Shell通过wait收集到的子进程退出状态是:
11111111 00000000
而高八位作为unsigned, 就是255了.
另外, 补充一下, 在Linux的内建Shell命令中, 很多都会遵守一个退出状态码的约定, 具体的值对应的意思[2]:
| Exit Code Number | Meaning | Example | Comments |
|---|---|---|---|
| 1 | Catchall for general errors | let "var1 = 1/0" | Miscellaneous errors, such as "divide by zero" and other impermissible operations |
| 2 | Misuse of shell builtins (according to Bash documentation) | empty_function() {} | Seldom seen, usually defaults to exit code 1 |
| 126 | Command invoked cannot execute | Permission problem or command is not an executable | |
| 127 | "command not found" | illegal_command | Possible problem with $PATH or a typo |
| 128 | Invalid argument to exit | exit 3.14159 | exit takes only integer args in the range 0 - 255 (see first footnote) |
| 128+n | Fatal error signal "n" | kill -9 $PPID of script | $? returns 137 (128 + 9) |
| 130 | Script terminated by Control-C | Control-C is fatal error signal 2, (130 = 128 + 2, see above) | |
| 255* | Exit status out of range | exit -1 | exit takes only integer args in the range 0 - 255 |
学c语言也遇到了这个问题，谢谢大佬
ltobet
Good I appreciated your work very thanks
truthbet
The best sites in Thailand and make good money
[…] 本文地址: http://www.laruence.com/2012/02/01/2503.html […]
可以的,这很鸟哥! 解释的很清楚
[…] references from the following: http://www.laruence.com/2012/02/01/2503.html […]
I’ve been exploring for a little for any
high-quality articles or weblog posts in this
kind of house . Explorring in Yahoo I eventually stumbled upon this
website. Studying this info So i’m glad too express that I have a very excellent uncanny eeling I found out exacgly what I needed.
I most indisputably will make ceretain to don?t put out of your mind this website
and give it a glance regularly.
膜拜，，西安罢餐，致敬！
虽然我看不懂···
但膜拜一下
请问退出之后的code是11是怎么回事？
这个不错
又充了下电啦
赞
哈 了解了
你太牛逼了。膜拜！！！