Why Does exit(-1) Give an Exit Code of 255?中文EN

Today someone asked a question on Weibo: using:

string exec ( string $command [, array &$output [, int &$return_var ]] )

to call a program, and the program exits with -1, but why does PHP get 255?
Simply put, the reason is that exit, or the return in the main function, can only use values between 0~255. The unsigned value of -1 is 255.
So how about the more complex explanation?
We know that in the Shell, running a command or a program is done by forking a child process (and then exec), and this program's exit code is collected by the Shell (the parent process) via wait, and then reported to us.

pid_t wait(int *statloc);

And for wait, for historical reasons, it returns a 16-bit integer through statloc (there are also 32-bit representations now, but they remain compatible with the existing design). In these 16 bits, the high 8 bits are the program's exit value (exit, or return), and the low 8 bits represent the signal that caused the program to exit (one of the bits indicates whether a Core file was produced). If the program exited normally, then the low 8 bits are 0[1].
So, if we return -1, and because we exited normally, the child-process exit status collected by the Shell through wait is:

11111111 00000000

And the high 8 bits, as unsigned, is 255.
In addition, as a supplement, in the Linux built-in Shell commands, many follow a convention for exit status codes, where the specific values correspond to meanings[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

• [1]: http://stackoverflow.com/questions/2726447/why-is-the-exit-code-255-instead-of-1-in-perl
• [2]: http://tldp.org/LDP/abs/html/exitcodes.html