Exit (-1) or return (-1) Why did the shell get an exit code of 255?

Write a Hello World:

Filename:main.c#include <stdio.h>int Main (void) {printf ("Hello wolrd!\n");   Return (-1); }

Compile execution: gcc main.c &&./a.out

Now let's see what is the return value of the previous execution in the current shell, which is "1"?

[Email protected]:~/Desktop $ gcc main.c &&./a.outhello [email protected]:~/desktop $ echo $?255

Ah, the result why "255"? Call a program, program exit-1, get the result is not "1"?

The following references are from: http://www.laruence.com/2012/02/01/2503.html

This is simply because the exit or return in the main function only uses the value between 0~255. -The unsigned value of 1 is 255.

What's so complicated about that?

We know that in the shell, running a command, a program, is to fork a child process (and then exec) to execute, and this program's exit code, by the shell (parent process), through wait to collect and then report to us.

1. pid_t wait (int *statloc);

For wait, for historical reasons, he will return a 16bit interge via Statloc (now also available in 32-bit notation, but will be compatible with existing designs). In this 16bits interge, the high 8 bits are the value of the program exit (exit, or return), and the low eight bits indicate the signal that caused the program to exit (one of them indicates whether a core file was generated), and if the program exits normally, the low eight bits are 0[1] .

So, if we return 1, and because we exit normally, the child process that the shell collects through wait exits the status:

1. 11111111 00000000

And the high eight-bit as unsigned, is 255.

In addition, many of the built-in shell commands in Linux will adhere to an exit status code that corresponds to the meaning of the value [2]:

Exit (-1) or return (-1) Why did the shell get an exit code of 255?