unistd.h库的getopt（）函数

Question

I'm currently trying to make my own version of getopt() function. But I do not know how it returns a character type as an int type.

Is there any way I can have a look into the source code of the getopt() function?

Answer 1

The source code of getopt() in glibc is here: https://github.com/lattera/glibc/blob/master/posix/getopt.c

Of course there are more implementations you might look at, but this is probably the most popular one. Here's another, from FreeBSD: https://github.com/lattera/freebsd/blob/master/lib/libc/stdlib/getopt.c

Answer 2

The return value of getopt(3) function is int to allow for an extra value (apart of all the possible char s it returns) to mark the end of options condition. This extra value is EOF (as in getchar(3) function) which must be different from any char possible value.

To deal with this and the possibility of different C compilers implement char either as signed or unsigned , both functions implement the return value as the character value as an unsigned byte from 0 to 255 (by mapping all the negative values to positive, this is adding to the negative values the constant 256 ---this is an example, as the language doesn't specify exactly how this is done---, so the negatives go in the range 128..255 ), and reserve EOF as the value -1 .

If you are writing a getopt(3) function to be integrated in your system's standard c library, just check what value is used for EOF (most probable is -1 ) and then implement it so the values returned for your default char type don't conflict/overlap with/ it.