A question about an include file.

[shaul@rakefet.debian.org]

Could someone explain what the following line, which is taken from 
/usr/include/errno.h, means :

	extern void     perror __P ((__const char* __s));

As I understand it, it means that perror is defined in an external file and 
returns a void type. But what are the __P, and __const ? And why there are 2 
sets of brackets, and not only 1 ?
Where does these constructs being documented ?

	Thank you.




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