GCC declarations: typedef __pid_t pid_t?

问题:

I am confused about the declaration of (for example) pid_t. What does __pid_t mean? Is it another type defined elsewhere? If yes, where? Why is my types.h in ubuntu 13.04 64bit defining pid_t like:

#ifndef __pid_t_defined
typedef __pid_t pid_t;
#define __pid_t_defined
#endif

and not something like

typedef int pid_t;

I saw some websites that have types.h headers with the declaration done the last way. This is one: http://www.sde.cs.titech.ac.jp/~gondow/dwarf2-xml/HTML-rxref/app/gcc-3.3.2/lib/gcc-lib/sparc-sun-solaris2.8/3.3.2/include/sys/types.h.html

UPDATE:

Ok I found out that a pid_t is an __pid_t which is an __PID_T_TYPE which is which is an __S32_TYPE which is an int. My question now is why is this? POSIX only states that pid_t has to be a signed integer, so why make the declaration enter soo deep in header files?

回答1:

If you are pulling up your types.h via 'man types' then at the top of the header file(under description in the man page) there should exist an include file that defines '__pid_t' at some point as a signed integer(if Ubuntu claims that their types are POSIX compliant; otherwise pid_t could be anything). The symbol ' __' is considered reserved(C standard, dunno about C++). If I had to take a wild guess as to why pid_t is defined as __pid_t and not some int is because __pid_t is what Debian's or the Linux Kernel's developers decided to use for the process ID variable name in all of their library functions; therefore only '__pid_t' needs to be changed to change the integer size for a process ID.

You should really look around before asking a question, similar stackoverflow questions are easily found: Size of pid_t, uid_t, gid_t on Linux .

回答2:

Ok I found what the __pid_t means on the answer to this question: Why PID of a process is represented by opaque data type?

QUOTE

typedef __pid_t pid_t;
...
# define __STD_TYPE     typedef
__STD_TYPE __PID_T_TYPE __pid_t;    /* Type of process identifications.  */
...
#define __PID_T_TYPE        __S32_TYPE
...
#define __S32_TYPE      int

UNQUOTE

So pid_t is an __pid_t which is an __PID_T_TYPE which is which is an __S32_TYPE which is an int.