2013-01-07 wcdj

如何獲取程序的PID（process ID）?

可以使用：

#include <unistd.h>
pid_t getpid(void);

通過檢視標頭檔案說明，可以得到更詳細的資訊：

find /usr/include -name unistd.h

/usr/include/asm/unistd.h
/usr/include/bits/unistd.h
/usr/include/linux/unistd.h
/usr/include/sys/unistd.h
/usr/include/unistd.h

cat /usr/include/unistd.h | grep getpid

/* Get the process ID of the calling process.  */
extern __pid_t getpid (void) __THROW;
 

如何獲取執行緒的TID（thread ID）?

通過檢視man得到如下描述：
(1) The gettid() system call first appeared on Linux in kernel 2.4.11.
(2) gettid() returns the thread ID of the current process. This is equal to the process ID (as returned by getpid(2)), unless the process is part of a thread group (created by specifying the CLONE_THREAD flag to the clone(2) system call). All processes in the same thread group have the same PID, but each one has a unique TID.
(3) gettid() is Linux specific and should not be used in programs that are intended to be portable. （如果考慮移植性，不應該使用此介面）

但是根據man的使用說明，測試後發現會報找不到此介面的錯誤“error： undefined reference to `gettid'”，通過下面連結可以找到更詳細的說明：

http://www.kernel.org/doc/man-pages/online/pages/man2/gettid.2.html
(1) Glibc does not provide a wrapper for this system call; call it using syscall(2).（說明Glibc並沒有提供此介面的宣告，此介面實際使用的是系統呼叫，使用者可以自己建立包裹函式）
(2) The thread ID returned by this call is not the same thing as a POSIX thread ID (i.e., the opaque value returned by pthread_self(3)).

然後檢視/usr/include/sys/syscall.h（實際在/usr/include/asm/unistd.h）可以找到我們需要的system call number：
#define __NR_gettid     224

因此，要獲取某個執行緒的TID，最nasty的方式是：

#include <sys/syscall.h>
printf("The ID of this thread is: %ld\n", (long int)syscall(224));

或者比較elegant的方式是：

#include <sys/syscall.h>
#define gettidv1() syscall(__NR_gettid)
#define gettidv2() syscall(SYS_gettid)
printf("The ID of this thread is: %ld\n", (long int)gettidv1());// 最新的方式
printf("The ID of this thread is: %ld\n", (long int)gettidv2());// traditional form

PS: 在/usr/include/sys/syscall.h中可以看到關於__NR_<name>和SYS_<name>兩個巨集的區別，實際最後使用的都是__NR_<name>。

// /usr/include/bits/syscall.h
#define SYS_gettid __NR_gettid

#ifndef _LIBC
/* The Linux kernel header file defines macros `__NR_<name>', but some     
   programs expect the traditional form `SYS_<name>'.  So in building libc
   we scan the kernel's list and produce <bits/syscall.h> with macros for
   all the `SYS_' names.  */
# include <bits/syscall.h>
#endif

驗證TID是否正確的方法：
檢視程序pid
(1) ps ux | grep prog_name
(2) pgrep prog_name
檢視執行緒tid
(1) ps -efL | grep prog_name
(2) ls /proc/pid/task