Linux學習
1．操作系統的實驗基本上都是以linux下面的系統編程為內容，進程的調度和創建都是很基本的內容。很幸運的是，linux為我們提供了很多的系統函數，供我們來使用。通過熟悉這些函數，使得我們可以能夠對進程和程序有一個更良好的認識。
2．Fork函數的使用。下面引用自百度百科：
|
fork()函數，Linux系統調用
頭文件：
#include <unistd.h>
函數定義：
int fork( void );
返回值：
子進程中返回0，父進程中返回子進程ID，出錯返回-1
函數說明：
一個現有進程可以調用fork函數創建一個新進程。由fork創建的新進程被稱為子進程（child process）。fork函數被調用一次但返回兩次。兩次返回的唯一區別是子進程中返回0值而父進程中返回子進程ID。
子進程是父進程的副本，它將獲得父進程數據空間、堆、棧等資源的副本。注意，子進程持有的是上述存儲空間的“副本”，這意味著父子進程間不共享這些存儲空間，它們之間共享的存儲空間只有代碼段。
示例代碼：
#include <unistd.h>
#include <stdio.h>
int main(int argc, void ** argv )
{
int pid = fork();
if(pid == -1 ) {
// print("error!");
} else if( pid = =0 ) {
// print("This is the child process!");
} else {
// print("This is the parent process! child process id = %d", pid);
}
return 0;
}
|
1. 下面來詳細的說說我的程序和問題，通過比較學習，相信對于linux的學習，特別是進程和程序直接的關系有一個更好的理解。
Process2.c
Code：
#include <stdio.h>
#include <unistd.h>
int main()
{
int p1;
p1=fork();
if (p1 == 0) //子進程，輸出它的父進程和當前進程的PID
{
printf("current PID is %d\n",getpid());
printf("father PID is %d\n",getppid());
printf("\n");
}
else //這里是父進程嗎？
{
printf("father PID is %d\n",getppid());
printf("current PID is %d\n",getpid());
printf("\n");
}
return 0;
}
分析：此進程的運行結果是？（思考后再回答）
是的，有兩種情況。首先，程序運行到fork語句之后，創建進程，此時有兩種情況，創建成功和不成功，而成功又有兩種情況，fork的返回值為0，則運行的是創建的子進程，fork的返回值為1，則運行的是父進程。用if語句來判斷，OK。
那么問題就是，if語句中沒有問題，p1為0，說明創建成功，運行的是子進程，可是else語句來說，卻又兩種情況，如果創建不成功也在else語句中，那么此時當前進程就是那個原始的進程，而它的父進程卻是1號PID，因為所有的進程都是1號進程的子進程，這個是linux下面的結論。操作系統的知識。
所以，此時的結果應該是：
|
father PID is 32667
current PID is 15254
current PID is 15255
father PID is 1
|
先放著，先解釋下面的這個，注意，這個對上面是有幫助的。
（看完之后回頭過來看看），看到沒有，首先輸出的那個father PID是什么，剛好就是下面的那個32667，這個不是偶然的，說明什么，當然，這個進程就是父進程，而創建它的那個進程的PID是32667，可以認為是系統創建的。就是說這種情況下面是，先運行的父進程。而它創建失敗了，那么它沒有子進程啊，那么那個下面的這個進程的父進程就不是上面的那個了，為何，從結果可以看到，它的PID是1，而不會是15254，說明不是上面的那個創建。而創建卻是1，說明是系統創建的，所以fork語句之后，沒有創建成功，而是由系統來親自創建一個進程，所以PID號當然是下一個，不過父進程就是1號了。（有興趣的可以查閱下資料，我們也是剛學，所以我還有繼續探究中！！）
|
current PID is 15302 //子進程
father PID is 15301[l1]
father PID is 32667 //父進程
current PID is 15301
|
系統創建了當前的那個父進程，就是程序運行的進程
PID = 32667
|
下面還想討論這樣一個問題，這個問題是從上面那個第一種情況引申來的。
如下：
Code：
|
#include <stdio.h>
int main()
{
int p1;
while (( p1 = fork())!= -1);
if (p1 == -1)
{
printf("current PID is %d\n",getpid());
printf("father PID is %d\n",getppid());
}
return 0;
}[l2]
|
結果如下，雖然比較多，不過很能說明問題：
|
current PID is 17600
father PID is 1
current PID is 17761
father PID is 1
current PID is 17602
father PID is 1
current PID is 18545
father PID is 1
current PID is 17608
father PID is 1
current PID is 17767
father PID is 1
current PID is 17624
father PID is 1
current PID is 17773
father PID is 1
current PID is 17616
father PID is 1
current PID is 18551
father PID is 1
current PID is 17630
father PID is 1
current PID is 18552
father PID is 1
current PID is 17636
father PID is 1
current PID is 17779
father PID is 1
current PID is 17642
father PID is 1
current PID is 17785
father PID is 1
current PID is 17648
father PID is 1
current PID is 17787
father PID is 1
current PID is 17654
father PID is 1
current PID is 17795
father PID is 1
current PID is 18510
father PID is 1
current PID is 17801
father PID is 1
current PID is 18511
father PID is 1
current PID is 17807
father PID is 1
current PID is 17656
father PID is 1
current PID is 17815
father PID is 1
current PID is 17384
father PID is 32667
current PID is 17819
father PID is 1
current PID is 17660
father PID is 1
father PID is 1
current PID is 17825
father PID is 1
current PID is 17668
father PID is 1
current PID is 18571
father PID is 1
father PID is 1
father PID is 1
father PID is 1
father PID is 1
current PID is 17672
father PID is 1
current PID is 18572
father PID is 1
current PID is 17678
father PID is 1
current PID is 17831
father PID is 1
father PID is 1
current PID is 17684
father PID is 1
current PID is 17839
father PID is 1
current PID is 17690
father PID is 1
current PID is 17845
father PID is 1
current PID is 17696
father PID is 1
current PID is 17851
father PID is 1
current PID is 17702
father PID is 1
current PID is 17857
father PID is 1
current PID is 17710
father PID is 1
current PID is 17863
father PID is 1
current PID is 18531
father PID is 1
current PID is 17786
father PID is 1
current PID is 17714
father PID is 1
current PID is 18566
father PID is 17786
current PID is 17720
father PID is 1
current PID is 18533
father PID is 1
current PID is 17728
father PID is 1
current PID is 18538
father PID is 1
current PID is 17730
father PID is 1
current PID is 18548
father PID is 17740
current PID is 17738
father PID is 1
current PID is 17794
father PID is 1
current PID is 17740
father PID is 1
current PID is 17804
father PID is 1
current PID is 17750
father PID is 1
current PID is 17810
father PID is 1
current PID is 17756
father PID is 1
current PID is 17814
father PID is 1
current PID is 17764
current PID is 17820
father PID is 1
father PID is 1
current PID is 17768
father PID is 1
current PID is 17826
father PID is 1
current PID is 17774
father PID is 1
current PID is 17832
father PID is 1
current PID is 17780
father PID is 1
current PID is 17838
father PID is 17913
father PID is 1
father PID is 17929
father PID is 17927
father PID is 1
father PID is 17941
father PID is 1
father PID is 17950
father PID is 1
|
看完了上面的那個例子，下面的這個例子就是小case了，當然，其實上面的那個例子就是由下面的這個例子引申來的，是因為寫錯了代碼而引起的。當然，根據錯誤，我們收獲不小啊！
Code：
|
#include <stdio.h>
#include <unistd.h>
int main()
{
int p1;
while ((p1=fork())==-1) ;
if (p1 == 0)
{
printf("current PID is %d\n",getpid());
printf("father PID is %d\n",getppid());
printf("\n");
}
else
{
printf("father PID is %d\n",getppid());
printf("current PID is %d\n",getpid());
printf("\n");
}
return 0;
}
|
結果如下：
|
current PID is 20312[l3]
father PID is 20311
father PID is 32667
current PID is 20311
----------------------------------------------------------------------------------------------------------------
father PID is 32667
current PID is 20309[l4]
current PID is 20310
father PID is 1
|
[l1]父進程的PID剛好就是下面的那個父進程的current PID，說明下面的那個進程就是父進程。
[l2]看看這個程序，雖然比較流氓，不過可以根據結果知道，如果創建失敗的話，那么也會有一個當前的PID，而且還是由一個1號進程創建。
[l3]當前是子進程，而它的父進程恰好是下面的這個進程！
[l4]父進程就是這個進程，而它的那個子進程卻是由系統創建的！
后來通過查閱資料知道，如果父進程創建一個子進程后其先結束，那么子進程就變成了孤兒進程，這樣的話，就會由初始化進程，即1號進程來領養。所以會顯示它的父進程為1.至此，完美的解決了。。
最近在看《Linux程序設計》（第三版）一書，收獲頗豐。本書是Linux下面程序設計的經典，值得一睹。。而且最近的感觸是，懂得linux的操作和會在Linux下面進程程序設計完全是兩回事，就像你會用windows和會開發windows下面的程序一樣。我輩任重而道遠啊。。
posted on 2009-12-10 14:41
deercoder 閱讀(314)
評論(0) 編輯 收藏 引用 所屬分類:
C/C++