Linux。 C 中的管道。将数据发送到另一个进程。写作和阅读

Posted 2023-02-16

【中文标题】Linux。 C 中的管道。将数据发送到另一个进程。写作和阅读

【英文标题】：Linux. Pipes in C. Sending data to another process. Writing and reading

【发布时间】：2016-12-16 12:43:35

【问题描述】：

（警告 - 不是完美的英语！）

在拼贴画上，他们给了我一项任务，我为此苦苦挣扎了几个小时，却无法解决。 我在将数据从一个进程发送到另一个进程时遇到问题。在下面的代码中，我将输入数据发送到子进程，但我没有从子进程接收它。各位大神能帮忙解决一下吗？

程序描述：我的工作是从用户那里获取输入，使用管道将其发送到子进程，进行一些计算，然后将这些计算进一步发送到下一个进程。最后一个过程对传入的数据求和并将其分配给最终结果。更具体地说，第一个子进程计算麦克劳林级数的单个项并将其发送到求和的进程。代码如下：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
#include <math.h>
#include <sys/stat.h>

int mkfifo(const char *s, mode_t m) 
  return 0;


 int i = 1;
 double FINAL_RESULT = 0 ;


double factorial(int N)

    if(N==1)return 1.0;

    return N*factorial(N - 1);


double Maclaurine(int i , double x , double eps)

    double result;

     result = (pow(x,(double)i) / (double)factorial(i));

    if( result < eps) return 0;

    return result;



void print(char * tab, int size)

    printf("\n");   

    for( int i = 0; i < size ; i++)

        printf("%c" , tab[i]);  

       
    printf("\n");   




int main()

    int pd;

    if(mkfifo("tmp/myfifo1", 0666) == -1)

        printf("Failed to create myfifo2\n");
    

    if(mkfifo("tmp/myfifo2", 0666) == -1)

        printf("Failed to create myfifo2\n");

    

    pd = open("tmp/myfifo1", O_WRONLY);

    double x1,eps1;

    printf("X Value : \n");
    scanf("%lf", &x1);
    printf("EPS Value : \n");
    scanf("%lf", &eps1);

    char tab1[128], tab2[128];

    //converting to char*
    sprintf(tab1,"%lf",x1);
    sprintf(tab2,"%lf",eps1);

    //printing to standard output
    print(tab1, strlen(tab1));
    print(tab2, strlen(tab2));

    //creating new process
    if(fork() == 0)

        //creating another process
        if(fork() == 0)

            int pd3; 
            pd3 = open("tmp/myfifo2", O_RDONLY);

            char tab5[128];

            double x3;

            //reading data from pipe , myfifo2
            read(pd3,tab5,sizeof(tab5));

            //converting char* to float
            x3 = atof(tab5);

            FINAL_RESULT += x3;
            printf("\nCurrent result = %lf" , FINAL_RESULT);

        
        int pd2;
        //opening myfifo1 for reading 
        pd2 = open("tmp/myfifo1", O_RDONLY);

        if(pd2 == - 1)

            printf("\nFailed to open myfifo1 for reading!\n");

        
        double x2, eps2;
        char tab3[128], tab4[128];

        //reading inputed parameters        
        read(pd2,tab3, sizeof(tab3) );

        read(pd2,tab4, sizeof(tab4) );
        //converting to floats
        x2 = atof(tab3);

        eps2 = atof(tab4);

        printf("\nReceived initial data x=%f, eps = %f \n" , x2, eps2);

        double singleTerm = 0;

        int pd4;

        char wyraz[128];

        //opening myfifo2 for writing 
        pd4 = open("tmp/myfifo2", O_WRONLY);

        if(pd4 == -1 )
            printf("\nfailed to open myfifo2\n");
        

        do
            //calculate single term of Maclaurin series
            singleTerm = Maclaurine(i,x2,eps2);
            printf("\n%d. Series term = %lf\n", i,singleTerm);

            sprintf(wyraz,"%lf",singleTerm);

            if(write(pd4, wyraz, sizeof(wyraz)) == -1 )
                printf("\nfailed to send singleTerm to myfifo2\n");
            

            i++;
            //if its less than eps, loop ends
        while(singleTerm);

        printf("\nFinal Result :  %lf", FINAL_RESULT);
        close(pd4);
        close(pd2);

        unlink("tmp/myfifo2");

    

    //write data to myfifo1
    write(pd, tab1, sizeof(tab1));

    write(pd, tab2, sizeof(tab2));

    close(pd);

    unlink("tmp/myfifo1");

    return 0;

这里是代码的结果：

pawel@IMPERIUMVB:~$ gcc pipes.c -lm -o x
pawel@IMPERIUMVB:~$ ./x
X Value : 
10
EPS Value : 
0.001

10.000000
0.001000

pawel@IMPERIUMVB:~$ 
Failed to open myfifo1 for reading!
Received initial data x=0.000000, eps = 0.000000 
failed to open myfifo2
1. Series term = 0.000000
failed to send singleTerm to myfifo2
Final Result :  0.000000
Current result = 0.000000
Failed to open myfifo1 for reading!
Received initial data x=0.000000, eps = 0.000000 
failed to open myfifo2
1. Series term = 0.000000
failed to send singleTerm to myfifo2
Final Result :  0.000000

【问题讨论】：

我只想补充一点，我没有完成这项任务，因为这是课堂作业。我发布它是因为我想了解这里发生了什么。

好的，谢谢，我会改正的。

好的，现在更正了。我希望它现在很好看。

在你的程序中包含 int mkfifo(const char *s, mode_t m) return 0; 意味着你不会创建 FIFO，所以你不能使用它们。您需要删除它才能使用系统调用。你应该总是检查open()的结果——仅仅因为你创建了一个文件（你还没有创建）并不意味着当你去打开它时它仍然可以访问或存在。

是的，你是对的。我只是复制粘贴这个功能，甚至没有考虑只是因为我用谷歌搜索它并且它给了我更少的错误。经过多次尝试，现在我有了解决方案。我将在下面发布代码。感谢您的回复:)

【参考方案1】：

好的，我解决了这个问题，程序给了我预期的结果。 第一个关键要素是擦除

int mkfifo(const char *s, mode_t m)  return 0; 

就像 Jonathan Leffler 说的 *“在你的程序中包含 int mkfifo(const char s, mode_t m) return 0; 意味着你不会创建 FIFO，所以你不能使用它们。你需要删除它以使用系统调用。”

第二件事是支持我的 fifo 在 lseek() 函数单次使用后一遍又一遍地开始。此外，我还必须使用 memset() 清除所有缓冲区。

这是我现在的代码：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <string.h>
#include <math.h>
#include <sys/stat.h>



 int i = 0;
 double FINAL_RESULT = 1.0 ;


double factorial(int N)

    if(N==1 || N == 0)return 1.0;

    return N*factorial(N - 1);


double Maclaurine(int i , double x , double eps)

    double result;

     result = (pow(x,(double)i) / (double)factorial(i));

    if( result < eps) return -1.0;

    return result;



void print(char * tab, int size)

    printf("\n");   

    for( int i = 0; i < size ; i++)

        printf("%c" , tab[i]);  

       
    printf("\n");   




int main()

    umask(0);

    if(mkfifo("myfifo1", 0666) == -1)

        printf("Failed to create myfifo2\n");
    

    if(mkfifo("myfifo2", 0666) == -1)

        printf("Failed to create myfifo2\n");

    


    if(fork() == 0)

        int pd = open("myfifo1", O_WRONLY);

        double x1,eps1;
        char tab1[256];
        //assigning tab1 data to 0's

        lseek(pd,0,0);
        memset(tab1, 0, 256);

        printf("X Value : \n");
        scanf("%lf", &x1);


        sprintf(tab1,"%lf",x1);
        //write data to myfifo1
        write(pd, tab1, strlen(tab1));

        printf("EPS Value : \n");
        scanf("%lf", &eps1);

        lseek(pd,0,0);
        memset(tab1, 0, 256);

        //converting to char*
        sprintf(tab1,"%lf",eps1);


        write(pd, tab1, strlen(tab1));

        lseek(pd,0,0);
        memset(tab1, 0, 256);

        close(pd);

        unlink("myfifo1");


        //printing to standard output
        //print(tab1, strlen(tab1));
        //print(tab2, strlen(tab2));

        //creating new process
    
    else if(fork() == 0)


        int pd2, pd3;
        //opening myfifo1 for reading 
        pd2 = open("myfifo1", O_RDONLY);
        //opening myfifo2 for writing 
        pd3 = open("myfifo2", O_WRONLY);



        if(pd2 == - 1)
            printf("\nFailed to open myfifo1 for reading!\n");
        


        if(pd3 == -1 )
            printf("\nfailed to open myfifo2\n");
        

        double x2, eps2;
        char tab2[256];

        lseek(pd2,0,0);
        memset(tab2, 0, 256);

        //reading inputed parameters        
        read(pd2,tab2, sizeof(tab2) );

        //converting to floats
        x2 = atof(tab2);

        lseek(pd2,0,0);
        memset(tab2, 0, 256);

        read(pd2,tab2, sizeof(tab2) );
        //converting to floats

        eps2 = atof(tab2);

        lseek(pd2,0,0);
        memset(tab2, 0, 256);

        //printf("\nReceived initial data x=%f, eps = %f \n" , x2, eps2);

        double singleTerm = 0.0;


        char wyraz[256];



        while(1)

            //calculate single term of Maclaurin series

            singleTerm = Maclaurine(i,x2,eps2);

            if(singleTerm > 0.0)printf("\n%d. Term of Mac Series = %lf", i+ 1,singleTerm);

            sprintf(wyraz,"%lf",singleTerm);


            lseek(pd3,0,0);

            if(write(pd3, wyraz, sizeof(wyraz)) == -1 )
                printf("\nfailed to send singleTerm to myfifo2\n");
            
            if(singleTerm < 0.0)

            //printf("\nI just broke the 1 loop\n");
            memset(wyraz, 0, 256);
            break;
            


            i=i+1;
        
        sleep(1);


        close(pd3);
        close(pd2);

        unlink("myfifo2");
        unlink("myfifo1");

    //creating another process
    else if(fork() == 0)

            int pd4; 
            pd4 = open("myfifo2", O_RDONLY);

            char tab3[256];

            double x3 = 0.0;

            while(1)

            x3=0.0;
            lseek(pd4,0,0);
            memset(tab3, 0, 256);
            //reading data from pipe , myfifo2
            read(pd4,tab3,sizeof(tab3));

            //converting char* to float
            x3 = atof(tab3);

            FINAL_RESULT += x3;


            if(x3<=0.0)

            printf("\nFinal Result :  %lf", FINAL_RESULT ); 
            break;
            //else
            //printf("\n x = %lf, Current result = %lf" ,x3, FINAL_RESULT);
            

            sleep(1);

            printf("\nFinal Result :  %lf", FINAL_RESULT);

            close(pd4);
            unlink("myfifo2");

        

    sleep(7);
    unlink("myfifo1");
    unlink("myfifo2");

    while(1)pause();

    return 0;

现在我可以通过使用麦克劳林级数看到 e^x 函数的数学估计之美。以下是结果：

pawel@IMPERIUMVB:~$ gcc rury.c -lm -o zad1
pawel@IMPERIUMVB:~$ ./zad1
X Value : 
8
EPS Value : 
0.0001

1. Term of Mac Series = 1.000000
2. Term of Mac Series = 8.000000
3. Term of Mac Series = 32.000000
4. Term of Mac Series = 85.333333
5. Term of Mac Series = 170.666667
6. Term of Mac Series = 273.066667
7. Term of Mac Series = 364.088889
8. Term of Mac Series = 416.101587
9. Term of Mac Series = 416.101587
10. Term of Mac Series = 369.868078
11. Term of Mac Series = 295.894462
12. Term of Mac Series = 215.195972
13. Term of Mac Series = 143.463982
14. Term of Mac Series = 88.285527
15. Term of Mac Series = 50.448873
16. Term of Mac Series = 26.906065
17. Term of Mac Series = 13.453033
18. Term of Mac Series = 6.330839
19. Term of Mac Series = 2.813706
20. Term of Mac Series = 1.184718
21. Term of Mac Series = 0.473887
22. Term of Mac Series = 0.180529
23. Term of Mac Series = 0.065647
24. Term of Mac Series = 0.022834
25. Term of Mac Series = 0.007611
26. Term of Mac Series = 0.002436
27. Term of Mac Series = 0.000749

Final Result :  2980.957900 // after adding all of above

e^8 ~ 2980.9579870417

估计是令人满意的:)