有人可以解释一下程序中下面两行的含义来实现管道吗？

Question

需要解释作者在这里试图做什么。为什么我不能使用一维数组而不是二维数组？

char writemessages[2][20]={"Hi", "Hello"};
char readmessage[20];

程序片段：

   int main() {
   int pipefds[2];
   int returnstatus;
   char writemessages[2][20]={"Hi", "Hello"}; //why can't i use singlr char array
   char readmessage[20];
   returnstatus = pipe(pipefds);

   if (returnstatus == -1) {
      printf("Unable to create pipe\n");
      return 1;
   }

   printf("Writing to pipe - Message 1 is %s\n", writemessages[0]);
   write(pipefds[1], writemessages[0], sizeof(writemessages[0]));
   read(pipefds[0], readmessage, sizeof(readmessage));
   printf("Reading from pipe – Message 1 is %s\n", readmessage);
   printf("Writing to pipe - Message 2 is %s\n", writemessages[0]);
   write(pipefds[1], writemessages[1], sizeof(writemessages[0]));
   read(pipefds[0], readmessage, sizeof(readmessage));
   printf("Reading from pipe – Message 2 is %s\n", readmessage);
   return 0;
}

Answer 1

explain the meaning of two lines below

行

char writemessages[2][20]={"Hi", "Hello"};

定义一个包含 2 行 20 列的字符数组并初始化它，因为空字符结束字符串，数组可以包含 2 个字符串，最多 19 个字符，不包括空字符。

初始化使第一个字符串成为 'H' 'i' 然后是空字符，第二个 'H' 'e' 'l' 'l' 'o' 然后是空字符

这是相当于 :

的简写方式

char writemessages[2][20]={ { 'H', 'i', 0 }, { 'H', 'e', 'l', 'l', 'o', 0 }};

或

char writemessages[2][20]= {
   { 'H', 'i', 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
   { 'H', 'e', 'l', 'l', 'o', 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
};

行

char readmessage[20];

定义一个大小为 20 的数组，因此空字符最多允许 19 个字符。

Why cannot I use Single dimension array instead od 2d array?

这只是将它们分组为二维数组的一种选择，您也可以使用一维数组:

 char writemessage1[20] = "Hi";
 char writemessage1[20] = "Hello";

 printf("Writing to pipe - Message 1 is %s\n", writemessage1);
 write(pipefds[1], writemessage1, sizeof(writemessage1));
 ...
 printf("Writing to pipe - Message 2 is %s\n", writemessage2);
 write(pipefds[1], writemessage2, sizeof(writemessage2));

注意代码总是发送 20 个字符，将定义更改为

 char writemessage1[] = "Hi";
 char writemessage1[] = "Hello";

由于数组的大小，

write(pipefds[1], writemessage1, sizeof(writemessage1)); 发送 3 个字符而 write(pipefds[1], writemessage2, sizeof(writemessage2)); 发送 6 个字符（它们会自动调整大小并初始化为包含结束空字符）

可能

printf("Writing to pipe - Message 2 is %s\n", writemessages[0]);
write(pipefds[1], writemessages[1], sizeof(writemessages[0]));

必须

printf("Writing to pipe - Message 2 is %s\n", writemessages[1]);
write(pipefds[1], writemessages[1], sizeof(writemessages[1]));

发送 "Hello" 而不是再次发送 "Hi"

有人可以解释一下程序中下面两行的含义来实现管道吗？

Can someone please explain the meaning of two lines below in the program to implement pipe?

c

posix

ipc

pipe