下面的简短程序旨在遍历从命令行传递的argv并执行每个参数。这不是我的作业，而是我正在做的准备作业。
第一个参数从STDIN和STDOUT获取输入，并将其写入管道。在每次迭代结束时(最后一次除外)，文件描述符都将交换，以便最后一个exec写入的管道将在下一个执行时读取。这样，我打算例如
./a.out /bin/pwd /usr/bin/wc
仅打印出工作目录的长度。代码如下
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <string.h>
main(int argc, char * argv[]) {
int i;
int left[2], right[2], nbytes; /* arrays for file descriptors */
/* pointers for swapping */
int (* temp);
int (* leftPipe) = left;
int (* rightPipe) = right;
pid_t childpid;
char readbuffer[80];
/* for the first iteration, leftPipe is STDIN */
leftPipe[0] = STDIN_FILENO;
leftPipe[1] = STDOUT_FILENO;
for (i = 1; i < argc; i++) {
/* reopen the right pipe (is this necessary?) */
pipe(rightPipe);
fprintf(stderr, "%d: %s\n", i, argv[i]);
fprintf(stderr, "%d %d %d %d\n", leftPipe[0], leftPipe[1], rightPipe[0], rightPipe[1]);
if ((childpid = fork()) == -1) {
perror("fork");
exit(1);
}
if (childpid == 0) {
/* read input from the left */
close(leftPipe[1]); /* close output */
dup2(leftPipe[0], STDIN_FILENO);
close(leftPipe[0]); /* is this necessary? A tutorial seemed to be doing this */
/* write output to the right */
close(rightPipe[0]); /* close input */
dup2(rightPipe[1], STDOUT_FILENO);
close(rightPipe[1]);
execl(argv[i], argv[i], NULL);
exit(0);
}
wait();
/* on all but the last iteration, swap the pipes */
if (i + 1 < argc) {
/* swap the pipes */
fprintf(stderr, "%d %d %d %d\n", leftPipe[0], leftPipe[1], rightPipe[0], rightPipe[1]);
temp = leftPipe;
leftPipe = rightPipe;
rightPipe = temp;
fprintf(stderr, "%d %d %d %d\n", leftPipe[0], leftPipe[1], rightPipe[0], rightPipe[1]);
}
}
/* read what was last written to the right pipe */
close(rightPipe[1]); /* the receiving process closes 1 */
nbytes = read(rightPipe[0], readbuffer, sizeof(readbuffer));
readbuffer[nbytes] = 0;
fprintf(stderr, "Received string: %s\n", readbuffer);
return 0;
}
更新:在以下所有测试用例中，我最初使用/bin/wc，但该wc揭示了抽水马桶根本不是我想的那样。我正在修改结果。
普通情况下(./a.out/bin/pwd)的输出是预期的:
1: /bin/pwd
Received string: /home/zeigfreid/Works/programmatical/Langara/spring_2012/OS/labs/lab02/play
使用第一个示例(./a.out/bin/pwd/usr/bin/wc)运行该程序的输出:
1: /bin/pwd
0 1 3 4
3 4 0 1
2: /bin/wc
此时，终端挂起(可能正在等待输入)。
如您所见，该字符串没有被接收。我想象的是，在交换指针时，或者在我不理解UNIX文件描述符的情况下，我在上面做错了什么。最后，我的任务是解释任意长的管道，这是我解决该问题的想法之一。我在判断我是否在树皮吠叫的正确轨道上遇到了麻烦。我了解UNIX文件描述符吗？
更新:
使用/bin/ls作为第二个参数运行它，我得到以下结果(数字是各个点的文件描述符):
1: /bin/pwd
0 1 3 4
0 1 3 4
3 4 0 1
2: /bin/ls
3 4 5 6
Received string: a.out
log
pipe2.c
play.c
@
最后还有一些垃圾，但是我现在更担心我不懂指针!尽管这两个命令彼此独立，但它们实际上并没有利用管道。
UPDATE :垃圾字符来自未关闭字符串。现在我将其关闭，没有任何垃圾。
最佳答案
挂起是由于以下事实造成的:在 fork 后的主要过程中，“右”管的书写端未正确关闭。因此，wc将永远不会停止读取(毕竟，主进程仍可以将内容写入管道!)。只有在写入端的文件描述符的所有副本都已关闭之后，它才会停止读取。
这是固定版本:
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char * argv[])
{
int i;
int left[2], right[2], nbytes; /* arrays for file descriptors */
/* pointers for swapping */
int (* temp);
int (* leftPipe) = left;
int (* rightPipe) = right;
pid_t childpid;
char readbuffer[80];
leftPipe[0] = STDIN_FILENO;
// no need to assign leftPipe[1] here, it will not be used
for (i = 1; i < argc; i++) {
pipe(rightPipe); // create new pipe
fprintf(stderr, "%d: %s\n", i, argv[i]);
fprintf(stderr, "%d %d %d %d\n", leftPipe[0], leftPipe[1], rightPipe[0], rightPipe[1]);
if ((childpid = fork()) == -1) {
perror("fork");
exit(1);
}
if (childpid == 0) {
// use the reading end of the left pipe as STDIN
dup2(leftPipe[0], STDIN_FILENO);
// use the writing end of the right pipe as STDOUT
dup2(rightPipe[1], STDOUT_FILENO);
// close reading end of the right pipe
close(rightPipe[0]);
execl(argv[i], argv[i], NULL);
exit(0);
}
// IMPORTANT!! close writing end of the right pipe, otherwise
// the program will hang (this is the main bug in your original
// implementation)
close(rightPipe[1]);
// wait properly!
waitpid(childpid, NULL, 0);
/* on all but the last iteration, swap */
if (i + 1 < argc) {
fprintf(stderr, "%d %d %d %d\n", leftPipe[0], leftPipe[1], rightPipe[0], rightPipe[1]);
temp = leftPipe;
leftPipe = rightPipe;
rightPipe = temp;
fprintf(stderr, "%d %d %d %d\n", leftPipe[0], leftPipe[1], rightPipe[0], rightPipe[1]);
}
}
nbytes = read(rightPipe[0], readbuffer, sizeof(readbuffer));
readbuffer[nbytes] = 0;
fprintf(stderr, "Received string: %s\n", readbuffer);
return 0;
}
输出:
>> ./a.out /bin/ls /bin/cat /usr/bin/wc
1: /bin/ls
0 32767 3 4
0 32767 3 4
3 4 0 32767
2: /bin/cat
3 4 4 5
3 4 4 5
4 5 3 4
3: /usr/bin/wc
4 5 5 6
Received string: 266 294 4280
如果您对此解决方案有特定疑问，请告诉我:)原始代码还存在其他一些小问题:
int代替size_t -Wall标志如果您有兴趣，这就是我的写法:
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char **argv) {
size_t i, nbytes;
int left[2], right[2], tmp[2];
pid_t childpid;
char readbuffer[80];
left[0] = STDIN_FILENO;
for (i = 1; i < argc; ++i) {
pipe(right);
switch ((childpid = fork())) {
case -1:
perror("fork");
exit(1);
case 0:
dup2(left[0], STDIN_FILENO);
dup2(right[1], STDOUT_FILENO);
close(right[0]);
execl(argv[i], argv[i], NULL);
default:
close(right[1]);
waitpid(childpid, NULL, 0);
}
if (i == argc - 1) break;
memcpy(tmp, left, sizeof tmp);
memcpy(left, right, sizeof left);
memcpy(right, tmp, sizeof right);
}
nbytes = read(right[0], readbuffer, sizeof readbuffer);
readbuffer[nbytes] = 0;
fprintf(stderr, "Received string: %s\n", readbuffer);
return 0;
}
关于c - 我了解Unix文件描述符如何在C中工作吗？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/9254198/