Zig*_*ggy 7 c pipe file-descriptor io-redirection
下面的简短程序旨在迭代从命令行传递的argv并执行每个参数.这不是我的功课,而是我正在做的准备工作.
第一个参数从STDIN和STDOUT获取输入,并写入管道.在每次迭代结束时(除了最后一次),交换文件描述符,以便下一个exec读取由最后一个exec写入的管道.以这种方式,我打算,例如,为
./a.out /bin/pwd /usr/bin/wc
只打印工作目录的长度.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:在我最初使用/ bin/wc的所有下面的测试案例中,但是他们知道水厕不是我认为的地方.我正在修改结果.
一个简单的情况下输出(./a.out/bin/pwd)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)运行此程序的输出:
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,未收到字符串.我想象的是,我在上面做了一些错误,无论是交换指针,还是我不理解unix文件描述符.最后,我的任务是解释任意长管道,这是我解决问题的一个想法.我无法判断我是否正在吠叫一棵树.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,但我现在更担心的是我不明白指针!这两个命令彼此独立,但它们并没有真正使用管道.
更新:垃圾字符来自未关闭字符串.现在我关闭它,没有垃圾.
挂起的原因是分叉后主进程中“右”管道的写入端未正确关闭。因此，wc永远不会停止读取（毕竟，主进程仍然可以向管道写入内容！）。只有在写入端的文件描述符的所有副本都被关闭后才会停止读取。
这是一个固定版本：
#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具体问题，请告诉我:)您的原始代码还存在一些其他小问题：
int被用来代替size_t-Wall标志进行编译时出现的所有警告如果你有兴趣，我会这样写：
#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;
}
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