有叉））用C麻烦（，管（），dup2（和exec（） [英] Having trouble with fork(), pipe(), dup2() and exec() in C
问题描述
下面是我的code：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <wait.h>
#include <readline/readline.h>
#define NUMPIPES 2
int main(int argc, char *argv[]) {
char *bBuffer, *sPtr, *aPtr = NULL, *pipeComms[NUMPIPES], *cmdArgs[10];
int fdPipe[2], pCount, aCount, i, status, lPids[NUMPIPES];
pid_t pid;
pipe(fdPipe);
while(1) {
bBuffer = readline("Shell> ");
if(!strcasecmp(bBuffer, "exit")) {
return 0;
}
sPtr = bBuffer;
pCount = -1;
do {
aPtr = strsep(&sPtr, "|");
pipeComms[++pCount] = aPtr;
} while(aPtr);
for(i = 0; i < pCount; i++) {
aCount = -1;
do {
aPtr = strsep(&pipeComms[i], " ");
cmdArgs[++aCount] = aPtr;
} while(aPtr);
cmdArgs[aCount] = 0;
if(strlen(cmdArgs[0]) > 0) {
pid = fork();
if(pid == 0) {
if(i == 0) {
close(fdPipe[0]);
dup2(fdPipe[1], STDOUT_FILENO);
close(fdPipe[1]);
} else if(i == 1) {
close(fdPipe[1]);
dup2(fdPipe[0], STDIN_FILENO);
close(fdPipe[0]);
}
execvp(cmdArgs[0], cmdArgs);
exit(1);
} else {
lPids[i] = pid;
/*waitpid(pid, &status, 0);
if(WIFEXITED(status)) {
printf("[%d] TERMINATED (Status: %d)\n",
pid, WEXITSTATUS(status));
}*/
}
}
}
for(i = 0; i < pCount; i++) {
waitpid(lPids[i], &status, 0);
if(WIFEXITED(status)) {
printf("[%d] TERMINATED (Status: %d)\n",
lPids[i], WEXITSTATUS(status));
}
}
}
return 0;
}
（将code的更新，以反映变化，他提出了以下两个答案，但还是不能如它应该......工作）
(The code was updated to reflect he changes proposed by two answers below, it still doesn't work as it should...)
下面是测试情况下失败：
Here's the test case where this fails:
nazgulled ~/Projects/SO/G08 $ ls -l
total 8
-rwxr-xr-x 1 nazgulled nazgulled 7181 2009-05-27 17:44 a.out
-rwxr-xr-x 1 nazgulled nazgulled 754 2009-05-27 01:42 data.h
-rwxr-xr-x 1 nazgulled nazgulled 1305 2009-05-27 17:50 main.c
-rwxr-xr-x 1 nazgulled nazgulled 320 2009-05-27 01:42 makefile
-rwxr-xr-x 1 nazgulled nazgulled 14408 2009-05-27 17:21 prog
-rwxr-xr-x 1 nazgulled nazgulled 9276 2009-05-27 17:21 prog.c
-rwxr-xr-x 1 nazgulled nazgulled 10496 2009-05-27 17:21 prog.o
-rwxr-xr-x 1 nazgulled nazgulled 16 2009-05-27 17:19 test
nazgulled ~/Projects/SO/G08 $ ./a.out
Shell> ls -l|grep prog
[4804] TERMINATED (Status: 0)
-rwxr-xr-x 1 nazgulled nazgulled 14408 2009-05-27 17:21 prog
-rwxr-xr-x 1 nazgulled nazgulled 9276 2009-05-27 17:21 prog.c
-rwxr-xr-x 1 nazgulled nazgulled 10496 2009-05-27 17:21 prog.o
问题是，我应该后回到我的壳，我应该能看到壳牌>等待更多的输入。您也可以注意到，你没有看到类似的消息为[4804]终止（状态：0）。（但使用不同的PID），这意味着第二个进程并没有终止
The problem is that I should return to my shell after that, I should see "Shell> " waiting for more input. You can also notice that you don't see a message similar to "[4804] TERMINATED (Status: 0)" (but with a different pid), which means the second process didn't terminate.
我认为这事做使用grep，因为这个作品：
I think it has something to do with grep, because this works:
nazgulled ~/Projects/SO/G08 $ ./a.out
Shell> echo q|sudo fdisk /dev/sda
[4838] TERMINATED (Status: 0)
The number of cylinders for this disk is set to 1305.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
(e.g., DOS FDISK, OS/2 FDISK)
Command (m for help):
[4839] TERMINATED (Status: 0)
您可以很容易地看到两个终止的消息...
You can easily see two "terminate" messages...
那么，什么是错我的code？
So, what's wrong with my code?
推荐答案
即使您的管道出口（和thust关闭的第一个命令后，标准输出=〜fdPipe [1] ），母公司仍然有 fdPipe [1] 打开。
Even after the first command of your pipeline exits (and thust closes stdout=~fdPipe[1]), the parent still has fdPipe[1] open.
这样，管道的第二个命令有一个标准输入=〜fdPipe [0] ，从来没有得到一个EOF，因为管道的另一个端点仍处于打开状态。
Thus, the second command of the pipeline has a stdin=~fdPipe[0] that never gets an EOF, because the other endpoint of the pipe is still open.
您需要创建一个新的管（fdPipe）每个 | ，并确保关闭两个端点在父;即。
You need to create a new pipe(fdPipe) for each |, and make sure to close both endpoints in the parent; i.e.
for cmd in cmds
if there is a next cmd
pipe(new_fds)
fork
if child
if there is a previous cmd
dup2(old_fds[0], 0)
close(old_fds[0])
close(old_fds[1])
if there is a next cmd
close(new_fds[0])
dup2(new_fds[1], 1)
close(new_fds[1])
exec cmd || die
else
if there is a previous cmd
close(old_fds[0])
close(old_fds[1])
if there is a next cmd
old_fds = new_fds
if there are multiple cmds
close(old_fds[0])
close(old_fds[1])
此外，为了更安全，你应该处理的情况下， fdPipe 和 {STDIN_FILENO，STDOUT_FILENO} 前重叠在进行任何的关闭和 dup2 操作。如果有人已经设法开始与标准输入或标准输出关闭你的shell这可能会发生，并且会导致与code在这里了极大的混乱。
Also, to be safer, you should handle the case of fdPipe and {STDIN_FILENO,STDOUT_FILENO} overlapping before performing any of the close and dup2 operations. This may happen if somebody has managed to start your shell with stdin or stdout closed, and will result in great confusion with the code here.
fdPipe1 fdPipe3
v v
cmd1 | cmd2 | cmd3 | cmd4 | cmd5
^ ^
fdPipe2 fdPipe4
在除了确保您关闭了管道的终点在父，我试图让一点， fdPipe1 ， fdPipe2 等的无法的是相同管道（）。
In addition to making sure you close the pipe's endpoints in the parent, I was trying to make the point that fdPipe1, fdPipe2, etc. cannot be the same pipe().
/* suppose stdin and stdout have been closed...
* for example, if your program was started with "./a.out <&- >&-" */
close(0), close(1);
/* then the result you get back from pipe() is {0, 1} or {1, 0}, since
* fd numbers are always allocated from the lowest available */
pipe(fdPipe);
close(0);
dup2(fdPipe[0], 0);
我知道你不使用关闭（0）在present code，但最后一段是警告你要提防这个案例。
I know you don't use close(0) in your present code, but the last paragraph is warning you to watch out for this case.
下面的最小的改变你code使得它在你提到的具体情况不及格的工作：
The following minimal change to your code makes it work in the specific failing case you mentioned:
@@ -12,6 +12,4 @@
pid_t pid;
- pipe(fdPipe);
-
while(1) {
bBuffer = readline("Shell> ");
@@ -29,4 +27,6 @@
} while(aPtr);
+ pipe(fdPipe);
+
for(i = 0; i < pCount; i++) {
aCount = -1;
@@ -72,4 +72,7 @@
}
+ close(fdPipe[0]);
+ close(fdPipe[1]);
+
for(i = 0; i < pCount; i++) {
waitpid(lPids[i], &status, 0);
这不会在管道多条命令工作;对于这一点，你需要这样的事情：（未经测试，因为你必须修复其他的东西也一样）
This won't work for more than one command in the pipeline; for that, you'd need something like this: (untested, as you have to fix other things as well)
@@ -9,9 +9,7 @@
int main(int argc, char *argv[]) {
char *bBuffer, *sPtr, *aPtr = NULL, *pipeComms[NUMPIPES], *cmdArgs[10];
- int fdPipe[2], pCount, aCount, i, status, lPids[NUMPIPES];
+ int fdPipe[2], fdPipe2[2], pCount, aCount, i, status, lPids[NUMPIPES];
pid_t pid;
- pipe(fdPipe);
-
while(1) {
bBuffer = readline("Shell> ");
@@ -32,4 +30,7 @@
aCount = -1;
+ if (i + 1 < pCount)
+ pipe(fdPipe2);
+
do {
aPtr = strsep(&pipeComms[i], " ");
@@ -43,11 +44,12 @@
if(pid == 0) {
- if(i == 0) {
- close(fdPipe[0]);
+ if(i + 1 < pCount) {
+ close(fdPipe2[0]);
- dup2(fdPipe[1], STDOUT_FILENO);
+ dup2(fdPipe2[1], STDOUT_FILENO);
- close(fdPipe[1]);
- } else if(i == 1) {
+ close(fdPipe2[1]);
+ }
+ if(i != 0) {
close(fdPipe[1]);
@@ -70,4 +72,17 @@
}
}
+
+ if (i != 0) {
+ close(fdPipe[0]);
+ close(fdPipe[1]);
+ }
+
+ fdPipe[0] = fdPipe2[0];
+ fdPipe[1] = fdPipe2[1];
+ }
+
+ if (pCount) {
+ close(fdPipe[0]);
+ close(fdPipe[1]);
}
这篇关于有叉））用C麻烦（，管（），dup2（和exec（）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
