我正在尝试编写一个实现命令的程序：
ls -al | tr 'a-z' 'A-Z' | grep X > testing
基本上我想学习如何在C中使用两次管道。有很多帖子我们只使用一个'|'。我根据只有一个'|'的例子做了一些尝试但遗憾的是它不起作用。有一个代码。
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#define MAX 512
int main(int argc,char** argv){
int pdesk[2];
int pipedesk[2];
if(pipe(pipedesk)==-1 || pipe(pdesk)==-1){
perror("Pipe");
exit(1);
}
switch(fork()){
case -1 :
perror("Creating process");
exit(1);
case 0:
dup2(pdesk[1],STDOUT_FILENO);
close(pdesk[1]);
execlp("ls","ls","-al",NULL);
perror("ls");
exit(1);
default: {
if(fork()==0){
if(fork()==0){
dup2(pdesk[0],STDIN_FILENO);
close(pdesk[0]);
dup2(pipedesk[1],STDOUT_FILENO);
close(pipedesk[1]);
execlp("tr","tr","a-z","A-Z",NULL);
perror("tr");
exit(1);
}
dup2(pipedesk[0],STDIN_FILENO);
close(pipedesk[0]);
int desk=open("testing",O_WRONLY|O_CREAT,0640);
if(desk==-1){
perror("opening file");
exit(1);
}
dup2(desk,STDOUT_FILENO);
if(close(desk)==-1){
perror("closing file");
exit(1);
}
execlp("grep","grep","X",NULL);
perror("grep");
exit(1);
}
// wait(NULL);
}
return 0;
}
}
答案 0 :(得分：2)
当管道的另一端死亡时，您的进程会收到一个SIGPIPE，默认会终止您的进程。
所以你至少应该忽略它！
signal(SIGPIPE, SIG_IGN);
为了清洁，您应该关闭当前过程中不会使用的所有管道末端。
例如：
case 0:
close(pdesk[0]);
close(pipedesk[0]);
close(pipedesk[1]);
答案 1 :(得分：0)
我不认为信号处理是相关的。有关的是确保管道正确关闭。如果有一个打开写入结束的进程，进程就不会在管道的读取端获得EOF。这包括当前的过程;如果一个进程同时打开了一个管道的读取和写入端，它将永远不会在管道的读取端获得EOF，因为理论上有一个进程可以写入它。因此，此过程将永久挂在管道上的read()。
此版本的代码适用于我：
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
int main(void)
{
int pdesk[2];
int pipedesk[2];
int pid;
if (pipe(pipedesk) == -1 || pipe(pdesk) == -1)
{
perror("Pipe");
exit(1);
}
//fprintf(stderr, "PID %d: controller\n", (int)getpid());
switch (pid = fork())
{
case -1:
perror("Creating process");
exit(1);
case 0:
//fprintf(stderr, "PID %d: will be 'ls'\n", (int)getpid());
dup2(pdesk[1], STDOUT_FILENO);
close(pdesk[0]); // JL
close(pdesk[1]);
close(pipedesk[0]); // JL
close(pipedesk[1]); // JL
execlp("ls", "ls", "-al", (char *)NULL);
perror("ls");
exit(1);
default:
//fprintf(stderr, "PID %d: ls process PID = %d\n", (int)getpid(), pid);
if ((pid = fork()) == 0)
{
//fprintf(stderr, "PID %d: about to fork 'tr'\n", (int)getpid());
if ((pid = fork()) == 0)
{
//fprintf(stderr, "PID %d: will be 'tr'\n", (int)getpid());
dup2(pdesk[0], STDIN_FILENO);
close(pdesk[0]);
close(pdesk[1]); // JL
dup2(pipedesk[1], STDOUT_FILENO);
close(pipedesk[0]); // JL
close(pipedesk[1]);
execlp("tr", "tr", "a-z", "A-Z", (char *)NULL);
perror("tr");
exit(1);
}
//fprintf(stderr, "PID %d: about to exec 'grep'\n", (int)getpid());
dup2(pipedesk[0], STDIN_FILENO);
close(pdesk[0]); // JL
close(pdesk[1]); // JL
close(pipedesk[0]);
close(pipedesk[1]); // JL
int desk = open("testing", O_WRONLY | O_CREAT, 0644);
if (desk == -1)
{
perror("opening file");
exit(1);
}
dup2(desk, STDOUT_FILENO);
if (close(desk) == -1)
{
perror("closing file");
exit(1);
}
execlp("grep", "grep", "X", (char *)NULL);
perror("grep");
exit(1);
}
//fprintf(stderr, "PID %d: closing pipes\n", (int)getpid());
close(pdesk[0]); // JL
close(pdesk[1]); // JL
close(pipedesk[0]); // JL
close(pipedesk[1]); // JL
break;
}
int status;
int corpse;
while ((corpse = wait(&status)) != -1)
fprintf(stderr, "PID %d: child %d died 0x%.4X\n", (int)getpid(), corpse, status);
return 0;
}
注意标记为// JL的所有行都是关闭操作。虽然你可以在没有其中一些的情况下离开（例如第一个孩子的那些，可能是＆＃39;可选＆＃39;），但你应该例行地关闭所有的管道。你没有使用。请注意，特别是原始父进程的关闭（后四个） - 这些都很重要。
在关闭多个管道时，未关闭足够的管道描述符是最常见的错误之一。通常，如果使用dup2()（或dup()）将管道描述符复制到标准输入或输出，则应关闭原始管道的两个端。如果你的进程根本没有使用管道描述符，你应该关闭它们。
程序的示例输出：
$ ./pp61
PID 35665: child 35666 died 0x0000
PID 35665: child 35667 died 0x0000
$
testing档案的示例内容：
DRWXR-XR-X 44 JLEFFLER STAFF 1496 NOV 23 17:28 .
DRWXR-XR-X 137 JLEFFLER STAFF 4658 NOV 23 17:28 ..
DRWXR-XR-X 18 JLEFFLER STAFF 612 OCT 23 20:00 .GIT
DR-XR-XR-X 4 JLEFFLER STAFF 136 AUG 14 23:27 SAFE
DRWXR-XR-X 35 JLEFFLER STAFF 1190 NOV 23 09:19 UNTRACKED
-RW-R--R-- 1 JLEFFLER STAFF 81 NOV 20 20:53 ANIMALS.TXT
DRWXR-XR-X 3 JLEFFLER STAFF 102 SEP 12 00:03 DOC
DRWXR-XR-X 8 JLEFFLER STAFF 272 NOV 10 20:58 ETC
DRWXR-XR-X 17 JLEFFLER STAFF 578 JUL 15 19:06 INC
-RW-R--R-- 1 JLEFFLER STAFF 1217 NOV 21 21:26 IX37.SQL
DRWXR-XR-X 5 JLEFFLER STAFF 170 JUL 9 23:47 LIB
-RWXR-XR-X 1 JLEFFLER STAFF 9052 NOV 19 13:01 LL73
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 6 15:40 LL73.DSYM
-RWXR-XR-X 1 JLEFFLER STAFF 8896 NOV 6 10:38 LL83
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 6 10:10 LL83.DSYM
-RW-R--R-- 1 JLEFFLER STAFF 108 NOV 20 20:53 NEWANIMAL.TXT
-RWXR-XR-X 1 JLEFFLER STAFF 9124 NOV 20 20:38 PD43
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 20 20:38 PD43.DSYM
-RWXR-XR-X 1 JLEFFLER STAFF 9148 NOV 23 17:28 PP61
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 23 14:11 PP61.DSYM
-RWXR-XR-X 1 JLEFFLER STAFF 9016 NOV 23 11:07 RS19
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 23 10:03 RS19.DSYM
DRWXR-XR-X 146 JLEFFLER STAFF 4964 OCT 9 17:06 SRC
-RWXR-XR-X 1 JLEFFLER STAFF 8760 NOV 23 13:12 STP83
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 23 13:04 STP83.DSYM
DRWXR-XR-X 6 JLEFFLER STAFF 204 NOV 6 21:52 TMP
-RWXR-XR-X 1 JLEFFLER STAFF 8808 NOV 23 10:38 TR37
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 23 10:38 TR37.DSYM
-RWXR-XR-X 1 JLEFFLER STAFF 26772 NOV 21 20:18 XY73
DRWXR-XR-X 3 JLEFFLER STAFF 102 NOV 21 20:18 XY73.DSYM
-RW-R--R-- 1 JLEFFLER STAFF 467 NOV 21 20:18 XY73.L
答案 2 :(得分：0)
这是一个易于理解的实现。
当每个过程关闭管道的另一端时，管道的一端会关闭。因此，关闭每个对当前过程无用的管道非常重要。
#include <unistd.h>
#include <stdio.h>
#include <memory.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
void closeAll(const int * fd_to_close){
int i = 0;
while(1){
int fd = fd_to_close[i];
if(fd == -1){
break;
}
close(fd);
i++;
}
}
pid_t forkExec(int my_stdin, int my_stdout, int my_stderr, char *const argv[], const int* fd_to_close){
pid_t child = fork();
if(child != 0){
return child;
}
if(my_stdin != STDIN_FILENO){
dup2(my_stdin, STDIN_FILENO);
}
if(my_stdout != STDOUT_FILENO){
dup2(my_stdout, STDOUT_FILENO);
}
if(my_stderr != STDERR_FILENO){
dup2(my_stderr, STDERR_FILENO);
}
closeAll(fd_to_close);
execvp(argv[0], argv);
perror("Executing ");
exit(1);
}
void waitFor(pid_t child){
int status;
pid_t w = waitpid(child, &status, 0);
if(w == -1){
perror("child ");
} else {
printf("Exit status of child %d was %d, killed by signal %d %s\n", (int) child, WEXITSTATUS(status), WTERMSIG(status), WCOREDUMP(status) ? "with coredump" : "");
}
}
void main(int argc, char**argv){
int pipe1[2];
int pipe2[2];
int fd_to_close[10];
signal(SIGPIPE, SIG_IGN);
if(pipe(pipe1)==-1 || pipe(pipe2)==-1){
perror("Pipe");
exit(1);
}
fd_to_close[0] = pipe1[0];
fd_to_close[1] = pipe1[1];
fd_to_close[2] = pipe2[0];
fd_to_close[3] = pipe2[1];
fd_to_close[4] = -1;
char* cmd1[] = {"ls", "-al", NULL};
char* cmd2[] = {"tr", "a-z", "A-Z", NULL};
char* cmd3[] = {"grep", "X", NULL};
pid_t child1 = forkExec(STDIN_FILENO, pipe1[1], STDERR_FILENO, cmd1, fd_to_close);
pid_t child2 = forkExec(pipe1[0], pipe2[1], STDERR_FILENO, cmd2, fd_to_close);
pid_t child3 = forkExec(pipe2[0], STDOUT_FILENO, STDERR_FILENO, cmd3, fd_to_close);
// Very important :
closeAll(fd_to_close);
printf("pid of %s is %d\n", cmd1[0], (int)child1);
printf("pid of %s is %d\n", cmd2[0], (int)child2);
printf("pid of %s is %d\n", cmd3[0], (int)child3);
waitFor(child1);
waitFor(child2);
waitFor(child3);
}