[英]No correct redirection between pipes
I'm writing a program that should work like the following unix command: seq 2 number | awk argument | grep argument
我正在编写一个程序,它应该像以下 unix 命令一样工作: seq 2 number | awk argument | grep argument
seq 2 number | awk argument | grep argument
#define READ 0
#define WRITE 1
void die(char* msg){
perror(msg);
exit(EXIT_FAILURE);
}
int main(int argc, char *argv[]) {
int seq_to_awk[2];
if(pipe(seq_to_awk) == -1)
die("pipe");
pid_t child = fork();
if(child == -1)
die("fork");
if(child == 0){
if(close(seq_to_awk[READ]) == -1)
die("close");
if(dup2(seq_to_awk[WRITE], STDOUT_FILENO) < 0)
die("dup2");
if(close(seq_to_awk[WRITE]) == -1)
die("close");
execlp("seq", "seq", "2", argv[1], NULL);
}
child = fork();
if(child == -1)
die("fork");
int awk_to_grep[2];
if(pipe(awk_to_grep) == -1)
die("pipe");
if(child == 0){
if(close(seq_to_awk[WRITE]) == -1)
die("close");
if(dup2(seq_to_awk[READ], STDIN_FILENO) < 0)
die("dup2");
if(close(seq_to_awk[READ]) == -1)
die("close");
/**/
if(close(awk_to_grep[READ]) < 0)
die("close");
if(dup2(awk_to_grep[WRITE], STDOUT_FILENO) < 0)
die("dup2");
if(close(awk_to_grep[WRITE]) == -1)
die("close");
/**/
execlp("awk", "awk", argv[2], NULL);
die("execlp");
}
if(close(seq_to_awk[READ]) == -1)
die("close");
if(close(seq_to_awk[WRITE]) == -1)
die("close");
/**/
child = fork();
if(child == -1)
die("fork");
if(child == 0){
if(close(awk_to_grep[WRITE]) == -1)
die("close");
if(dup2(awk_to_grep[READ], STDIN_FILENO) < 0)
die("dup2");
if(close(awk_to_grep[READ]) == -1)
die("close");
execlp("grep", "grep", argv[3], NULL);
die("execlp");
}
/**/
if(close(awk_to_grep[READ]) == -1)
die("close");
if(close(awk_to_grep[WRITE]) == -1)
die("close");
wait(NULL);
wait(NULL);
wait(NULL);
exit(EXIT_SUCCESS);
}
When I run the following code with the command: ./program 26 {print} 5
I get nothing as output, when the correct output should be:当我使用以下命令运行以下代码时: ./program 26 {print} 5
我什么也得不到 output,而正确的 output 应该是:
5
15
25
I don't understand what exactly I'm doing wrong here.我不明白我到底做错了什么。 When I leave out one of the programs(either awk or grep) the program runs as expected.当我遗漏其中一个程序(awk 或 grep)时,程序会按预期运行。
The problem is when setting up the awk_to_grep
pipe. You fork
before creating the pipe, so you in fact create two separate pipes.问题是在设置awk_to_grep
pipe 时。您在创建 pipe之前fork
,因此您实际上创建了两个单独的管道。 The pipe in your child process is not available to the parent and other children created after that.您的子进程中的 pipe 对父进程和之后创建的其他子进程不可用。
Swap the order to this:将订单交换为:
int awk_to_grep[2];
if(pipe(awk_to_grep) == -1)
die("pipe");
child = fork();
if(child == -1)
die("fork");
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