这个问题源自我尝试实施以下指令：
Linux Pipes as Input and Output
How to send a simple string between two programs using pipes?
http://tldp.org/LDP/lpg/node11.html
我的问题与Linux Pipes as Input and Output中的问题类似，但更具体。
基本上，我正在尝试替换：
/directory/program < input.txt > output.txt
在C ++中使用管道以避免使用硬盘驱动器。这是我的代码：
//LET THE PLUMBING BEGIN
int fd_p2c[2], fd_pFc[2], bytes_read;
// "p2c" = pipe_to_child, "pFc" = pipe_from_child (see above link)
pid_t childpid;
char readbuffer[80];
string program_name;// <---- includes program name + full path
string gulp_command;// <---- includes my line-by-line stdin for program execution
string receive_output = "";
pipe(fd_p2c);//create pipe-to-child
pipe(fd_pFc);//create pipe-from-child
childpid = fork();//create fork
if (childpid < 0)
{
cout << "Fork failed" << endl;
exit(-1);
}
else if (childpid == 0)
{
dup2(0,fd_p2c[0]);//close stdout & make read end of p2c into stdout
close(fd_p2c[0]);//close read end of p2c
close(fd_p2c[1]);//close write end of p2c
dup2(1,fd_pFc[1]);//close stdin & make read end of pFc into stdin
close(fd_pFc[1]);//close write end of pFc
close(fd_pFc[0]);//close read end of pFc
//Execute the required program
execl(program_name.c_str(),program_name.c_str(),(char *) 0);
exit(0);
}
else
{
close(fd_p2c[0]);//close read end of p2c
close(fd_pFc[1]);//close write end of pFc
//"Loop" - send all data to child on write end of p2c
write(fd_p2c[1], gulp_command.c_str(), (strlen(gulp_command.c_str())));
close(fd_p2c[1]);//close write end of p2c
//Loop - receive all data to child on read end of pFc
while (1)
{
bytes_read = read(fd_pFc[0], readbuffer, sizeof(readbuffer));
if (bytes_read <= 0)//if nothing read from buffer...
break;//...break loop
receive_output += readbuffer;//append data to string
}
close(fd_pFc[0]);//close read end of pFc
}
我绝对确定上述字符串已正确初始化。但是，发生了两件对我没有意义的事情：
（1）我正在执行的程序报告“输入文件为空”。因为我没有用“＆lt;”来调用程序它不应该期望输入文件。相反，应该期待键盘输入。此外，它应该阅读“gulp_command。”中包含的文字。
（2）程序的报告（通过标准输出提供）出现在终端中。这很奇怪，因为这个管道的目的是将stdout传输到我的字符串“receive_output”。但由于它出现在屏幕上，这表明信息没有通过管道正确传递给变量。如果我在if语句的末尾实现以下内容，
cout << receive_output << endl;
我什么都没得到，好像字符串是空的。我感谢您给我的任何帮助！
编辑：澄清
我的程序目前使用文本文件与另一个程序通信。我的程序写入一个文本文件（例如input.txt），由外部程序读取。该程序然后生成output.txt，由我的程序读取。所以它是这样的：
my code -> input.txt -> program -> output.txt -> my code
因此，我的代码目前使用
system("program < input.txt > output.txt");
我想用管道替换这个过程。我希望将输入作为标准输入传递给程序，并让我的代码将该程序的标准输出读入字符串。
答案 0 :(得分：7)
您的主要问题是您有dup2()的参数被颠倒了。你需要使用：
dup2(fd_p2c[0], 0); // Duplicate read end of pipe to standard input
dup2(fd_pFc[1], 1); // Duplicate write end of pipe to standard output
我误解了你所写的内容，直到我对设置代码进行错误检查并从dup2()调用得到了意外的值，这告诉我问题是什么。当出现问题时，请插入您之前吝啬的错误检查。
您也没有确保从孩子那里读取数据的空终止;这段代码确实如此。
工作代码（带诊断），使用cat作为最简单的“其他命令”：
#include <unistd.h>
#include <string>
#include <iostream>
using namespace std;
int main()
{
int fd_p2c[2], fd_c2p[2], bytes_read;
pid_t childpid;
char readbuffer[80];
string program_name = "/bin/cat";
string gulp_command = "this is the command data sent to the child cat (kitten?)";
string receive_output = "";
if (pipe(fd_p2c) != 0 || pipe(fd_c2p) != 0)
{
cerr << "Failed to pipe\n";
exit(1);
}
childpid = fork();
if (childpid < 0)
{
cout << "Fork failed" << endl;
exit(-1);
}
else if (childpid == 0)
{
if (dup2(fd_p2c[0], 0) != 0 ||
close(fd_p2c[0]) != 0 ||
close(fd_p2c[1]) != 0)
{
cerr << "Child: failed to set up standard input\n";
exit(1);
}
if (dup2(fd_c2p[1], 1) != 1 ||
close(fd_c2p[1]) != 0 ||
close(fd_c2p[0]) != 0)
{
cerr << "Child: failed to set up standard output\n";
exit(1);
}
execl(program_name.c_str(), program_name.c_str(), (char *) 0);
cerr << "Failed to execute " << program_name << endl;
exit(1);
}
else
{
close(fd_p2c[0]);
close(fd_c2p[1]);
cout << "Writing to child: <<" << gulp_command << ">>" << endl;
int nbytes = gulp_command.length();
if (write(fd_p2c[1], gulp_command.c_str(), nbytes) != nbytes)
{
cerr << "Parent: short write to child\n";
exit(1);
}
close(fd_p2c[1]);
while (1)
{
bytes_read = read(fd_c2p[0], readbuffer, sizeof(readbuffer)-1);
if (bytes_read <= 0)
break;
readbuffer[bytes_read] = '\0';
receive_output += readbuffer;
}
close(fd_c2p[0]);
cout << "From child: <<" << receive_output << ">>" << endl;
}
return 0;
}
示例输出：
Writing to child: <<this is the command data sent to the child cat (kitten?)>>
From child: <<this is the command data sent to the child cat (kitten?)>>
请注意，您需要小心确保不会因代码而陷入僵局。如果你有一个严格同步的协议（所以父母写一条消息并以锁定步骤读取一个响应），你应该没事，但如果父母试图写一个太大而不适合孩子的管道的消息当孩子正在尝试写一条太大而无法容纳管道回到父母的信息时，每个人都会被阻止写作，同时等待对方阅读。
答案 1 :(得分：1)
听起来你正在寻找coprocesses。您可以使用C / C ++对它们进行编程，但由于它们已经在（bash）shell中可用，因此更容易使用shell，对吧？
首先使用内置coproc内置程序启动外部程序：
coproc external_program
coproc在后台启动程序并存储文件描述符以在数组shell变量中与它通信。现在你只需要启动程序将它连接到那些文件描述符：
your_program <&${COPROC[0]} >&${COPROC[1]}
答案 2 :(得分：1)
#include <stdio.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <string.h>
#include <iostream>
using namespace std;
int main() {
int i, status, len;
char str[10];
mknod("pipe", S_IFIFO | S_IRUSR | S_IWUSR, 0); //create named pipe
pid_t pid = fork(); // create new process
/* Process A */
if (pid == 0) {
int myPipe = open("pipe", O_WRONLY); // returns a file descriptor for the pipe
cout << "\nThis is process A having PID= " << getpid(); //Get pid of process A
cout << "\nEnter the string: ";
cin >> str;
len = strlen(str);
write(myPipe, str, len); //Process A write to the named pipe
cout << "Process A sent " << str;
close(myPipe); //closes the file descriptor fields.
}
/* Process B */
else {
int myPipe = open("pipe", O_RDONLY); //Open the pipe and returns file descriptor
char buffer[21];
int pid_child;
pid_child = wait(&status); //wait until any one child process terminates
int length = read(myPipe, buffer, 20); //reads up to size bytes from pipe with descriptor fields, store results
// in buffer;
cout<< "\n\nThis is process B having PID= " << getpid();//Get pid of process B
buffer[length] = '\0';
cout << "\nProcess B received " << buffer;
i = 0;
//Reverse the string
for (length = length - 1; length >= 0; length--)
str[i++] = buffer[length];
str[i] = '\0';
cout << "\nRevers of string is " << str;
close(myPipe);
}
unlink("pipe");
return 0;
}