我在5年前完成了正确的工作计划。那时我停止使用它，我升级了操作系统，时间过去了，灰尘覆盖了代码，最后我把它挖出来发现它不再与子进程通信了。
这是代码（简化，但它显示了问题）：
#include <stdio.h>
#include <signal.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/types.h>
#include <string.h>
#include <sys/time.h>
#include <sys/stat.h>
#include <iostream>
#include <string>
#include <cassert>
int main()
{
std::cout << "Creating the pipe for reading from the process" << std::endl;
const std::string pipe_name = "/tmp/proc_comm";
{
int res = mkfifo(pipe_name.c_str(),0777);
assert(res==0);
}
std::cout << "Launching subprocess" << std::endl;
FILE *cmd_handle = popen(("espeak -x -q -z 1> "+pipe_name+" 2> /dev/null").c_str(), "w");
assert(cmd_handle!=0);
std::cout << "Opening the pipe" << std::endl;
int pipe_id = open(pipe_name.c_str(),O_RDONLY);
assert(pipe_id!=-1);
const std::string message = "hello\n";
std::cout << "Sending the message" << std::endl;
if (!fwrite(message.c_str(),sizeof(char),message.length(),cmd_handle))
assert(0);
if (ferror(cmd_handle))
assert(0);
if (fflush(cmd_handle)!=0)
assert(0);
fd_set output_set;
FD_ZERO(&output_set);
FD_SET(pipe_id,&output_set);
static timeval timeout;
timeout.tv_sec = 10;
timeout.tv_usec = 0;
std::cout << "Selecting the pipe for reading" << std::endl;
const int inputs = select(pipe_id+1, // max of pipe ids + 1
&output_set,0,0,&timeout);
if (inputs==-1) // error
assert(0);
else if (inputs==0) // nothing to read
assert(0); // HERE (*)
else
{
// we can only read from our pipe
assert(inputs==1);
assert(FD_ISSET(pipe_id,&output_set));
const int bufsize = 20000;
char char_buf[bufsize];
memset(char_buf,0,sizeof(char_buf));
std::cout << "Reading from the pipe" << std::endl;
const int count = read(pipe_id,char_buf,bufsize-1);
if (count==-1)
assert(0);
std::cout << "Read " << count << std::endl;
}
return 0;
}
它编译，运行，创建并打开从进程读取的管道，启动子进程，发送消息但是没有任何东西可以从进程中读取（行HERE (*)）。 / p>
那么你现在如何阅读这个过程？如果可能的话，我想保留一般工作流程，即使用管道进行阅读，以及用于处理写入的流程句柄。
答案 0 :(得分：2)
FILE *cmd_handle = popen (("echo begin; espeak -x -q -z 1>" + pipe_name + " 2>/dev/null; echo end;").c_str (), "w");。cat < /tmp/proc_commn &; ./a.out; 输出为：
创建用于从流程中读取的管道
启动子流程
打开管子
开始
发送消息
选择用于读取的管道
a.out：main.cpp：64：int main（）：断言'0'失败。
h @ l'oU ####＆lt; ---断言后仅在FIFO中的数据。
结束
添加pclose()电话后，问题就会消失：
if (!fwrite(message.c_str(),sizeof(char),message.length(),cmd_handle))
assert(0);
if (ferror(cmd_handle))
assert(0);
if (fflush(cmd_handle)!=0)
assert(0);
/* pclose added. */
if (pclose (cmd_hanlde) < 0)
{
/* handle error. */
}
fd_set output_set;
我不认为你的代码在任何意义上都已经过时了。我不是专家，所以我对这个问题没有好的答案。但我希望，这篇文章可以帮助您解决问题。
使用strace（strace -f ./a.out）和任何select()超时运行代码：
Porcess 31985是父进程（主进程）
Porcess 31987是子进程（espeak）。
# Child process starts trying to read from stdin.
[pid 31987] read(0, <unfinished ...>
[pid 31985] <... sync resumed> ) = 0
[pid 31985] write(1, "Sending the message\n", ...
[pid 31985] fstat64(4, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
[pid 31985] mmap2(NULL, 4096, PROT_READ|PROT_WRITE, ...
# Main process writes message to the pipe.
[pid 31985] write(4, "hello\n", 6) = 6
# Child process reads message from the pipe.
[pid 31987] <... read resumed> "hello\n", 4096) = 6
[pid 31985] write(1, "Selecting the pipe for reading:3"..., 3 ...
) = 33
[pid 31987] fstat64(1, <unfinished ...>
# Main process calls select().
[pid 31985] select(4, [3], NULL, NULL, {10, 0} <unfinished ...>
[pid 31987] <... fstat64 resumed> {st_mode=S_IFIFO|0755, st_size=0, ...}) = 0
[pid 31987] mmap2(NULL, 4096, PROT_READ ...
# Child process calls read() again (second time).
# Seems like it waits another portion of input.
# Note: The previous input don't processed yet.
[pid 31987] read(0, <unfinished ...>
# Despite big value of `select()` timeout, there is no data processing here.
# Select returns by timeout.
[pid 31985] <... select resumed> ) = 0 (Timeout)
[pid 31985] write(2, "a.out: main.cpp:69: i ...
) = 54
[pid 31985] rt_sigprocmask(SIG_UNBLOCK, [ABRT], NULL, 8) = 0
[pid 31985] gettid() = 31985
[pid 31985] tgkill(31985, 31985, SIGABRT) = 0
[pid 31985] --- SIGABRT (Aborted) @ 0 (0) ---
让我们看一下espeak的{{3}}。
这是从stdin（src / espeak.cpp）读取输入的代码（假设在一组内部变量上找到命令行参数的映射的正确性）：
728 // line by line input on stdin
729 while(fgets(p_text,max,stdin) != NULL) // <--- * here first and second
730 { // <--- * read() call.
731 p_text[max-1] = 0;
732 espeak_Synth(p_text,max,0,POS_CHARACTER,0,synth_flags,NULL,NULL);
733
734 }
关于espeak_Synth()函数的评论（src / speak_lib.h：268）：
/*
* Synthesize speech for the specified text.
* The speech sound data is passed to the calling
* program in buffers by means of the callback function specified by
* espeak_SetSynthCallback(). The command is asynchronous:
* it is internally buffered and returns as soon as possible.
* ...
*/
espeak_Synth()是异步函数（这就是为什么我们连续看到两个read()调用）。
我假设espeak_Synth()的调用应该导致数据写入stdout（经过一段时间的延迟）。但即使select()超时值很大，也不会发生这种情况。这里有些奇怪。