【发布时间】：2014-01-02 02:10:52
【问题描述】：
基于 stackoverflow 中位于 here 的类似示例， 我有三个命名管道，管道_a、管道_b 和管道_c，它们是从外部进程提供的。我想要一个读取器进程，它输出到控制台，无论写入任何这些管道。
下面的程序是一个一体式c程序，应该以非阻塞的方式读取三个管道，并在其中任何一个管道获得新数据时显示输出。
但是，它不起作用 - 它正在阻塞！如果 pipe_a 得到数据，它会显示它，然后等待新数据到达 pipe_b 等...
select() 应该允许监视多个文件描述符，直到一个准备好，此时我们应该进入管道的读取函数并获取数据。
谁能帮助确定为什么管道的行为就像处于阻塞模式？
/*
* FIFO example using select.
*
* $ mkfifo /tmp/fifo
* $ clang -Wall -o test ./test.c
* $ ./test &
* $ echo 'hello' > /tmp/fifo
* $ echo 'hello world' > /tmp/fifo
* $ killall test
*/
#include <sys/types.h>
#include <sys/select.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
// globals
int fd_a, fd_b, fd_c;
int nfd_a, nfd_b, nfd_c;
fd_set set_a, set_b, set_c;
char buffer_a[100*1024];
char buffer_b[100*1024];
char buffer_c[100*1024];
int readPipeA()
{
ssize_t bytes;
size_t total_bytes;
if (FD_ISSET(fd_a, &set_a)) {
printf("\nDescriptor %d has new data to read.\n", fd_a);
total_bytes = 0;
for (;;) {
printf("\nDropped into read loop\n");
bytes = read(fd_a, buffer_a, sizeof(buffer_a));
if (bytes > 0) {
total_bytes += (size_t)bytes;
printf("%s", buffer_a);
} else {
if (errno == EWOULDBLOCK) {
printf("\ndone reading (%ul bytes)\n", total_bytes);
break;
} else {
perror("read");
return EXIT_FAILURE;
}
}
}
}
}
int readPipeB()
{
ssize_t bytes;
size_t total_bytes;
if (FD_ISSET(fd_b, &set_b)) {
printf("\nDescriptor %d has new data to read.\n", fd_b);
total_bytes = 0;
for (;;) {
printf("\nDropped into read loop\n");
bytes = read(fd_b, buffer_b, sizeof(buffer_b));
if (bytes > 0) {
total_bytes += (size_t)bytes;
printf("%s", buffer_b);
} else {
if (errno == EWOULDBLOCK) {
printf("\ndone reading (%ul bytes)\n", total_bytes);
break;
} else {
perror("read");
return EXIT_FAILURE;
}
}
}
}
}
int readPipeC()
{
ssize_t bytes;
size_t total_bytes;
if (FD_ISSET(fd_c, &set_c)) {
printf("\nDescriptor %d has new data to read.\n", fd_c);
total_bytes = 0;
for (;;) {
printf("\nDropped into read loop\n");
bytes = read(fd_c, buffer_c, sizeof(buffer_c));
if (bytes > 0) {
total_bytes += (size_t)bytes;
printf("%s", buffer_c);
} else {
if (errno == EWOULDBLOCK) {
printf("\ndone reading (%ul bytes)\n", total_bytes);
break;
} else {
perror("read");
return EXIT_FAILURE;
}
}
}
}
}
int main(int argc, char* argv[])
{
// create pipes to monitor (if they don't already exist)
system("mkfifo /tmp/PIPE_A");
system("mkfifo /tmp/PIPE_B");
system("mkfifo /tmp/PIPE_C");
// open file descriptors of named pipes to watch
fd_a = open("/tmp/PIPE_A", O_RDWR | O_NONBLOCK);
if (fd_a == -1) {
perror("open");
return EXIT_FAILURE;
}
FD_ZERO(&set_a);
FD_SET(fd_a, &set_a);
fd_b = open("/tmp/PIPE_B", O_RDWR | O_NONBLOCK);
if (fd_b == -1) {
perror("open");
return EXIT_FAILURE;
}
FD_ZERO(&set_b);
FD_SET(fd_b, &set_b);
fd_c = open("/tmp/PIPE_C", O_RDWR | O_NONBLOCK);
if (fd_c == -1) {
perror("open");
return EXIT_FAILURE;
}
FD_ZERO(&set_c);
FD_SET(fd_c, &set_c);
for(;;)
{
// check pipe A
nfd_a= select(fd_a+1, &set_a, NULL, NULL, NULL);
if (nfd_a) {
if (nfd_a == -1) {
perror("select");
return EXIT_FAILURE;
}
readPipeA();
}
// check pipe B
nfd_b= select(fd_b+1, &set_b, NULL, NULL, NULL);
if (nfd_b) {
if (nfd_b == -1) {
perror("select");
return EXIT_FAILURE;
}
readPipeB();
}
// check pipe C
nfd_c= select(fd_c+1, &set_c, NULL, NULL, NULL);
if (nfd_c) {
if (nfd_c == -1) {
perror("select");
return EXIT_FAILURE;
}
readPipeC();
}
}
return EXIT_SUCCESS;
}
--- 更新代码---
根据此处的反馈修改了应用程序，并阅读了更多内容：
/*
* FIFO example using select.
*
* $ mkfifo /tmp/fifo
* $ clang -Wall -o test ./test.c
* $ ./test &
* $ echo 'hello' > /tmp/fifo
* $ echo 'hello world' > /tmp/fifo
* $ killall test
*/
#include <sys/types.h>
#include <sys/select.h>
#include <sys/time.h>
#include <sys/types.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
int readPipe(int fd)
{
ssize_t bytes;
size_t total_bytes = 0;
char buffer[100*1024];
printf("\nDropped into read pipe\n");
for(;;) {
bytes = read(fd, buffer, sizeof(buffer));
if (bytes > 0) {
total_bytes += (size_t)bytes;
printf("%s", buffer);
} else {
if (errno == EWOULDBLOCK) {
printf("\ndone reading (%d bytes)\n", (int)total_bytes);
break;
} else {
perror("read");
return EXIT_FAILURE;
}
}
}
return EXIT_SUCCESS;
}
int main(int argc, char* argv[])
{
int fd_a, fd_b, fd_c; // file descriptors for each pipe
int nfd; // select() return value
fd_set read_fds; // file descriptor read flags
struct timeval tv;
tv.tv_sec = 0;
tv.tv_usec = 0;
// create pipes to monitor (if they don't already exist)
system("mkfifo /tmp/PIPE_A");
system("mkfifo /tmp/PIPE_B");
system("mkfifo /tmp/PIPE_C");
// open file descriptors of named pipes to watch
fd_a = open("/tmp/PIPE_A", O_RDWR | O_NONBLOCK);
if (fd_a == -1) {
perror("open");
return EXIT_FAILURE;
}
fd_b = open("/tmp/PIPE_B", O_RDWR | O_NONBLOCK);
if (fd_b == -1) {
perror("open");
return EXIT_FAILURE;
}
fd_c = open("/tmp/PIPE_C", O_RDWR | O_NONBLOCK);
if (fd_c == -1) {
perror("open");
return EXIT_FAILURE;
}
FD_ZERO(&read_fds);
FD_SET(fd_a, &read_fds); // add pipe to the read descriptor watch list
FD_SET(fd_b, &read_fds);
FD_SET(fd_c, &read_fds);
for(;;)
{
// check if there is new data in any of the pipes
nfd = select(fd_a+1, &read_fds, NULL, NULL, &tv);
if (nfd != 0) {
if (nfd == -1) {
perror("select");
return EXIT_FAILURE;
}
if (FD_ISSET(fd_a, &read_fds)) {
readPipe(fd_a);
}
}
nfd = select(fd_b+1, &read_fds, NULL, NULL, &tv);
if (nfd != 0) {
if (nfd == -1) {
perror("select");
return EXIT_FAILURE;
}
if (FD_ISSET(fd_b, &read_fds)){
readPipe(fd_b);
}
}
nfd = select(fd_c+1, &read_fds, NULL, NULL, &tv);
if (nfd != 0) {
if (nfd == -1) {
perror("select");
return EXIT_FAILURE;
}
if (FD_ISSET(fd_c, &read_fds)){
readPipe(fd_c);
}
}
usleep(10);
}
return EXIT_SUCCESS;
}
当任何一个受监视管道中有数据等待时，选择返回零 (0) 仍然存在问题？我一定不能正确使用select() 和fd_isset()。你能看出我做错了什么吗？谢谢。
【问题讨论】：
-
缩小你的问题。做一个测试用例。这代码太多了。例如，是否只使用两个管道会再次出现问题？那么你的代码将只有这个大小的 66%。
-
问题是选择功能被阻塞了。我理解 select() 检查标志以查看读取是否“将”阻塞，以便可以决定是否执行读取。管道正在以 RDWR 和 NONBLOCK 模式打开。 select的参数可能需要更改，我接下来会检查。
-
请阅读我最初的评论。如果唯一的问题是你有一个阻塞选择应该是一个非阻塞管道，你可能会生成一个 10 行的程序来证明这一点。
-
顺便说一句，这是很多不必要的重复代码！
标签： c++ c linux named-pipes