Question

下面的程序在OS X下运行良好，但在linux中运行不正常。它继续循环遍历perror（“读错误”）行，读缓冲区中没有字节，EWOULDBLOCK不是errno（errno = 0）;

在OS X中，程序按预期工作，即从三个命名管道读取，并将任何数据打印到控制台。

        #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("\nReading pipe descriptor # %d\n",fd);
            for(;;) {
                bytes = read(fd, buffer, sizeof(buffer));
                if (bytes > 0) {
                    total_bytes += (size_t)bytes;
                    printf("%s", buffer);
                } 
                else {
                    if (errno == EWOULDBLOCK) {
                        break; // recieve buffer is empty so return to main loop
                    } 
                    else {
                        perror("read error"); 
                        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 = 10000;

            // 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");
            system("chmod 666 /tmp/PIPE_*");

            // open file descriptors of named pipes to watch
            fd_a = open("/tmp/PIPE_A", O_RDONLY | O_NONBLOCK); // the O_RDWR flag is undefined on a FIFO.
            if (fd_a == -1) {
                perror("open error");
                return EXIT_FAILURE;
            }

            fd_b = open("/tmp/PIPE_B", O_RDONLY | O_NONBLOCK);
            if (fd_b == -1) {
                perror("open error");
                return EXIT_FAILURE;
            }

            fd_c = open("/tmp/PIPE_C", O_RDONLY | O_NONBLOCK);
            if (fd_c == -1) {
                perror("open error");
                return EXIT_FAILURE;
            }

            // check for new data in each of the pipes
            for(;;)
            {
                // clear fds read flags
                FD_ZERO(&read_fds);

                // PIPE_A
                FD_SET(fd_a, &read_fds);
                nfd = select(fd_a+1, &read_fds, NULL, NULL, &tv);
                if (nfd != 0) {
                    if (nfd == -1) {
                        perror("select error");
                        return EXIT_FAILURE;
                    }
                    if (FD_ISSET(fd_a, &read_fds)) {
                        readPipe(fd_a);
                    }
                }

                // PIPE_B
                FD_SET(fd_b, &read_fds);
                nfd = select(fd_b+1, &read_fds, NULL, NULL, &tv);
                if (nfd != 0) {
                    if (nfd == -1) {
                        perror("select error");
                        return EXIT_FAILURE;
                    }
                    if (FD_ISSET(fd_b, &read_fds)){
                        readPipe(fd_b);
                    }
                }

                // PIPE_C
                FD_SET(fd_c, &read_fds);
                nfd = select(fd_c+1, &read_fds, NULL, NULL, &tv);
                if (nfd != 0) {
                    if (nfd == -1) {
                        perror("select error");
                        return EXIT_FAILURE;
                    }
                    if (FD_ISSET(fd_c, &read_fds)){
                        readPipe(fd_c);
                    }
                }
            }
            return EXIT_SUCCESS;
        }

Answer 1

允许（并且预期）读取可以返回0.这意味着管道返回EOF。你没有处理这个条件。除非调用失败并返回-1，否则errno的内容无关紧要。

for (;;)
{
    bytes = read(fd, buffer, sizeof(buffer));

    if (bytes > 0)
    {
        total_bytes += (size_t)bytes;
        printf("%s", buffer);
    }

    if (bytes == 0)
        return //something appropriate

    if (bytes == -1)
    {
        if (errno == EWOULDBLOCK)
           break; // recieve buffer is empty so return to main loop
        else
        {
            perror("read error");
            return EXIT_FAILURE;
        }
    }
}

您正在努力返回不同的代码，但您在main中没有注意到它们。

另外，3 select语句又是什么？我以为在前一个问题中已经解决了这个问题。

修改

for (;;) { // clear fds read flags FD_ZERO(&read_fds); FD_SET(fd_a, &read_fds); FD_SET(fd_b, &read_fds); FD_SET(fd_c, &read_fds); tv.tv_sec = 0; tv.tv_usec = 10000; nfd = select(fd_c + 1, &read_fds, NULL, NULL, &tv); if (nfd == 0) //timeout - continue or do something else for a bit continue; if (nfd == -1) { perror("select error"); return EXIT_FAILURE; } if (FD_ISSET(fd_a, &read_fds)) readPipe(fd_a); if (FD_ISSET(fd_b, &read_fds)) readPipe(fd_b); if (FD_ISSET(fd_c, &read_fds)) readPipe(fd_c); }

从linux和OS X中的命名管道读取

1 个答案: