C程序从FIFO读取数据

Question

我有两个程序来写入和读取 FIFO。一个是读取（O_RDONLY）一个 FIFO。另一个是将数据写入这个 FIFO。这是代码：

读取：可执行文件名为读取。

#include<errno.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<stdio.h>
#include<string.h>

int main(int argc, char *argv[])
{
    int fd, nread;
    const char *pipe_file = "/tmp/test_fifo";

    char buf[0x100] ;

    fd = open(pipe_file, O_RDONLY);

    while(1) {
        printf("\n"); /*  */
        memset(buf, 0, 100);
        nread = read(fd, buf, 0x100);
        printf("data is %s", buf);
        sleep(1);
    }

    return 0;
}

Write：可执行文件名为write。

#include<errno.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<stdio.h>
#include<string.h>

int main(int argc, char *argv[])
{
    int fd;
    int num = 1234;
    char *s = "changzhi";
    const char *pipe_file = "/tmp/test_fifo";


    fd = open(pipe_file, O_WRONLY);
    write(fd, s, strlen(s));

    return 0;
}

名为 /tmp/test_fifo 的 FIFO 已存在。当我运行 read 读取 FIFO 并运行 write 写入 FIFO 时，一切正常。 但是，当读取代码中没有printf("\n"); 时，read 无法读取数据。我不知道为什么。有人可以帮助我吗？非常感谢！

Answer 1

你的第二个printf() 被缓冲，直到\n 被写入。读取没有被阻塞:-)

重写你的循环：

while(1) {
    memset(buf, 0, 100);
    nread = read(fd, buf, 0x100);
    printf("data is %s\n", buf);
    sleep(1);
}

Answer 2

在严格的编译选项下干净地编译代码：

$ gcc -O3 -g -std=c11 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
>     -Wold-style-definition -Werror write.c -o write
$ gcc -O3 -g -std=c11 -Wall -Wextra -Wmissing-prototypes -Wstrict-prototypes \
>     -Wold-style-definition -Werror read.c -o read
$

代码基本上对我有用。我添加了错误检查。稍微奇怪的argv[argc-argc] 打印argv[0]，但避免了关于argc 未使用的警告（由于-Werror 而导致的错误）。

read.c

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char **argv)
{
    int fd, nread;
    const char *pipe_file = "/tmp/test_fifo";

    char buf[0x100];

    if ((fd = open(pipe_file, O_RDONLY)) < 0)
    {
        fprintf(stderr, "%s: Failed to open FIFO %s\n", argv[argc-argc], pipe_file);
        return 1;
    }
    printf("%s: open successful\n", argv[0]);

    while(1) {
        printf("\n"); /*  */
        memset(buf, 0, 100);
        nread = read(fd, buf, 0x100-1);
        if (nread <= 0)
            break;
        printf("data is %s\n", buf);
        sleep(1);
    }

    return 0;
}

write.c

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
    int fd;
    char *s = "changzhi";
    const char *pipe_file = "/tmp/test_fifo";

    if ((fd = open(pipe_file, O_WRONLY)) < 0)
    {
        fprintf(stderr, "%s: failed to open FIFO %s\n", argv[argc-argc], pipe_file);
        return 1;
    }
    printf("%s: open successful\n", argv[0]);
    if (write(fd, s, strlen(s)) <= 0)
    {
        fprintf(stderr, "%s: failed to open FIFO %s\n", argv[argc-argc], pipe_file);
        return 1;
    }
    printf("%s: write successful\n", argv[0]);

    return 0;
}

样本输出

$ ./read & ./write
[1] 85542
./write: open successful
./write: write successful
./read: open successful

data is changzhi
$ 

[1]+  Done                    ./read
$ ./write & ./read
[1] 85544
./write: open successful
./read: open successful

./write: write successful
data is changzhi

[1]+  Done                    ./write
$

如果 FIFO 不存在，程序会明智地报告错误。