FIFO

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FIFO指代先进先出(first in, first out)，Unix中的FIFO类似于管道。它是一个单向(半双工)数据流。不同于管道的是每个FIFO有一个路径名与之关联，从而允许无亲缘关系的进程访问同一个FIFO。FIFO也称谓有名管道（named pipe）。

FIFO由mkfifo函数创建。

<span style="font-size:14px">#include <sys/types.h>#include <sys/stat.h>int mkfifo(const char *pathname, mode_t mode);        返回：成功时为0，出错时为-1</span>

FIFO创建完后，它必须或者打开来读或者打开来写。可以使用open或fopen函数打开。FIFO不能打开来既读又写，因为它是半双工的。

对管道或FIFO的write总是往末尾添加数据，对它们的read则总是从开通返回数据，如果对管道或FIFO调用lseek，那就会返回ESPIPE错误。
下面是一个简单的例子：

/* * main.cpp * *  Created on: 2013-10-26 *      Author: Richard */#include <unistd.h>#include <sys/types.h>#include <sys/wait.h>#include <stdlib.h>#include <stdio.h>#include <string.h>#include <sys/stat.h>#include <fcntl.h>#include <errno.h>#define FIFO1 "/tmp/fifo.1"#define FIFO2 "/tmp/fifo.2"void client(int readfd, int writefd){    size_t len;    size_t n;    char buff[1024];    fgets(buff, 1024, stdin);    len = strlen(buff);    if (buff[len - 1] == '\n')    {        len--;    }    write(writefd, buff, len);    while ((n = read(readfd, buff, 1024)) > 0)    {        write(STDOUT_FILENO, buff, n);    }}void server(int readfd, int writefd){    int fd;    ssize_t n;    char buff[1025];    if ((n = read(readfd, buff, 1024)) == 0)    {        printf("End of file while reading pathname");        exit(0);    }    buff[n] = '\0';    if ((fd = open(buff, O_RDONLY)) < 0)    {        snprintf(buff + n, sizeof(buff) - n, "Cannot open, %s\n",                strerror(errno));        n = strlen(buff);        write(writefd, buff, n);    }    else    {        while ((n = read(fd, buff, 1024)) > 0)        {            write(writefd, buff, n);        }        close(fd);    }}int main(int argc, char **argv){    int readfd, writefd;    pid_t childpid;    if ((mkfifo(FIFO1, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH) < 0)            && (errno != EEXIST))    {        printf("cannot create %s\n", FIFO1);        return 0;    }    if ((mkfifo(FIFO2, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH) < 0)            && (errno != EEXIST))    {        unlink(FIFO1);        printf("cannot create %s\n", FIFO2);        return 0;    }    if ((childpid = fork()) == 0)    {        readfd = open(FIFO1, O_RDONLY, 0);        writefd = open(FIFO2, O_WRONLY, 0);        server(readfd, writefd);        exit(0);    }    writefd = open(FIFO1, O_WRONLY, 0);    readfd = open(FIFO2, O_RDONLY, 0);    client(readfd, writefd);    waitpid(childpid, NULL, 0);    close(readfd);    close(writefd);    unlink(FIFO1);    unlink(FIFO2);    return 0;}