好久没更新了,今天看一下第三种新的fd:eventfd类似于管道的概念,可以实现线程间的事件通知,所不同的是eventfd的缓冲区大小是sizeof(uint64_t)也就是8字节,它是一个64位的计数器,写入递增计数器,读取将得到计数器的值,并且清零。看一下代码:
#include <sys/eventfd.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h> /* Definition of uint64_t */
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
uint64_t u;
int efd = eventfd(10, 0);
if (efd == -1)
handle_error("eventfd");
int ret = fork();
if(ret == 0)
{
for (int j = 1; j < argc; j++) {
printf("Child writing %s to efd\n", argv[j]);
u = atoll(argv[j]);
ssize_t s = write(efd, &u, sizeof(uint64_t));
if (s != sizeof(uint64_t))
handle_error("write");
}
printf("Child completed write loop\n");
exit(EXIT_SUCCESS);
}
else
{
sleep(2);
ssize_t s = read(efd, &u, sizeof(uint64_t));
if (s != sizeof(uint64_t))
handle_error("read");
printf("Parent read %llu from efd\n",(unsigned long long)u);
exit(EXIT_SUCCESS);
}
}
比较简单,不做过解释。子进程写入命令行中传入的参数,父进程读取其中计数器的值。
运行结果:
./eventfd 10 20 30
Child writing 10 to efd
Child writing 20 to efd
Child writing 30 to efd
Child completed write loop
Parent read 70 from efd
命令行传入的是10、20、30其和应为60,为啥读取的是70呢?请看15行调用eventfd时第一个参数是10,这个参数是创建eventfd时初始化计数器的值。