在标准输入上测试select
实现参考了《UNIX环境高级编程》14.4.1和《UNIX系统编程手册》63.2.1。
#include <cstdio>
#include <iostream>
#include <unistd.h>
#include <fcntl.h>
#include <sys/select.h>
using namespace std;
const int BUF_SIZE = 128;
int main(int argc, char **argv) {
fd_set rs;
FD_ZERO(&rs);
FD_SET(STDIN_FILENO, &rs);
for (int i = 0; i < 5; i++) {
timeval tv = {.tv_sec=2, .tv_usec=0};
fd_set rs_temp = rs;
int ret = select(STDIN_FILENO + 1, &rs_temp, NULL, NULL, &tv);
if (ret == -1) {
printf("select ret: %d, errno: %d\n", ret, errno);
return 1;
}
printf("select ret: %d\n", ret);
printf("if can read data from fd: %d is %s\n", STDIN_FILENO, FD_ISSET(STDIN_FILENO, &rs_temp) ? "true" : "false");
if (FD_ISSET(STDIN_FILENO, &rs_temp)) {
char buf[BUF_SIZE];
int n = read(STDIN_FILENO, buf, 5);
if (n == -1) {
printf("read failed, errno: %d\n", errno);
return 1;
}
buf[n] = '\0';
printf("success read %d bytes, buf: %s\n", n, buf);
}
}
return 0;
}
下面运行结果中第三行的hello world是在第一次循环结果打印后，第二次调用select阻塞时输入的。
运行结果：
select ret: 0 if can read data from fd: 0 is false hello world select ret: 1 if can read data from fd: 0 is true success read 5 bytes, buf: hello select ret: 1 if can read data from fd: 0 is true success read 5 bytes, buf: worl select ret: 1 if can read data from fd: 0 is true success read 2 bytes, buf: d select ret: 0 if can read data from fd: 0 is false Process finished with exit code 0
非阻塞I/O
一个描述符阻塞与否并不影响select是否阻塞。1
#include <cstdio>
#include <cstring>
#include <iostream>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
const int BUF_SIZE = 128;
using namespace std;
void set_fl(int fd, int flags) {
int val;
if ((val = fcntl(fd, F_GETFL, 0)) < 0) {
fprintf(stderr, "fcntl F_GETFL error\n");
exit(1);
}
val |= flags;
if (fcntl(fd, F_SETFL, val) < 0) {
fprintf(stderr, "fcntl F_SETFL error\n");
exit(1);
}
}
int main(int argc, char **argv) {
int n;
char buf[128];
set_fl(STDIN_FILENO, O_NONBLOCK);
n = read(STDIN_FILENO, buf, BUF_SIZE - 1);
printf("n: %d, errno: %d, str: %s\n", n, errno, strerror(errno));
fd_set rs;
FD_ZERO(&rs);
FD_SET(STDIN_FILENO, &rs);
for (int i = 0; i < 5; i++) {
fd_set rs_temp = rs;
timeval tv = {.tv_sec=2, .tv_usec=0};
int ret = select(STDIN_FILENO + 1, &rs_temp, NULL, NULL, &tv);
if (ret == -1) {
fprintf(stderr, "select ret: %d, errno: %d, str: %s", ret, errno, strerror(errno));
return 1;
}
printf("select ret: %d\n", ret);
printf("if can read data from fd: %d is %s\n", STDIN_FILENO, FD_ISSET(STDIN_FILENO, &rs_temp) ? "true" : "false");
}
return 0;
}
执行结果：
n: -1, errno: 35, str: Resource temporarily unavailable select ret: 0 if can read data from fd: 0 is false select ret: 0 if can read data from fd: 0 is false select ret: 0 if can read data from fd: 0 is false select ret: 0 if can read data from fd: 0 is false select ret: 0 if can read data from fd: 0 is false Process finished with exit code 0
可以看到代码首先对设置了非阻塞标志的标准输入读，read函数返回-1，每次select的返回结果也是0。
select阻塞中被信号中断
#include <cstdio>
#include <cstring>
#include <iostream>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/select.h>
using namespace std;
void handler(int sig) {
printf("caught signal: %d, str: %s\n", sig, strsignal(sig));
}
int main(int argc, char **argv) {
signal(SIGUSR1, handler);
int ret = select(0, NULL, NULL, NULL, NULL);
printf("select ret: %d\n", ret);
if (ret == -1) {
printf("errno: %d, str: %s\n", errno, strerror(errno));
}
return 0;
}
运行后给进程发SIGUSR1信号。
caught signal: 30, str: User defined signal 1: 30 select ret: -1 errno: 4, str: Interrupted system call
select支持的最大数量
#include <cstdio>
#include <cstring>
#include <iostream>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/select.h>
using namespace std;
int main(int argc, char **argv) {
timeval tv;
int ret;
printf("FD_SETSIZE: %d\n", FD_SETSIZE);
tv = {.tv_sec=0, .tv_usec=0};
ret = select(FD_SETSIZE, NULL, NULL, NULL, &tv);
printf("select ret: %d\n", ret);
if (ret == -1) {
printf("errno: %d, str: %s\n", errno, strerror(errno));
}
tv = {.tv_sec=0, .tv_usec=0};
ret = select(FD_SETSIZE + 1, NULL, NULL, NULL, &tv);
printf("select ret: %d\n", ret);
if (ret == -1) {
printf("errno: %d, str: %s\n", errno, strerror(errno));
}
return 0;
}
FD_SETSIZE: 1024 select ret: 0 select ret: -1 errno: 22, str: Invalid argument Process finished with exit code 0
在本机的macOs上$ uname -a的结果为Darwin localhost 19.0.0 Darwin Kernel Version 19.0.0: Thu Oct 17 16:17:15 PDT 2019; root:xnu-6153.41.3~29/RELEASE_X86_64 x86_64
The behavior of these macros is undefined if a descriptor value is less than zero or greater than or equal to FD_SETSIZE, which is normally at least equal to the maximum number of descriptors supported by the system.2