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C/C++：Linux select 1024 文件描述符限制
通常来说，Linux下select调用要求文件描述符的值小于1024，也就是说，fd set中的每个文件描述符的值域为：[0,1023]。
如果超过，Linux下select调用会发生什么？
环境：
[test1280@localhost ~]$ uname -a
Linux localhost.localdomain 3.10.0-327.el7.x86_64 #1 SMP Thu Nov 19 22:10:57 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
方法：
通过dup2生成指定数值的文件描述符，并将其与标准输入关联，select的读集合包含且只包含指定数值文件描述符。
代码：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>int main(int argc, char *argv[])
{if (argc != 2){fprintf(stderr, "usage: %s fd\n", argv[0]);exit(1);}int fd = atoi(argv[1]);if (fd < 3) // <0 0 1 2{fprintf(stderr, "fd domain: (2, fd_max]\n");exit(1);}fprintf(stdout, "fd size: %d\n", FD_SETSIZE);// 创建指定文件描述符，与标准输入关联if (dup2(0, fd) == fd){fprintf(stdout, "dup2 ok! new fd: %d\n", fd);}else{fprintf(stderr, "dup2 error: %d(%s)\n", errno, strerror(errno));exit(1);}// 设为非阻塞long flags = fcntl(fd, F_GETFL); fcntl(fd, F_SETFL, flags|O_NONBLOCK);fprintf(stdout, "will into main loop...\n");while (1){fd_set readSet;FD_ZERO(&readSet);FD_SET(fd, &readSet);struct timeval tv;tv.tv_sec = 3;tv.tv_usec = 0;int res = select(fd + 1, &readSet, NULL, NULL, &tv);if (res == -1) {fprintf(stderr, "select error: %d(%s)\n", errno, strerror(errno));exit(1);} else if (res == 0) {fprintf(stdout, "timeout...\n");} else {static char buff[1024];int count = read(fd, buff, sizeof(buff));fprintf(stdout, "select return %d, read count %d\n", res, count);}}exit(0);
}编译：
[test1280@localhost ~]$ gcc -o main main.c -g -Wall
运行：
case 1（正常态）：
[test1280@localhost ~]$ ./main 10
fd size: 1024
dup2 ok! new fd: 10
will into main loop...
timeout...
timeout...
test1280timeout...select return 1, read count 9
timeout...
^C
case 2（异常态）：
[test1280@localhost ~]$ ./main 1280
fd size: 1024
dup2 ok! new fd: 1280
will into main loop...
select error: 9(Bad file descriptor)
可读、可写、异常文件描述符集合包含大于1024的文件描述符时，执行select将返回EBADF错误。
但是，并不总是这样的！
在另一个程序中，将一个大于1024的FD加入到select可读监听集合执行select时，发生：
此FD没有输入数据，不可读，select并没有等待指定的时间返回0，而是立刻返回一个正数指代存在可读FD。
我没有能在以上的代码中复现这个问题。（以上代码执行select发现非法FD，直接返回EBADF错误）
在网络上别人的博客中，曾出现过段错误的异常：
https://my.oschina.net/u/1780368/blog/491863
man 2 select：
POSIX allows an implementation to define an upper limit, advertised
via the constant FD_SETSIZE, on the range of file descriptors that
can be specified in a file descriptor set. The Linux kernel imposes
no fixed limit, but the glibc implementation makes fd_set a fixed-
size type, with FD_SETSIZE defined as 1024, and the FD_*() macros
operating according to that limit. To monitor file descriptors
greater than 1023, use poll(2) instead.
According to POSIX, select() should check all specified file
descriptors in the three file descriptor sets, up to the limit
nfds-1. However, the current implementation ignores any file
descriptor in these sets that is greater than the maximum file
descriptor number that the process currently has open. According to
POSIX, any such file descriptor that is specified in one of the sets
should result in the error EBADF.
An fd_set is a fixed size buffer. Executing FD_CLR() or FD_SET()
with a value of fd that is negative or is equal to or larger than
FD_SETSIZE will result in undefined behavior.
Although there are some ways to increase this limit (because of you’re using Linux, everything is possible), select() call can only listen 1024 file descriptor at the same time and all of the file descriptor number must be less than 1024.
总结：
1.select要求三个入参被监听集合的FD（取值）不超过1024；由此可推2、4：
2.select要求三个入参被监听集合的FD（数量）不超过1024；
3.若超过1024，执行select可能发生段错误，可能发生立刻返回不阻塞（以及错误的返回值结果），等等。
4.即使select监听集合中只有一个FD，如果FD值大于1024，同样有问题。
参考：
1.https://linux-tips.com/t/is-it-possible-to-listen-file-descriptor-greater-than-1024-with-select/45
2.https://my.oschina.net/u/1780368/blog/491863
3.http://man7.org/linux/man-pages/man2/select.2.html
4.http://bbs.chinaunix.net/thread-1791112-1-1.html
5.https://blog.codingnow.com/2014/02/select_bug.html
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