一直不知道为什么select的文件描述符限制是比较小的?
于是去查看内核5.6的版本，其中在/include/uapi/linux/posix_types.h有说明
/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef _LINUX_POSIX_TYPES_H
#define _LINUX_POSIX_TYPES_H
#include <linux/stddef.h>
/*
* This allows for 1024 file descriptors: if NR_OPEN is ever grown
* beyond that you'll have to change this too. But 1024 fd's seem to be
* enough even for such "real" unices like OSF/1, so hopefully this is
* one limit that doesn't have to be changed [again].
*
* Note that POSIX wants the FD_CLEAR(fd,fdsetp) defines to be in
* <sys/time.h> (and thus <linux/time.h>) - but this is a more logical
* place for them. Solved by having dummy defines in <sys/time.h>.
*/
/*
* This macro may have been defined in <gnu/types.h>. But we always
* use the one here.
*/
#undef __FD_SETSIZE
#define __FD_SETSIZE 1024
typedef struct {
unsigned long fds_bits[__FD_SETSIZE / (8 * sizeof(long))];
} __kernel_fd_set;
/* Type of a signal handler. */
typedef void (*__kernel_sighandler_t)(int);
/* Type of a SYSV IPC key. */
typedef int __kernel_key_t;
typedef int __kernel_mqd_t;
#include <asm/posix_types.h>
#endif /* _LINUX_POSIX_TYPES_H */
其中__FD_SETSIZE 为select 的文件描述符的最大数量