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The C and C++ Include Header Files

cat -n /usr/include/linux/posix_types.h

     1	/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
     2	#ifndef _LINUX_POSIX_TYPES_H
     3	#define _LINUX_POSIX_TYPES_H
     4	
     5	#include <linux/stddef.h>
     6	
     7	/*
     8	 * This allows for 1024 file descriptors: if NR_OPEN is ever grown
     9	 * beyond that you'll have to change this too. But 1024 fd's seem to be
    10	 * enough even for such "real" unices like OSF/1, so hopefully this is
    11	 * one limit that doesn't have to be changed [again].
    12	 *
    13	 * Note that POSIX wants the FD_CLEAR(fd,fdsetp) defines to be in
    14	 * <sys/time.h> (and thus <linux/time.h>) - but this is a more logical
    15	 * place for them. Solved by having dummy defines in <sys/time.h>.
    16	 */
    17	
    18	/*
    19	 * This macro may have been defined in <gnu/types.h>. But we always
    20	 * use the one here.
    21	 */
    22	#undef __FD_SETSIZE
    23	#define __FD_SETSIZE	1024
    24	
    25	typedef struct {
    26		unsigned long fds_bits[__FD_SETSIZE / (8 * sizeof(long))];
    27	} __kernel_fd_set;
    28	
    29	/* Type of a signal handler.  */
    30	typedef void (*__kernel_sighandler_t)(int);
    31	
    32	/* Type of a SYSV IPC key.  */
    33	typedef int __kernel_key_t;
    34	typedef int __kernel_mqd_t;
    35	
    36	#include <asm/posix_types.h>
    37	
    38	#endif /* _LINUX_POSIX_TYPES_H */