device tree compiler源码中的一个看不懂的地方？

Question

项目源码参考https://github.com/dgibson/dtc

在fdt.h中用类型fdt32_t定义了很多变量，这个类型是在libfdt_env.h中定义的，fdt.h文件里并没包含libfdt_env.h，为什么能使用fdt32_t定义变量？

fdt.h:

/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-Clause) */
#ifndef FDT_H
#define FDT_H
/*
 * libfdt - Flat Device Tree manipulation
 * Copyright (C) 2006 David Gibson, IBM Corporation.
 * Copyright 2012 Kim Phillips, Freescale Semiconductor.
 */

#ifndef __ASSEMBLY__

struct fdt_header {
    fdt32_t magic;             /* magic word FDT_MAGIC */
    fdt32_t totalsize;         /* total size of DT block */
    fdt32_t off_dt_struct;         /* offset to structure */
    fdt32_t off_dt_strings;         /* offset to strings */
    fdt32_t off_mem_rsvmap;         /* offset to memory reserve map */
    fdt32_t version;         /* format version */
    fdt32_t last_comp_version;     /* last compatible version */

    /* version 2 fields below */
    fdt32_t boot_cpuid_phys;     /* Which physical CPU id we're
                        booting on */
    /* version 3 fields below */
    fdt32_t size_dt_strings;     /* size of the strings block */

    /* version 17 fields below */
    fdt32_t size_dt_struct;         /* size of the structure block */
};

struct fdt_reserve_entry {
    fdt64_t address;
    fdt64_t size;
};

struct fdt_node_header {
    fdt32_t tag;
    char name[0];
};

struct fdt_property {
    fdt32_t tag;
    fdt32_t len;
    fdt32_t nameoff;
    char data[0];
};

#endif /* !__ASSEMBLY */

#define FDT_MAGIC    0xd00dfeed    /* 4: version, 4: total size */
#define FDT_TAGSIZE    sizeof(fdt32_t)

#define FDT_BEGIN_NODE    0x1        /* Start node: full name */
#define FDT_END_NODE    0x2        /* End node */
#define FDT_PROP    0x3        /* Property: name off,
                       size, content */
#define FDT_NOP        0x4        /* nop */
#define FDT_END        0x9

#define FDT_V1_SIZE    (7*sizeof(fdt32_t))
#define FDT_V2_SIZE    (FDT_V1_SIZE + sizeof(fdt32_t))
#define FDT_V3_SIZE    (FDT_V2_SIZE + sizeof(fdt32_t))
#define FDT_V16_SIZE    FDT_V3_SIZE
#define FDT_V17_SIZE    (FDT_V16_SIZE + sizeof(fdt32_t))

#endif /* FDT_H */

libfdt_env.h

/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-Clause) */
#ifndef LIBFDT_ENV_H
#define LIBFDT_ENV_H
/*
 * libfdt - Flat Device Tree manipulation
 * Copyright (C) 2006 David Gibson, IBM Corporation.
 * Copyright 2012 Kim Phillips, Freescale Semiconductor.
 */

#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>

#ifdef __CHECKER__
#define FDT_FORCE __attribute__((force))
#define FDT_BITWISE __attribute__((bitwise))
#else
#define FDT_FORCE
#define FDT_BITWISE
#endif

typedef uint16_t FDT_BITWISE fdt16_t;
typedef uint32_t FDT_BITWISE fdt32_t;
typedef uint64_t FDT_BITWISE fdt64_t;

#define EXTRACT_BYTE(x, n)    ((unsigned long long)((uint8_t *)&x)[n])
#define CPU_TO_FDT16(x) ((EXTRACT_BYTE(x, 0) << 8) | EXTRACT_BYTE(x, 1))
#define CPU_TO_FDT32(x) ((EXTRACT_BYTE(x, 0) << 24) | (EXTRACT_BYTE(x, 1) << 16) | 
             (EXTRACT_BYTE(x, 2) << 8) | EXTRACT_BYTE(x, 3))
#define CPU_TO_FDT64(x) ((EXTRACT_BYTE(x, 0) << 56) | (EXTRACT_BYTE(x, 1) << 48) | 
             (EXTRACT_BYTE(x, 2) << 40) | (EXTRACT_BYTE(x, 3) << 32) | 
             (EXTRACT_BYTE(x, 4) << 24) | (EXTRACT_BYTE(x, 5) << 16) | 
             (EXTRACT_BYTE(x, 6) << 8) | EXTRACT_BYTE(x, 7))

static inline uint16_t fdt16_to_cpu(fdt16_t x)
{
    return (FDT_FORCE uint16_t)CPU_TO_FDT16(x);
}
static inline fdt16_t cpu_to_fdt16(uint16_t x)
{
    return (FDT_FORCE fdt16_t)CPU_TO_FDT16(x);
}

static inline uint32_t fdt32_to_cpu(fdt32_t x)
{
    return (FDT_FORCE uint32_t)CPU_TO_FDT32(x);
}
static inline fdt32_t cpu_to_fdt32(uint32_t x)
{
    return (FDT_FORCE fdt32_t)CPU_TO_FDT32(x);
}

static inline uint64_t fdt64_to_cpu(fdt64_t x)
{
    return (FDT_FORCE uint64_t)CPU_TO_FDT64(x);
}
static inline fdt64_t cpu_to_fdt64(uint64_t x)
{
    return (FDT_FORCE fdt64_t)CPU_TO_FDT64(x);
}
#undef CPU_TO_FDT64
#undef CPU_TO_FDT32
#undef CPU_TO_FDT16
#undef EXTRACT_BYTE

#ifdef __APPLE__
#include <AvailabilityMacros.h>

/* strnlen() is not available on Mac OS < 10.7 */
# if !defined(MAC_OS_X_VERSION_10_7) || (MAC_OS_X_VERSION_MAX_ALLOWED < 
                                         MAC_OS_X_VERSION_10_7)

#define strnlen fdt_strnlen

/*
 * fdt_strnlen: returns the length of a string or max_count - which ever is
 * smallest.
 * Input 1 string: the string whose size is to be determined
 * Input 2 max_count: the maximum value returned by this function
 * Output: length of the string or max_count (the smallest of the two)
 */
static inline size_t fdt_strnlen(const char *string, size_t max_count)
{
    const char *p = memchr(string, 0, max_count);
    return p ? p - string : max_count;
}

#endif /* !defined(MAC_OS_X_VERSION_10_7) || (MAC_OS_X_VERSION_MAX_ALLOWED <
          MAC_OS_X_VERSION_10_7) */

#endif /* __APPLE__ */

#endif /* LIBFDT_ENV_H */

Answer 1

h 文件一般是不会直接进编译器得，被编译得一般都是 .c 。

只要经过预编译之后 .c 文件里所有得符号在使用之前都被定义了就可以。

比如，在 .c 里可以这样写：

#include <libfdt_env.h>
#include <fdt.h>

编译就不会有问题。