c - 为什么在 c 中对结构使用下划线？

Question

在下面的源代码中，有人可以解释理性(为什么好的编程习惯 typedef 结构 _lwm2m_object_t 具有新名称 lwm2m_object_t ？它所做的只是删除下划线？为什么是下划线一审使用？

typedef struct _lwm2m_object_t lwm2m_object_t;

typedef uint8_t (*lwm2m_read_callback_t) (lwm2m_uri_t * uriP, char ** bufferP, int * lengthP, lwm2m_object_t * objectP);
typedef uint8_t (*lwm2m_write_callback_t) (lwm2m_uri_t * uriP, char * buffer, int length, lwm2m_object_t * objectP);
typedef uint8_t (*lwm2m_execute_callback_t) (lwm2m_uri_t * uriP, char * buffer, int length, lwm2m_object_t * objectP);
typedef uint8_t (*lwm2m_create_callback_t) (lwm2m_uri_t * uriP, char * buffer, int length, lwm2m_object_t * objectP);
typedef uint8_t (*lwm2m_delete_callback_t) (uint16_t id, lwm2m_object_t * objectP);
typedef void (*lwm2m_close_callback_t) (lwm2m_object_t * objectP);


struct _lwm2m_object_t
{
    uint16_t                 objID;
    lwm2m_list_t *           instanceList;
    lwm2m_read_callback_t    readFunc;
    lwm2m_write_callback_t   writeFunc;
    lwm2m_execute_callback_t executeFunc;
    lwm2m_create_callback_t  createFunc;
    lwm2m_delete_callback_t  deleteFunc;
    lwm2m_close_callback_t   closeFunc;
    void *                   userData;
};

Answer 1

实际上如果你使用

typedef struct lwm2m_object_t lwm2m_object_t;

struct lwm2m_object_t {
//
}

C 仍然允许它。这是因为结构名称和 typedef 标识符具有不同的 namespace 。请看Why doesn't ANSI C have namespaces?有关 C 中命名空间的信息。

然而，许多专业用户避免这种情况，并为结构和类型定义使用不同的名称。 MISRA¹ 也不允许这样做。

Rule 5.6 (advisory): No identifier in one name space should have the same spelling as an identifier in another name space, with the exception of structure member and union member names. [MISRA C 2004]

下划线的使用只是一些人遵循的约定。您可以遵循其他约定，例如

typedef struct sTag_lwm2m_object_t lwm2m_object_t;

struct sTag_lwm2m_object_t {
//
}

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¹ https://en.wikipedia.org/wiki/MISRA_C