Learn open62541 -- [66] UA_ Generation method of string

UA_String It is a special type , Its prototype is as follows , It's a structure ,

/** * String * ^^^^^^ * A sequence of Unicode characters. Strings are just an array of UA_Byte. */
typedef struct {
    
    size_t length; /* The length of the string */
    UA_Byte *data; /* The content (not null-terminated) */
} UA_String;

Usually use C/C++ when , Strings are generally used char Type array or std::string To express , And UA_String not quite the same , Sometimes it is easy to fall into the pit when using .

This paper mainly describes UA_String Related generation methods .

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relevant API

Generate UA_String Of API There are three , as follows ,

• UA_STRING
• UA_STRING_ALLOC
• UA_STRING_STATIC

Here are the introduction ,

1. UA_STRING

Its prototype is as follows ,

/** * ``UA_STRING`` returns a string pointing to the original char-array. * ``UA_STRING_ALLOC`` is shorthand for ``UA_String_fromChars`` and makes a copy * of the char-array. */
static UA_INLINE UA_String
UA_STRING(char *chars) {
    
    UA_String s; s.length = 0; s.data = NULL;
    if(!chars)
        return s;
    s.length = strlen(chars); s.data = (UA_Byte*)chars; return s;
}

The string needs to exist in advance , Then send its address to UA_STRING(), How to use it is as follows ,

const char *aa = "hello";
UA_String ustring1 = UA_STRING((char*)aa);

std::string tt = "world";
UA_String ustring2 = UA_STRING((char*)tt.data());

char arr[128] = "hhheee";
UA_String ustring3 = UA_STRING(arr);

It should be noted that , Because the string needs to exist in advance , Therefore, in the subsequent use, we should ensure that the space is effective , Otherwise, there will be a segment error .

2. UA_STRING_ALLOC

This is a macro definition , as follows ,

#define UA_STRING_ALLOC(CHARS) UA_String_fromChars(CHARS)

It's right UA_String_fromChars() Redefinition of , This function will copy the incoming string （ Use dynamic memory ）, Will not affect the original string

UA_String ustring = UA_STRING_ALLOC("hello");

If you don't want subsequent operations, it will affect the original string , Then use this macro to generate UA_String

3. UA_STRING_STATIC

This is also a macro definition , as follows ,

/* Define strings at compile time (in ROM) */
#define UA_STRING_STATIC(CHARS) {
      sizeof(CHARS)-1, (UA_Byte*)CHARS}

The string needs to exist in advance , No copying . This macro only applies to the following 2 In the form of ,

char arr[128] = "hhheee";
UA_String ustring1 = UA_STRING_STATIC(arr);

UA_String ustring2 = UA_STRING_STATIC("wwweee");

You can't write it like that ,

const char *aa = "hello";
UA_String ustring1 = UA_STRING_STATIC(aa);

Why? ？ Because it uses sizeof To get the number of bytes of the string , This shows the difference between array names and pointers , Like the following example ,

char aa[128];
sizeof(aa); //  be equal to 128

char *ptr = aa;
sizeof(aa); //  Equal to pointer length , by 4 perhaps 8

This is a potential pit , Accidentally fell in . Again , Because the string needs to exist in advance , Therefore, in the subsequent use, we should ensure that the space is effective , Otherwise, there will be a segment error .

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Summary

This article mainly describes the generation UA_String One of the three API, And precautions .