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I need to hard code 8 byte addresses into char arrays of length 8. This must be done many places in my code (in function scope), so I have tried to come up with a one-liner. The following works perfectly in C when compiling with gcc.

char a[8];

void cpaddr(char target[], char *source) {
    int i;
    for (i=0; i<8; i++)
        target[i] = source[i];
}

int main() {
    char b[] = {0x00, 0x10, 0xFF, 0xCA, 0x00, 0x00, 0xA2, 0x7D};
    cpaddr(a, b);

    // line below does not compile with Arduino IDE
    cpaddr(a, (char[]) {0x00, 0x10, 0xFF, 0xCA, 0x00, 0x00, 0xA2, 0x7D});
}

When compiling on Arduino the last line, which is the one-liner I was aiming for, does not compile. It gives:

/home/bob/Desktop/ate/Ate.ino: In function 'int main()':
Ate:101: error: taking address of temporary array
cpaddr(a, (char[]) {0x00, 0x10, 0xFF, 0xCA, 0x00, 0x00, 0xA2, 0x7D});
^
exit status 1
taking address of temporary array

What is the problem here, that apparently is not a problem with gcc?

How do I fix it?

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Have you tried void cpaddr(char target[], const char *source) – jantje Mar 2 '16 at 13:25
    
I tried your exact code with g++ (and gcc) version 4.8.4 and got the exact same error message you got. – Nick Gammon♦ Mar 2 '16 at 20:33
    
I'm using 4.9.3. Screenshot of result: i.imgur.com/2RJbGwg.png – Mads Skjern Mar 3 '16 at 8:19
    
( type[] ){val, ues} is not a valid way to create a temporary in C++: stackoverflow.com/questions/15458883/… The solution is to typedef it. – underscore_d Jun 26 '16 at 7:13
up vote 3 down vote accepted

It is quite right, using that kind of syntax is not allowed. It's a bit of a pain, but it's ok since there is an alternative method - kind of a "trick" if you will.

That trick is to use a string, not an array. After all, a string is just an array, it's just handled slightly differently by the compiler.

Instead of using {...} use "..." and use the hexadecimal character escape sequence \xNN, such as:

cpaddr(a, "\x00\x10\xFF\xCA\x00\x00\xA2\x7D");

You could even lose your custom function and use a standard library function - memcpy():

memcpy(a, "\x00\x10\xFF\xCA\x00\x00\xA2\x7D", 8);

On the 8-bit AVRs you can save RAM by using the progmem variant and the F() macro:

memcpy_P(a, F("\x00\x10\xFF\xCA\x00\x00\xA2\x7D"), 8);
share|improve this answer
    
I think that it should be PSTR() instead of F(). F() is a very special Arduino String hack. – Mikael Patel Mar 2 '16 at 22:07
    
F() just imposes a type to PSTR() for overloading. You can use either. – Majenko Mar 2 '16 at 22:17
    
Because of the type you cannot use F() directly without a cast. The compiler will say "cannot convert 'const __FlashStringHelper*' to 'const char*'". – Mikael Patel Mar 2 '16 at 22:30
    
Mine didn't. It compiled perfectly fine. I always test before posting code. It's not a cast, it's a reinterpret_cast. – Majenko Mar 2 '16 at 22:31
    
But apparently not on memcpy_P as it takes whatever, i.e. const void*. – Mikael Patel Mar 2 '16 at 22:32
up vote 2 down vote

See Using array init list as temporary in C++11?

You can solve it by using const. This compiles:

char a[8];

void cpaddr(char target[], const byte *source) {
    int i;
    for (i=0; i<8; i++)
        target[i] = source[i];
}

int main() {
    byte b[] = {0x00, 0x10, 0xFF, 0xCA, 0x00, 0x00, 0xA2, 0x7D};
    cpaddr(a, b);
    cpaddr(a, (const byte[]) {0x00, 0x10, 0xFF, 0xCA, 0x00, 0x00, 0xA2, 0x7D});
}

Note I had to change your array from char to byte because I was getting (valid) warnings that things like 0xCA don't fit into a char.

share|improve this answer
    
As Majenko said, memcpy would be better than doing a byte-by-byte copy. – Nick Gammon♦ Mar 2 '16 at 20:41
    
I hate it when people use non-standard arduino specific types like byte, when there is a perfectly good and standard uint8_t available... – Majenko Mar 3 '16 at 20:14
    
As explained in the very thread you linked, ( type[] ){val, ues} is not a valid way to create a temporary in C++: stackoverflow.com/questions/15458883/… The solution is to typedef it. – underscore_d Jun 26 '16 at 7:12
    
Recent versions of the IDE use C++11, so whether or not it is valid in C++ doesn't really matter. – Nick Gammon♦ Jun 26 '16 at 9:25

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