Works with gcc, not with Arduino. error: taking address of temporary array

Question

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?

• jantje: Have you tried void cpaddr(char target[], const char *source)
• Nick Gammon: I tried your exact code with g++ (and gcc) version 4.8.4 and got the exact same error message you got.
• Mads Skjern: I'm using 4.9.3. Screenshot of result: i.imgur.com/2RJbGwg.png
• underscore_d: ( type[] ){val, ues} is not a valid way to create a temporary in C++: stackoverflow.com/questions/15458883/… The solution is to typedef it.

Answer 1

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);

• Mikael Patel: I think that it should be PSTR() instead of F(). F() is a very special Arduino String hack.
• Majenko: F() just imposes a type to PSTR() for overloading. You can use either.
• Mikael Patel: Because of the type you cannot use F() directly without a cast. The compiler will say "cannot convert 'const __FlashStringHelper*' to 'const char*'".
• Majenko: Mine didn't. It compiled perfectly fine. I always test before posting code. It's not a cast, it's a reinterpret_cast.
• Mikael Patel: But apparently not on memcpy_P as it takes whatever, i.e. const void*.

Answer 2

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.

• Nick Gammon: As Majenko said, memcpy would be better than doing a byte-by-byte copy.
• Majenko: I hate it when people use non-standard arduino specific types like byte, when there is a perfectly good and standard uint8_t available...
• underscore_d: 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.
• Nick Gammon: Recent versions of the IDE use C++11, so whether or not it is valid in C++ doesn't really matter.

Answer 3

How do I fix it?

"Char []" shouldn't compile under c. It is just invalid.

You can use string, cast to char * . or just another pointer as you did already.