Undefined reference to ... const! Why?

[roberto.bizzarri]

Hello, I’am having trouble with const data.
It should be easy, but…
(working with ESP32 on PlatformIO of course)

This is OK:

*main.cpp*
const char test[] = { 1,2,3 };

main() {
   char buff[10];
   strcpy(buff, (char *) test);
}

…but if the const data are on another file, linker returns Undefined Reference to test:

*main.cpp*
extern const char test[];

main() {
   char buff[10];
   strcpy(buff, (char *) test);
}

--------------------------

*file.cpp*
const char test[] = { 1,2,3 };

Thanks for help!

[maxgerhardt]

@roberto.bizzarri

*main.cpp*
const char test[] = { 1,2,3 };

main() {
   char buff[10];
   strcpy(buff, (char *) test);
}

How is this fine? strcpy will look for a zero-terminated string, and the array you point it to has ano explicit '\0' character in it – so copying from it is undefined behavior, and it may copy data beyond the boundaries of the array.

@roberto.bizzarri

*file.cpp*
const char test[] = { 1,2,3 };

Where is this file located?

[roberto.bizzarri]

both files are in /src directory

[maxgerhardt]

file.cpp must also contain an extern declaration of the variable.

[roberto.bizzarri]

Why? Is it a rule of C++ ?
I did it so many times with the standard C without that error…
Thanks!

[maxgerhardt]

Good question, I do not know this. Maybe try asking this at stackoverflow.com/.