How do I use the C preprocessor to make a substitution with an environment variable

Question

In the code below, I would like the value of THE_VERSION_STRING to be taken from the value of the environment variable MY_VERSION at compile time

Answer 1

If I recall correctly, you can use the command line parameter -D with gcc to #define a value at compile time.

i.e.:

$ gcc file.c -o file -D"THE_VERSION_STRING=${THE_VERSION_STRING}"

Answer 2

In the code below, I would like the value of THE_VERSION_STRING to be taken from the value of the environment variable MY_VERSION at compile time

No, you can't do it like this. The only way to extract environment variables is at runtime with the getenv() function. You will need to explicitly extract the value and copy it to pluginRequires.

If you want the effect of a compile-time constant, then you'll have to specify the definition on the compiler commandline as Seth suggests.