Makefile variables and getenv()

There are three types of variables in the Makefile

• makes’ environment variables
• command-line
• variables defined in the Makefile

Priority

1. command-line
2. variables defined in the Makefile
3. environment variable

There are an exception (-eflag):

Variables in make can come from the environment in which make is run. Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value. However, an explicit assignment in the makefile, or with a command argument, overrides the environment. (If the ‘-e’ flag is specified, then values from the environment override assignments in the makefile. See Summary of Options. But this is not recommended practice.)

getenv()

When make runs a recipe, some variables defined in the makefile are placed into the environment of each command make invokes. By default, only variables that came from the make’s environment or set on its command line are placed into the environment of the commands. You can use the export directive to pass other variables. See Communicating Variables to a Sub-make, for full details.

So according to the text above, we can use getenv in C file to get the variable passed by the command line. Here is an example:

C file:

#include <stdio.h>
#include <stdlib.h>

int main() {
  char *mainargs = NULL;
  mainargs = getenv("mainargs");
  if(mainargs)
        printf("mainargs = %s!!!\n", mainargs);
  mainargs = getenv("hello");
  if(mainargs)
        printf("hello = %s!!!\n", mainargs);
  return 0;
}

Makefile:

hello=123

run: getenv
        @./getenv
        @echo $(hello)

getenv : getenv.c
        @gcc -o getenv getenv.c

we can see that the command-line variable can be gotten through getenv, but the hello variable defined in the Makefile cannot be gotten through getenv.