Question

Hi I’m working with C++ on Linux platform, i have to get the value of a environment variable(user defined) and use it further.
Following is the code I’m using,

const char *show_line = getenv ("MY_SHOW_LINES");

bool myFlag = (strcmp(show_line, "1") == 0) ? false : true;

Above code executes properly when i set the value of environment variable(MY_SHOW_LINES) equal to 1 but when i unset the value of it(i.e. unset MY_SHOW_LINES).
Above code gives memory fault.
Any suggestion on above UN-expected behavior??
Thanks in advance

Answer 1

If you pass a NULL pointer to strcmp, you get undefined behavior, in this case a very likely crash.

Are you aware that pointers can be NULL, i.e. purposely invalid?

You should guard against this possibility:

bool myFlag = false; // set default value
if ( show_line != NULL ) {
    myFlag = ( strcmp(show_line, "1") != 0 ); // "? false : true" same as NOT…
}