Memory fault with the use of const char*?

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

See http://stackoverflow.com/questions/4932848/strcmp-behaviour — Rohit Karlupia, Apr 11 '12 at 06:07

score 4 · Accepted Answer · answered Apr 11 '12 at 06:02

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…
}

score 0 · Answer 2 · answered Apr 11 '12 at 06:03

0

According to the man page,

The getenv() function returns a pointer to the value in the environment, or NULL if there is no match.

So if it doesn't exist, it returns NULL, which causes digestion problems for strcmp().

answered Apr 11 '12 at 06:03

glglgl

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Memory fault with the use of const char*?

2 Answers2