According to tutorialsonpoint.com:

The *getenv function searches for the environment string pointed to by name and returns the associated value to the string.

Where name is the C string containing the name of the requested variable

That page provides a simple example after that description, but such example doesn't explain step by step how this function actually works. So I seeked the code of this function in the net, and I managed to get a "replica" of the *getenv function called *_getenv, here it is:

char *_getenv(const char *name)
{
    int i, j;
    int status;

for (i = 0; environ[i] != NULL; i  )
{
    status = 1;
    for (j = 0; environ[i][j] != '='; j  )
    {
        if (name[j] != environ[i][j])
        {
            status = 0;
            break;
        }
    }

    if (status)
    {
        return (&environ[i][j   1]);
    }
}
return (NULL);
}

getEnv = _getenv("PATH");

I know that the variable environ points to an array of pointers to strings called the "environment". The last pointer in this array has the value NULL. Meaning that, in the first for cycle this function will loop over the array until it finds a NAME called NULL

What I don't get is what happens in the second for cycle, where the final condition to be met is environ[i][j] != '=' and also the if statement where (&environ[i][j 1]) is returned, I mean, why is it adding 1 to the j counter when returning the address?

CodePudding user response：

what happens in the second for cycle,

The function checks if the value before = is equal to name.

name       = "somevariable"
environ[i] = "somevariable=somevalue"
              ^^^^^^^^^^^^
              ^^^^^^^^^^^^ ----------- are these parts equal?

So compare if each byte of name is equal to environ[i] up until environ[i][??] = '=', so:

bool are_equal = true; // assume they are equal
for (j = 0; environ[i][j] != '='; j  ) { // for each character in environ[i] up until equal sign
    if (name[j] != environ[i][j]) { // check if bytes are equal
        are_equal = false; // if they are not, we notify and break
        break;
    }
}
if (are_equal) { do_smthg() }

why is it adding 1 to the j counter when returning the address?

environ[i][j] points to '='.

environ[i] = "somevariable=somevalue"
                          ^ ------------ [j]
                           ^ ----------  [j   1]

We want to return the pointer to the value, excluding the = sign, so the function returns a pointer to envron[i][j 1].