Question

I am new to Linux. I came across this piece of code to print environmental variables. It is kind of confusing me. How can this code print the environmental variables?

#include <stdio.h>

extern char **environ;
int main()
{
    char **var;
    for(var=environ; *var!=NULL;++var)
        printf("%s\n",*var);

    return 0;
}

what is extern here?

Answer 1

If you don’t know what extern means, please find a book to learn C from. It simply means ‘defined somewhere else, but used here’.

The environ global variable is unique amongst POSIX global variables in that it is not declared in any header. It is like the argv array to the program, an array of character pointers each of which points at an environment variable in the name=value format. The list is terminated by a null pointer, just like argv is. There is no count for the environment, though.

for (var = environ; *var != NULL; ++var)
    printf("%s\n", *var);

So, on the first iteration, var points at the first environment variable; then it is incremented to the next, until the value *var (a char *) is NULL, indicating the end of the list.

That loop could also be written as:

char **var = environ;
while (*var != 0)
    puts(*var++);