Section 7.5. Environment List

7.5. Environment List

Each program is also passed an environment list. Like the argument list, the environment list is an array of character pointers, with each pointer containing the address of a null-terminated C string. The address of the array of pointers is contained in the global variable environ:

     extern char **environ; 

For example, if the environment consisted of five strings, it could look like Figure 7.5. Here we explicitly show the null bytes at the end of each string. We'll call environ the environment pointer, the array of pointers the environment list, and the strings they point to the environment strings.

Figure 7.5. Environment consisting of five C character strings

By convention, the environment consists of

name=value

strings, as shown in Figure 7.5. Most predefined names are entirely uppercase, but this is only a convention.

Historically, most UNIX systems have provided a third argument to the main function that is the address of the environment list:

     int main(int argc, char *argv[], char *envp[]); 

Because ISO C specifies that the main function be written with two arguments, and because this third argument provides no benefit over the global variable environ, POSIX.1 specifies that environ should be used instead of the (possible) third argument. Access to specific environment variables is normally through the getenv and putenv functions, described in Section 7.9, instead of through the environ variable. But to go through the entire environment, the environ pointer must be used.