Print the environment variables using environ

Question

How to print the environment variables in a C program using "environ".

extern char **environ

Solution 1

#include <unistd.h>
#include <stdio.h>

extern char **environ;
//...

int i = 0;
while(environ[i]) {
  printf("%s\n", environ[i++]); // prints in form of "variable=value"
}

Solution 2

Do you mean

int main(int argc, char **argv, char **envp)
{
    while(*envp!=null) {
        printf("%s\n", *envp);
        envp++;
    }
    return 0;
}

Comments

[nitin_cherian]

exactly what i needed? So environ is actually an array of character pointers right? and each character pointer in the array points to an environment variable right??

[sje397]

Each pointer points to a string (yeah, char array) of the form "variable=value". The last entry is a null-pointer, which ends the loop above.

[R.. GitHub STOP HELPING ICE]

You forgot extern char **environ;. While some nonstandard systems may declare it in a header file, POSIX specifies that it is not available unless you declare it yourself manually.

[R.. GitHub STOP HELPING ICE]

No, extern char **environ; is POSIX standard. This signature for main is completely nonstandard.

[jim mcnamara]

You are completely correct. However. That was why I asked. It compiles and runs correctly under gcc on Solaris 9 & 10, under cc on HPUX 10i, 11.0, and 11i. It also compiled and ran on older versions of DEC Unix and DGUX. The point being that people get **environ and the old int main( int, char **, char *) confused. I don't have my K&R around to verify or I would claim it in there as well. What does your K&R say?