C program to print environment variables

In C programming, environment variables are global system variables that contain information about the operating system and user environment. These variables can affect how running processes behave on the system. C provides access to environment variables through the third parameter of the main() function.

Syntax

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

Parameters:

• argc − Number of command-line arguments
• argv[] − Array of command-line argument strings
• envp[] − Array of environment variable strings (NULL-terminated)

Example: Print All Environment Variables

This program iterates through the envp array to print all environment variables −

#include <stdio.h>

int main(int argc, char *argv[], char *envp[]) {
    int i;
    
    printf("Environment Variables:<br>");
    printf("=====================<br>");
    
    for (i = 0; envp[i] != NULL; i++) {
        printf("%s<br>", envp[i]);
    }
    
    printf("\nTotal environment variables: %d<br>", i);
    return 0;
}

Environment Variables:
=====================
ALLUSERSPROFILE=C:\ProgramData
CommonProgramFiles=C:\Program Files\Common Files
HOMEDRIVE=C:
NUMBER_OF_PROCESSORS=2
OS=Windows_NT
PATHEXT=.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC
PROCESSOR_ARCHITECTURE=x86
PROCESSOR_IDENTIFIER=x86 Family 6 Model 42 Stepping 7, GenuineIntel
PROCESSOR_LEVEL=6
PROCESSOR_REVISION=2a07
ProgramData=C:\ProgramData
ProgramFiles=C:\Program Files
PUBLIC=C:\Users\Public
SESSIONNAME=Console
SystemDrive=C:
SystemRoot=C:\Windows
windir=C:\Windows

Total environment variables: 17

Alternative Method: Using extern char **environ

Another way to access environment variables is using the global environ variable −

#include <stdio.h>

extern char **environ;

int main() {
    int i = 0;
    
    printf("Environment Variables (using extern environ):<br>");
    printf("=============================================<br>");
    
    while (environ[i] != NULL) {
        printf("%s<br>", environ[i]);
        i++;
    }
    
    return 0;
}

Key Points

• Environment variables are stored as name=value pairs
• The envp array is NULL-terminated like argv
• Environment variables are inherited from the parent process
• Common variables include PATH, HOME, USER, and system-specific variables

Conclusion

C programs can access environment variables through the envp parameter or the global environ variable. This allows programs to adapt their behavior based on system configuration and user preferences.