In this article, I will discuss some of the most simple ways to access environment variables in C language programming.

Approach 1

In this method, we simply used theenviron variable by extending it using the extern keyword.

Code

#include<stdio.h>// https://man7.org/linux/man-pages/man7/environ.7.htmlexternchar**environ;intmain(intargc,char**argv){// list out all the environment variablesfor(inti=0;environ[i]!=NULL;i++){printf("%d: %s\n",i,envp[i]);}return0;}

Check outenvironman page for more info.

Approach 2

Most of the students don't know that themain function can also receive a third argument which is nothing but the array of environment variables.

NOTE: The third argument may not support all compilers. But it should work on the latest compiler versions.

Code

#include<stdio.h>intmain(intargc,char**argv,char**envp){// list out all the environment variablesfor(inti=0;envp[i]!=NULL;i++){printf("%d: %s\n",i,envp[i]);}return0;}

Approach 3

In this method, we use thegetenv andsetenv functions defined

getenv takes one argument which is the environment variable name and returns a null-terminated char pointer. If the passed name does not exist, it returnsNULL.

setenv takes three arguments. The first one is the environment variable name, the second is its value and the third one is an int flag which if non-zero overwrites the already existing environment variable with the same name.

NOTE:setenv doesn't set the environment variable permanently but for the current process and its child process only. It meanssetenv the environment variables are visible only in the current process and its child ones.

Check out some other functions likeclearenv, andputenv inman page for more operations.

Code

#include<stdio.h>#include<stdlib.h> // setenv, getenvintmain(intargc,char**argv){char*user=getenv("USER"),*hosttype=getenv("HOSTTYPE"),*hostname=getenv("HOSTNAME");printf("USER: %s\n",user?user:"null");printf("HOSTTYPE: %s\n",hosttype?hosttype:"null");printf("HOSTNAME: %s\n",hostname?hostname:"null");setenv("USER","namantam1",1);printf("updated USER: %s\n",getenv("USER"));return0;}

Output

USER: namanHOSTTYPE: x86_64HOSTNAME: nullupdated USER: namantam1

Application

• We can set log verbosity based on the environment variable.
• One can set the environment variable in the parent process and access the same in its child process. Check outthis article to know how to clone the current process into a child process.

❤️Thank you so much for reading this article. I'm a passionate engineering student learning new things so If you find any mistakes or have any suggestions please let me know in the comments.

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