How do I access environment variables in C?

Environment variables are in the global variable environ. This points to an array of pointers to strings. To access the variable, we declare it with extern. We can then iterate over it like a normal array.

#include <stdio.h>
extern char** environ;
int main(void) {
  for (size_t i = 0; environ[i] != NULL; i++) {
    printf("%s\n", environ[i]);
  }
  return 0;
}

This prints some interesting entries:

% ./a.out
TERM_SESSION_ID=w2t0p0:383E38D0-AD02-44CC-8549-291D5A58E4B2
SSH_AUTH_SOCK=/private/tmp/com.apple.launchd.Wl0phK4Ypi/Listeners
Apple_PubSub_Socket_Render=/private/tmp/com.apple.launchd.MBf7K7qlp1/Render
COLORFGBG=0;15
ITERM_PROFILE=Default
XPC_FLAGS=0x0
LANG=en_GB.UTF-8
PWD=/Users/jhf/dev/tmp/environ
SHELL=/bin/zsh
SECURITYSESSIONID=186a7
TERM_PROGRAM_VERSION=3.0.13
TERM_PROGRAM=iTerm.app
PATH=/Users/jhf/.opam/system/bin:/Users/jhf/Library/Python/2.7/bin:/Users/jhf/Library/Android/sdk/platform-tools:/Users/jhf/Library/Android/sdk/tools:/Users/jhf/go/bin:/Users/jhf/bin/Nim/bin:/Users/jhf/bin/depot_tools:/Users/jhf/.local/bin:/Users/jhf/Library/Haskell/bin:/Users/jhf/bin:/Users/jhf/.rbenv/shims:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
DISPLAY=/private/tmp/com.apple.launchd.XUm95WeFlF/org.macosforge.xquartz:0
COMMAND_MODE=unix2003
TERM=xterm-256color
HOME=/Users/jhf
TMPDIR=/var/folders/cl/wwjnrg0x34n20fcz0wp4c1sh0000gn/T/
USER=jhf
XPC_SERVICE_NAME=0
LOGNAME=jhf
__CF_USER_TEXT_ENCODING=0x1F5:0x0:0x2
ITERM_SESSION_ID=w2t0p0:383E38D0-AD02-44CC-8549-291D5A58E4B2
SHLVL=1
OLDPWD=/Users/jhf/dev/tmp
ZSH=/Users/jhf/.oh-my-zsh
GOPATH=/Users/jhf/go
PAGER=less
LESS=-R
LC_CTYPE=en_GB.UTF-8
LSCOLORS=Gxfxcxdxbxegedabagacad
OCAML_TOPLEVEL_PATH=/Users/jhf/.opam/system/lib/toplevel
PERL5LIB=/Users/jhf/.opam/system/lib/perl5:
MANPATH=:/Users/jhf/.opam/system/man
OPAMUTF8MSGS=1
CAML_LD_LIBRARY_PATH=/Users/jhf/.opam/system/lib/stublibs:/usr/local/lib/ocaml/stublibs
_=/Users/jhf/dev/tmp/environ/./a.out

Tagged #environment-variables, #c, #programming, #posix.

👋 I'm Jim, a full-stack product engineer. Want to build an amazing product and a profitable business? Read more about me or Get in touch!

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