Which C header defines the common variable names (PATH, HOME, IFS...)? - c

Question

Chapter 8 of POSIX standard define a list of commonly used environment variables "that are frequently exported by widely used command interpreters and applications".
However I cannot find any C header providing their names in any of my unix-like systems.
I'm looking for something like:
#define ENV_PATH "PATH"
#define ENV_USER "USER"
#define ENV_IFS "IFS"
...
Where I can find such header? Any OS-specific header would work: I just don't want to invent names for the constants myself.
edit
If you are used to only mainstream operating systems, you might ask: why you want to use constants here? $PATH is always $PATH everywhere!
This is not actually true.
In Plan 9 from Bell Labs, environment variables are usually lowercase (apparently due to aesthetics).
In Jehanne, a new operating system derived by Plan 9, I'm reconsidering this design choice, to ease the integration of POSIX tools. However, since I like the lowercase environment variables, I'd like to be able to easily switch back to lowercase names when Jehanne will be "the one true operating system" :-D

Answer 1

As stated in the comments, there is no header file that provides any POSIX-specified list of environment variables used by applications and utilities.
A list of "certain variables that are frequently exported by widely used command interpreters and applications" can be found at http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08. (The actual environment variable list requires reformatting but here it is anyway...)
It is unwise to conflict with certain variables that are frequently
exported by widely used command interpreters and applications:
ARFLAGS IFS MAILPATH PS1
CC LANG MAILRC PS2
CDPATH LC_ALL MAKEFLAGS PS3
CFLAGS LC_COLLATE MAKESHELL PS4
CHARSET LC_CTYPE MANPATH PWD
COLUMNS LC_MESSAGES MBOX RANDOM
DATEMSK LC_MONETARY MORE SECONDS
DEAD LC_NUMERIC MSGVERB SHELL
EDITOR LC_TIME NLSPATH TERM
ENV LDFLAGS NPROC TERMCAP
EXINIT LEX OLDPWD TERMINFO
FC LFLAGS OPTARG TMPDIR
FCEDIT LINENO OPTERR TZ
FFLAGS LINES OPTIND USER
GET LISTER PAGER VISUAL
GFLAGS LOGNAME PATH YACC
HISTFILE LPDEST PPID YFLAGS
HISTORY MAIL PRINTER
HISTSIZE MAILCHECK PROCLANG
HOME MAILER PROJECTDIR
To access the value of an environment variable, use the getenv() function.
The exec() function documentation specifies the char **environ variable:
In addition, the following variable, which must be declared by the
user if it is to be used directly:
extern char **environ;
is initialized as a pointer to an array of character pointers to the
environment strings. The argv and environ arrays are each terminated
by a null pointer. . The null pointer terminating the argv array is not counted in argc.
Applications can change the entire environment in a single operation
by assigning the environ variable to point to an array of character
pointers to the new environment strings. After assigning a new value
to environ, applications should not rely on the new environment
strings remaining part of the environment, as a call to getenv(),
putenv(), setenv(), unsetenv(), or
any function that is dependent on an environment variable may, on
noticing that environ has changed, copy the environment strings to a
new array and assign environ to point to it.
Any application that directly modifies the pointers to which the
environ variable points has undefined behavior.
Conforming multi-threaded applications shall not use the environ
variable to access or modify any environment variable while any other
thread is concurrently modifying any environment variable. A call to
any function dependent on any environment variable shall be considered
a use of the environ variable to access that environment variable.

Answer 2

You can do something like that and in get_env_variables function you modify what you want. Just create something like a strncmp function for check if you want to modify this variable or not.
int main(int ac, char **av, char **env){
int i = 0;
while (env[i] != NULL){
env[i] = get_env_variables(env[i]);
i++;
}
}
char *get_env_variables(char *str) {
// PUT SOME CODE HERE
}
EDIT : don't forgot to return new env[i].