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Showing content from https://en.cppreference.com/w/cpp/language/../../cpp/../c/header/../program/getenv.html below:

getenv, getenv_s - cppreference.com

char *getenv( const char *name );

(1) errno_t getenv_s( size_t *restrict len, char *restrict value,
                  rsize_t valuesz, const char *restrict name ); (2) (since C11)

1) Searches for an environmental variable with name name in the host-specified environment list and returns a pointer to the string that is associated with the matched environment variable. The set of environmental variables and methods of altering it are implementation-defined.

This function is not required to be thread-safe. Another call to

getenv

, as well as a call to the POSIX functions

setenv()

,

unsetenv()

, and

putenv()

may invalidate the pointer returned by a previous call or modify the string obtained from a previous call.

Modifying the string returned by getenv invokes undefined behavior.

2)

Same as

(1)

, except that the values of the environment variable is written to the user-provided buffer

value

(unless null) and the number of bytes written is stored in the user-provided location

*len

(unless null). If the environment variable is not set in the environment, zero is written to

*len

(unless null) and

'\0'

is written to

value[0]

(unless null). In addition, the following errors are detected at runtime and call the currently installed

constraint handler

function:

As with all bounds-checked functions, getenv_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdlib.h>.
[edit] Parameters name - null-terminated character string identifying the name of the environmental variable to look for len - pointer to a user-provided location where getenv_s will store the length of the environment variable value - pointer to a user-provided character array where getenv_s will store the contents of the environment variable valuesz - maximum number of characters that getenv_s is allowed to write to dest (size of the buffer) [edit] Return value

1) character string identifying the value of the environmental variable or null pointer if such variable is not found.

2) zero if the environment variable was found, non-zero if it was not found or if a runtime constraint violation occurred. On any error, writes zero to *len (unless len is a null pointer).

[edit] Notes

On POSIX systems, the environment variables are also accessible through the global variable environ, declared as extern char **environ; in <unistd.h>, and through the optional third argument, envp, of the main function.

The call to getenv_s with a null pointer for value and zero for valuesz is used to determine the size of the buffer required to hold the entire result.

[edit] Example 
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    const char *name = "PATH";
    const char *env_p = getenv(name);
    if (env_p)
        printf("Your %s is %s\n", name, env_p);
}

Possible output:

Your PATH is /home/gamer/.local/bin:/usr/local/bin:/usr/bin:/bin:/usr/share/games
[edit] References
[edit] See also

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