The environment in which a C program is executing may provide to it a set of named environment strings which it can use to determine its behaviour.

#include <stdlib.h>
char *getenv(const char *name);

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
errno_t getenv_s(size_t *lenp,
                 char *val,
                 rsize_t max,
                 const char *name);

getenv returns a pointer to the null-terminated environment string identified by the null-terminated string name. If no such string is found, it returns NULL. Although the string appears to be modifiable, the result should not be modified, so it's usually best to assign it straight to a const char * at once.

getenv_s does the same, but copies the environment string into the array of max bytes starting at val, including a null terminator, and writes the number of bytes (excluding the null) to *lenp, if lenp is not null, and finally returns zero. If the string (including the terminating null) is too long, or the named string is not found, *lenp is set to zero if lenp is not null, and the function returns non-zero. val[0] is also set to a null character if the named string is not found and max is not zero.