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C99 defines {{getenv()}} to have the following behavior: \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]:

The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function.

Consequently, it is best not to store this pointer as it may be overwritten by a subsequent call to the getenv() function or invalidated as a result of changes made to the environment list through calls to putenv(), setenv(), or other means. Storing the pointer for later use can result in a dangling pointer or a pointer to incorrect data. This string should be referenced immediately and discarded, or copied so that the copy may be referenced safely at a later time.

The getenv() function is not thread-safe. Make sure to address any possible race conditions resulting from the use of this function.

...

char *tmpvar;
char *tempvar;
size_t requiredSize;

getenv_s(&requiredSize, NULL, 0, "TMP");
tmpvar = (char *)malloc(requiredSize * sizeof(char));
if (!tmpvar) {
   /* Handle Errorerror */
}
getenv_s(&requiredSize, tmpvar, requiredSize, "TMP" );

getenv_s(&requiredSize, NULL, 0, "TEMP");
tempvar = (char *)malloc(requiredSize * sizeof(char));
if (!tempvar) {
   free(tmpvar);
   tmpvar = NULL;
   /* Handle Errorerror */
}
getenv_s(&requiredSize, tempvar, requiredSize, "TEMP" );

if (strcmp(tmpvar, tempvar) == 0) {
  puts("TMP and TEMP are the same.\n");
}
else {
  puts("TMP and TEMP are NOT the same.\n");
}
free(tmpvar);
tmpvar = NULL;
free(tempvar);
tempvar = NULL;

...

It is the calling program's responsibility to free the memory by calling free()any allocated buffers returned by these functions.

char *tmpvar;
char *tempvar;
size_t len;

errno_t err = _dupenv_s(&tmpvar, &len, "TMP");
if (err) return -1;
errno_t err = _dupenv_s(&tempvar, &len, "TEMP");
if (err) {
  free(tmpvar);
  tmpvar = NULL;
  return -1;
}

if (strcmp(tmpvar, tempvar) == 0) {
  puts("TMP and TEMP are the same.\n");
}
else {
  puts("TMP and TEMP are NOT the same.\n");
}
free(tmpvar);
tmpvar = NULL;
free(tempvar);
tempvar = NULL;

...