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The getenv() function searches an environment list for a string that matches a specified name, and returns a pointer to a string associated with the matched list member.

Section 7.20.4.5 of C99 states: \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]

The set of environment names and the method for altering the environment list are implementation-defined.

The getenv() function searches an environment list for a string that matches a specified name, and returns a pointer to a string associated with the matched list member. Depending on the implementation, multiple environment variables with the same name may be allowed and can cause unexpected results if a program cannot consistently choose the same value. The GNU glibc library addresses this issue in getenv() and setenv() by always using the first variable it encounters and ignoring the rest. Other implementations are following suit, although it is unwise to rely on this.

...

extern char ** environ;

int main(void) {
  if (multiple_vars_with_same_name()) {
    printf("Someone may be tampering.\n");
    return 1;
  }

  /* ... */

  return 0;
}

int multiple_vars_with_same_name(void) {
  size_t i;
  size_t j;
  size_t k;
  size_t l;
  size_t len_i;
  size_t len_j;

  for(i = 0; environ[i] != NULL; i++) {
    for(j = i; environ[j] != NULL; j++) {
      if (i != j) {
        k = 0;
        l = 0;

        len_i = strlen(environ[i]);
        len_j = strlen(environ[j]);

        while (k < len_i && l < len_j) {
          if (environ[i][k] != environ[j][l])
            break;

          if (environ[i][k] == '=')
            return 1;

          k++;
          l++;
        }
      }
    }
  }
  return 0;
}

...

The following non-compliant code behaves differently when compiled under test and run on Linux and Microsoft Windows implementationsplatforms.

char *temp;

if (putenv("TEST_ENV=foo") != 0) {
  /* Handle Error */
}
if (putenv("Test_ENV=bar") != 0) {
  /* Handle Error */
}

temp = getenv("TEST_ENV");

if (temp == NULL) {
  /* Handle Error */
}

printf("%s\n", temp);

On a test an IA-32 Linux machine with GCC Compiler Version 3.4.4, this code prints:

foo

Whereas, on a test an IA-32 Windows XP machine with Microsoft Visual C++ 2008 Express, it prints:

...