Page History

The C Standard, subclause 7.22.4.6, paragraph 4 [ISO/IEC 9899:2011] states that getenv() has the following behavior, states:

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.

This paragraph gives an implementation the latitude, for example, to return a pointer to a statically allocated buffer of static storage duration.   Consequently, do not store this pointer because it may be overwritten by a subsequent call to the getenv() function or invalidated as a result of changes made by modifications to the environment list through calls to the POSIX functions putenv() or  setenv(), or other implementation-specific 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; if . If later use is anticipated, the string should be copied so the copy can be safely referenced as needed.

...

This noncompliant code example compares attempts compare the value of the TMP and TEMP environment variables to determine if they are the same:

...

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
void func(void) {
  char *tmpvar;
  char *tempvar;

  const char *temp = getenv("TMP");
  if (temp != NULL) {
    tmpvar = (char *)malloc(strlen(temp)+1);
    if (tmpvar != NULL) {
      strcpy(tmpvar, temp);
    } else {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }

  temp = getenv("TEMP");
  if (temp != NULL) {
    tempvar = (char *)malloc(strlen(temp)+1);
    if (tempvar != NULL) {
      strcpy(tempvar, temp);
    } else {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }

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

Compliant Solution (Annex K)

The C Standard, Annex K, provides the getenv_s() function for getting a value from the current environment. However, according to the standard, getenv_s() can still have data races with other threads of execution that modify the environment list.

#define __STDC_WANT_LIB_EXT1__ 1
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
void func(void) {
  char *tmpvar;
  char *tempvar;
  size_t requiredSize;
  errno_t err;
  err = getenv_s(&requiredSize, NULL, 0, "TMP");

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

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

  if (err) {
    /* Handle error */
  }
  if (strcmp(tmpvar, tempvar) == 0) {
    printf("TMP and TEMP are the same.\n");
  } else {
    printf("TMP and TEMP are NOT the same.\n");
  }
  free(tmpvar);
  tmpvar = NULL;
  free(tempvar);
  tempvar = NULL;
}

...

Microsoft Windows provides the _dupenv_s() and wdupenv_s() functions for getting a value from the current environment [MSDN]. The _dupenv_s() function searches the list of environment variables for a specified name. If the name is found, a buffer is allocated; the variable's value is copied into the buffer, and the buffer's address and number of elements are returned. By allocating the buffer itself, The _dupenv_s() and _wdupenv_s() functions provide a more convenient alternative alternatives to getenv_s() and _wgetenv_s() because each function handles buffer allocation directly.

The calling program caller is responsible for freeing any allocated buffers returned by these functions by calling free().

...

Storing the pointer to the string returned by getenv(), localeconv(), setlocale(), or strerror() can result in overwritten data.

...

Related Guidelines

Bibliography

...