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Wiki MarkupC99 says {{getenv()}} has the following behavior \ [[ISO/IEC 9899:1999|AA. Bibliography#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.

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Compliant Solution (Windows)

Wiki MarkupWindows provides the [{{getenv_s()}} and {{\_wgetenv_s()}}|http://msdn.microsoft.com/en-us/library/tb2sfw2z(VS.80).aspx] functions for getting a value from the current environment \[ [MSDN|AA. Bibliography#MSDN]\].

char *tmpvar;
char *tempvar;
size_t requiredSize;

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

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

Compliant Solution (Windows)

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Windows also provides the [{{\_dupenv_s()}} and {{\_wdupenv_s()}}|http://msdn.microsoft.com/en-us/library/ms175774.aspx] functions for getting a value from the current environment \[ [MSDN|AA. Bibliography#MSDN]\].

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, _dupenv_s() and _wdupenv_s() provide a more convenient alternative to getenv_s() and _wgetenv_s().

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Compliant Solution (POSIX)

Wiki MarkupPOSIX provides the [{{strdup()}}|http://www.opengroup.org/onlinepubs/009695399/functions/strdup.html] function, which can make a copy of the environment variable string \ [[Open Group 2004|AA. Bibliography#Open Group 04]\]. The {{strdup()}} function is also included in ISO/IEC PDTR 24731-2 \ [[ISO/IEC PDTR 24731-2|AA. Bibliography#ISO/IEC ISO/IEC PDTR 24731-2]\].

char *tmpvar;
char *tempvar;

const char *temp = getenv("TMP");
if (temp != NULL) {
  tmpvar = strdup(temp);
  if (tmpvar == NULL) {
    /* Handle error */
  }
}
else {
  return -1;
}

temp = getenv("TEMP");
if (temp != NULL) {
  tempvar = strdup(temp);
  if (tempvar == NULL) {
    free(tmpvar);
    tmpvar = NULL;
    /* Handle error */
  }
}
else {
  free(tmpvar);
  tmpvar = NULL;
  return -1;
}

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

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ISO/IEC PDTR 24731-2

Bibliography

Wiki Markup\[[MSDN|AA. Bibliography#MSDN]\] [{{\_dupenv_s()}} and {{\_wdupenv_s()}}|http://msdn.microsoft.com/en-us/library/ms175774.aspx], [{{getenv_s()}}, {{\_wgetenv_s()}}|http://msdn.microsoft.com/en-us/library/tb2sfw2z(VS.80).aspx] \[[Open Group 2004|AA. Bibliography#Open Group 04]\] Chapter 8, and "Environment Variables", [{{strdup}}|http://www.opengroup.org/onlinepubs/009695399/functions/strdup.html] \[[Viega 2003|AA. Bibliography#Viega 03]\] Section 3.6, "Using Environment Variables
[Open Group 2004] Chapter 8, and "Environment Variables", strdup
[Viega 2003] Section 3.6, "Using Environment Variables Securely"

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10. Environment (ENV)      10. Environment (ENV)