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C99 says The C standard says getenv() has the following behavior [ISO/IEC 9899:19992011]:

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; if later use is anticipated, the string should be copied , so the copy can be safetly safely referenced as needed.

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

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Code Block

bgColor	#FFcccc
lang	c


char *tmpvar;
char *tempvar;

tmpvar = getenv("TMP");
if (!tmpvar) return -1;
tempvar = getenv("TEMP");
if (!tempvar) 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 */
  }
}

...

Code Block

bgColor	#ccccff
lang	c


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;

...

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

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Code Block

bgColor	#ccccff
lang	c


char *tmpvar;
char *tempvar;
size_t len;

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

...

POSIX provides the strdup() function, which can make a copy of the environment variable string [Open Group 2004]. The strdup() function is also included in ISO/IEC PDTR 24731-2 in Extensions to the C Library—Part II [ISO/IEC PDTR TR 24731-2:2010].

Code Block

bgColor	#ccccff
lang	c


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;

...

This compliant solution uses only the C99 C malloc() and strcpy() functions to copy the string returned by getenv() into a dynamically allocated buffer.

Code Block

bgColor	#ccccff
lang	c


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 {
  return -1;
}

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

Tool	Version	Checker	Description

Section

Related Guidelines

CERT C++ Secure Coding Standard: ENV00-CPP. Do not store the pointer to the string returned by getenv()

ISO/IEC 9899:19992011 Section 7.2022.4, "Communication with the environment"

ISO/IEC TR 17961 (Draft) Using an object overwritten by getenv, localeconv, setlocale, and strerror [libuse]

ISO/IEC PDTR 24731-2

Bibliography

[MSDN] _dupenv_s() and _wdupenv_s(), getenv_s(), _wgetenv_s()
[Open Group 2004] Chapter 8, and "Environment Variables," , strdup
[Viega 2003] Section 3.6, "Using Environment Variables Securely"

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