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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 allows an implementation, for example, to copy the environmental variable to an internal static buffer and return a pointer to that buffer.
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char *tmpvar;
char *tempvar;
tmpvar = getenv("TMP");
if (!tmpvar) return -1;
tempvar = getenv("TEMP");
if (!tempvar) 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");
}
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Compliant Solution (Windows)
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char *tmpvar;
char *tempvar;
size_t requiredSize;
getenv_s(&requiredSize, NULL, 0, "TMP");
tmpvar= malloc(requiredSize * sizeof(char));
if (!tmpvar) {
/* handle error condition */
}
getenv_s(&requiredSize, tmpvar, requiredSize, "TMP" );
getenv_s(&requiredSize, NULL, 0, "TEMP");
tempvar= malloc(requiredSize * sizeof(char));
if (!tempvar) {
/* handle error condition */
}
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");
}
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Compliant Solution (Windows)
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Microsoft Visual Studio 2005 provides provides the {{\_dupenv_s()}} and {{\_wdupenv_s()}} functions for getting a value from the current environment. \[[Microsoft Visual Studio 2005/.NET Framework 2.0 help pages|http://msdn2.microsoft.com/en-us/library/ms175774(VS.80).aspx]\]. |
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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);
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);
free(tempvar);
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Compliant Solution (POSIX)
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char *tmpvar = strdup(getenv("TMP"));
char *tempvar = strdup(getenv("TEMP"));
if (!tmpvar) return -1;
if (!tempvar) 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");
}
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If an environmental variable does not exist, the call to {{getenv()}} returns a NULL pointer. In these cases, the call to {{strdup()}} should also return a NULL pointer, but it is important to verify this as this behavior is not guaranteed by POSIX \[[Open Group 04|AA. C References#Open Group 04]\] |
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char *tmpvar;
char *tempvar;
char *temp;
if ( (temp = getenv("TMP")) != NULL) {
tmpvar= malloc(strlen(temp)+1);
if (tmpvar != NULL) {
strcpy(tmpvar, temp);
}
else {
/* handle error condition */
}
}
else {
return -1;
}
if ( (temp = getenv("TEMP")) != NULL) {
tempvar= malloc(strlen(temp)+1);
if (tempvar != NULL) {
strcpy(tempvar, temp);
}
else {
/* handle error condition */
}
}
else {
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");
}
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Risk Assessment
Storing the pointer to the string returned by getenv() can result in overwritten environmental data.
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\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.20.4, "Communication with the environment"
\[[Open Group 04|AA. C References#Open Group 04]\] Chapter 8, "Environment Variables", [strdup|http://www.opengroup.org/onlinepubs/009695399/functions/strdup.html]
\[[Viega 03|AA. C References#Viega 03]\] Section 3.6, "Using Environment Variables Securely" |
