+#define MAXEVPSIZE 16
+
+static char __fallback_evp[MAXEVPSIZE];
+static char __stdout_evp[MAXEVPSIZE];
+
+static void
+__loadevp(const char* key, char destination[MAXEVPSIZE])
+{
+ char* evp = getenv(key);
+ if (evp != NULL) {
+ strlcpy(destination, evp, MAXEVPSIZE);
+ } else {
+ destination[0] = '\0';
+ }
+}
+
+static void
+__evpinit(void* __unused unused)
+{
+ __loadevp("STDBUF", __fallback_evp);
+ __loadevp("STDBUF1", __stdout_evp);
+}
+
+static char*
+__getevp(int fd)
+{
+ static os_once_t predicate;
+ os_once(&predicate, NULL, __evpinit);
+
+ if (fd == STDOUT_FILENO && __stdout_evp[0] != '\0') {
+ return __stdout_evp;
+ } else if (__fallback_evp[0] != '\0') {
+ return __fallback_evp;
+ } else {
+ return NULL;
+ }
+
+}
+
+/*
+ * Internal routine to determine environment override buffering for a file.
+ *
+ * Sections of the below taken from NetBSD's version of this file under the same license.
+ */
+static int
+__senvbuf(FILE *fp, size_t *bufsize, int *couldbetty)
+{
+ char* evp;
+ char* end;
+ int flags;
+ long size;
+
+ flags = 0; // Default to fully buffered
+
+ if ((evp = __getevp(fp->_file)) == NULL) {
+ return flags;
+ }
+
+ // Look at the first character only to determine buffering mode
+ switch (*evp) {
+ case 'u':
+ case 'U':
+ flags |= __SNBF;
+ break;
+ case 'l':
+ case 'L':
+ flags |= __SLBF;
+ break;
+ case 'f':
+ case 'F':
+ // Default flags is fully buffered
+ break;
+ default:
+ // Unexpected buffering mode, use default fully buffered
+ return flags;
+ }
+ // User specified envrionment defaults have higher priority than tty defaults
+ *couldbetty = 0;
+
+ // Advance the envrionment variable pointer, so we can attempt to parse the number
+ evp++;
+ if (!isdigit(*evp)) {
+ return flags; // No number found, this protects us from negative size values
+ }
+
+ size = strtol_l(evp, &end, 10, LC_C_LOCALE);
+ if (*end != '\0') {
+ return flags;
+ }
+
+ if (size <= 0) {
+ return __SNBF; // Override with unbuffered if the buffer size is 0
+ }
+
+ // We had a non zero buffer, cap it and return the flags;
+ *bufsize = size > MAXBUFSIZE ? MAXBUFSIZE : size;
+ return flags;
+}