Second try at a portable unsetenv().

src/port/unsetenv.c

@@ -0,0 +1,56 @@
+/*-------------------------------------------------------------------------
+ *
+ * unsetenv.c
+ *	  unsetenv() emulation for machines without it
+ *
+ * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  $PostgreSQL: pgsql/src/port/unsetenv.c,v 1.1 2004/05/05 21:18:29 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+
+void
+unsetenv(const char *name)
+{
+	char  *envstr;
+
+	if (getenv(name) == NULL)
+		return;					/* no work */
+
+	/*
+	 * The technique embodied here works if libc follows the Single Unix Spec
+	 * and actually uses the storage passed to putenv() to hold the environ
+	 * entry.  When we clobber the entry in the second step we are ensuring
+	 * that we zap the actual environ member.  However, there are some libc
+	 * implementations (notably recent BSDs) that do not obey SUS but copy
+	 * the presented string.  This method fails on such platforms.  Hopefully
+	 * all such platforms have unsetenv() and thus won't be using this hack.
+	 *
+	 * Note that repeatedly setting and unsetting a var using this code
+	 * will leak memory.
+	 */
+
+	envstr = (char *) malloc(strlen(name) + 2);
+	if (!envstr)				/* not much we can do if no memory */
+		return;
+
+	/* Override the existing setting by forcibly defining the var */
+	sprintf(envstr, "%s=", name);
+	putenv(envstr);
+
+	/* Now we can clobber the variable definition this way: */
+	strcpy(envstr, "=");
+
+	/*
+	 * This last putenv cleans up if we have multiple zero-length names
+	 * as a result of unsetting multiple things.
+	 */
+	putenv(envstr);
+}