Allow nil value in setenv.

* sysif.c (setenv_wrap): If value is nil, and overwrite is
missing or t, call unsetenv.

* txr.1: Documented.

2 files changed, 51 insertions, 3 deletions

diff --git a/sysif.c b/sysif.c
index 3b8341cc..d0a06ca0 100644
--- a/sysif.c
+++ b/sysif.c
@@ -703,8 +703,12 @@ val getenv_wrap(val name)
 static val setenv_wrap(val name, val value, val overwrite)
 {
   char *nameu8 = utf8_dup_to(c_str(name));
-  char *valu8 = utf8_dup_to(c_str(value));
-  setenv(nameu8, valu8, default_arg(overwrite, t) != nil);
+  char *valu8 = value ? utf8_dup_to(c_str(value)) : 0;
+  int ovw = default_arg(overwrite, t) != nil;
+  if (valu8)
+    setenv(nameu8, valu8, ovw);
+  else if (ovw)
+    unsetenv(nameu8);
   free(valu8);
   free(nameu8);
   return value;
diff --git a/txr.1 b/txr.1
index fd4dd621..5d817140 100644
--- a/txr.1
+++ b/txr.1
@@ -35353,6 +35353,19 @@ function creates or modifies the environment variable indicated by
 The
 .meta value
 string argument specifies the new value for the variable.
+If
+.meta value
+is
+.codn nil ,
+then
+.code setenv
+behaves like
+.codn unsetenv ,
+except that it observes the
+.meta overwrite-p
+argument. That is to say, the meaning of a null
+.meta value
+is that the variable is to be removed.
 
 If the
 .meta overwrite-p
@@ -35366,11 +35379,22 @@ effectively giving rise to a two-argument form of
 .code setenv
 which creates or overwrites environment variables.
 
+A variable removal is deemed to be an overwrite.
+Thus if both
+.meta value
+and
+.meta overwrite-p
+are
+.codn nil ,
+then
+.code setenv
+does nothing.
+
 The
 .code setenv
 function unconditionally returns
 .meta value
-regardless of whether or not it overwrites an existing variable.
+regardless of whether or not it overwrites or removes an existing variable.
 
 The
 .code unsetenv
@@ -35380,6 +35404,26 @@ specified by
 if it exists. On some platforms, it instead sets the environment variable
 to the empty string.
 
+Note: supporting removal semantics in
+.code setenv
+allows for the following simple save/modify/restore pattern:
+
+.cblk
+  (let* ((old-val (getenv "SOME-VAR")))
+    (unwind-protect
+      (progn (setenv "SOME-VAR" new-val)
+             ...)
+      (setenv "SOME-VAR" old-val)))
+.cble
+
+This works in the case when
+.code SOME-VAR
+exists, as well as in the case that it doesn't exist.
+In both cases, its previous value or, respectively, non-existence,
+is restored by the
+.code unwind-protect
+cleanup form.
+
 .SS* System Programming
 .coNP Accessor @ errno
 .synb