New function: replace-env.

Using this new function together with env, it's now possible
to save the set of environment variables, clobber it to a
specified set (possibly empty) and then restore it.

Useful for improved security in running child processes.

* lib.[ch] (chk_substrdup_utf8): New function.

* sysif.c (replace_env): New function.
(sysif_init): Register replace-env intrinsic.

* sysif.h (replace_env): Declared.

* txr.1: Documented.

* stdlib/doc-syms.tl: Updated.

1 files changed, 15 insertions, 0 deletions

diff --git a/lib.c b/lib.c
index 534013eb..cf2d6cf8 100644
--- a/lib.c
+++ b/lib.c
@@ -4293,6 +4293,21 @@ char *chk_strdup_utf8(const char *str)
   return copy;
 }
 
+char *chk_substrdup_utf8(const char *str, size_t off, size_t len)
+{
+  size_t size = strlen(str) + 1, nchar;
+  char *copy;
+  if (off >= size - 1)
+    return chk_strdup_utf8("");
+  if (off + len < off)
+    uw_throw(error_s, lit("string size overflow"));
+  nchar = min(size - off, len + 1);
+  copy = coerce(char *, chk_malloc(nchar));
+  memcpy(copy, str, nchar - 1);
+  copy[nchar - 1] = 0;
+  return copy;
+}
+
 unsigned char *chk_strdup_8bit(const wchar_t *str)
 {
   size_t nchar = wcslen(str) + 1, i;