Patchwork [U-Boot,v3,07/18] env: Add support for callbacks to environment vars

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Submitter Joe Hershberger
Date Nov. 1, 2012, 4:39 p.m.
Message ID <1351787996-24560-8-git-send-email-joe.hershberger@ni.com>
Download mbox | patch
Permalink /patch/196293/
State Superseded
Delegated to: Tom Rini
Headers show

Comments

Joe Hershberger - Nov. 1, 2012, 4:39 p.m.
Add support for per-variable callbacks to the "hashtable" functions.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
---
Changes in v3:
- Use Marek's linker lists instead of implementing it directly
- Rebase onto latest master
- Add flags parameter to callbacks
- Implement reverse search in env_attr_lookup()
- Fix space skipping in env_attr_lookup()
- All errors coming back from hsearch_r() are no longer fatal.  Don't
abort import on failed ENTER action.
- Removed checkpatch.pl warning

Changes in v2:
- Added much-needed documentation
- Factored out prevch and nextch in env_attr_lookup()

 README                 |  30 +++++++
 common/Makefile        |   4 +
 common/env_attr.c      | 222 +++++++++++++++++++++++++++++++++++++++++++++++++
 common/env_callback.c  | 132 +++++++++++++++++++++++++++++
 include/env_attr.h     |  55 ++++++++++++
 include/env_callback.h |  62 ++++++++++++++
 include/env_default.h  |   5 ++
 include/environment.h  |   2 +
 include/search.h       |   5 ++
 lib/hashtable.c        |  67 ++++++++++++++-
 10 files changed, 580 insertions(+), 4 deletions(-)
 create mode 100644 common/env_attr.c
 create mode 100644 common/env_callback.c
 create mode 100644 include/env_attr.h
 create mode 100644 include/env_callback.h

Patch

diff --git a/README b/README
index 22fd6b7..2834b52 100644
--- a/README
+++ b/README
@@ -4096,6 +4096,36 @@  Please note that changes to some configuration parameters may take
 only effect after the next boot (yes, that's just like Windoze :-).
 
 
+Callback functions for environment variables:
+---------------------------------------------
+
+For some environment variables, the behavior of u-boot needs to change
+when their values are changed.  This functionailty allows functions to
+be associated with arbitrary variables.  On creation, overwrite, or
+deletion, the callback will provide the opportunity for some side
+effect to happen or for the change to be rejected.
+
+The callbacks are named and associated with a function using the
+U_BOOT_ENV_CALLBACK macro in your board or driver code.
+
+These callbacks are associated with variables in one of two ways.  The
+static list can be added to by defining CONFIG_ENV_CALLBACK_LIST_STATIC
+in the board configuration to a string that defines a list of
+associations.  The list must be in the following format:
+
+	entry = variable_name[:callback_name]
+	list = entry[,list]
+
+If the callback name is not specified, then the callback is deleted.
+Spaces are also allowed anywhere in the list.
+
+Callbacks can also be associated by defining the ".callbacks" variable
+with the same list format above.  Any association in ".callbacks" will
+override any association in the static list. You can define
+CONFIG_ENV_CALLBACK_LIST_DEFAULT to a list (string) to define the
+".callbacks" envirnoment variable in the default or embedded environment.
+
+
 Command Line Parsing:
 =====================
 
diff --git a/common/Makefile b/common/Makefile
index 9e43322..af6e8f9 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -43,6 +43,8 @@  COBJS-y += cmd_nvedit.o
 COBJS-y += cmd_version.o
 
 # environment
+COBJS-y += env_attr.o
+COBJS-y += env_callback.o
 COBJS-y += env_common.o
 COBJS-$(CONFIG_ENV_IS_IN_DATAFLASH) += env_dataflash.o
 COBJS-$(CONFIG_ENV_IS_IN_EEPROM) += env_eeprom.o
@@ -197,6 +199,8 @@  endif
 ifdef CONFIG_SPL_BUILD
 COBJS-$(CONFIG_SPL_YMODEM_SUPPORT) += xyzModem.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += cmd_nvedit.o
+COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_attr.o
+COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_callback.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_common.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += env_nowhere.o
 COBJS-$(CONFIG_SPL_NET_SUPPORT) += miiphyutil.o
diff --git a/common/env_attr.c b/common/env_attr.c
new file mode 100644
index 0000000..7d330a5
--- /dev/null
+++ b/common/env_attr.c
@@ -0,0 +1,222 @@ 
+/*
+ * (C) Copyright 2012
+ * Joe Hershberger, National Instruments, joe.hershberger@ni.com
+ *
+ * See file CREDITS for list of people who contributed to this
+ * project.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ * MA 02111-1307 USA
+ */
+
+#include <common.h>
+#include <env_attr.h>
+#include <errno.h>
+#include <linux/string.h>
+#include <malloc.h>
+
+/*
+ * Iterate through the whole list calling the callback for each found element.
+ * "attr_list" takes the form:
+ *	attributes = [^,:\s]*
+ *	entry = name[:attributes]
+ *	list = entry[,list]
+ */
+int env_attr_walk(const char *attr_list,
+	int (*callback)(const char *name, const char *attributes))
+{
+	const char *entry, *entry_end;
+	char *name, *attributes;
+
+	if (!attr_list)
+		/* list not found */
+		return 1;
+
+	entry = attr_list;
+	do {
+		char *entry_cpy = NULL;
+
+		entry_end = strchr(entry, ENV_ATTR_LIST_DELIM);
+		/* check if this is the last entry in the list */
+		if (entry_end == NULL) {
+			int entry_len = strlen(entry);
+
+			if (entry_len) {
+				/*
+				 * allocate memory to copy the entry into since
+				 * we will need to inject '\0' chars and squash
+				 * white-space before calling the callback
+				 */
+				entry_cpy = malloc(entry_len + 1);
+				if (entry_cpy)
+					/* copy the rest of the list */
+					strcpy(entry_cpy, entry);
+				else
+					return -ENOMEM;
+			}
+		} else {
+			int entry_len = entry_end - entry;
+
+			if (entry_len) {
+				/*
+				 * allocate memory to copy the entry into since
+				 * we will need to inject '\0' chars and squash
+				 * white-space before calling the callback
+				 */
+				entry_cpy = malloc(entry_len + 1);
+				if (entry_cpy) {
+					/* copy just this entry and null term */
+					strncpy(entry_cpy, entry, entry_len);
+					entry_cpy[entry_len] = '\0';
+				} else
+					return -ENOMEM;
+			}
+		}
+
+		/* check if there is anything to process (e.g. not ",,,") */
+		if (entry_cpy != NULL) {
+			attributes = strchr(entry_cpy, ENV_ATTR_SEP);
+			/* check if there is a ':' */
+			if (attributes != NULL) {
+				/* replace the ':' with '\0' to term name */
+				*attributes++ = '\0';
+				/* remove white-space from attributes */
+				attributes = strim(attributes);
+			}
+			/* remove white-space from name */
+			name = strim(entry_cpy);
+
+			/* only call the callback if there is a name */
+			if (strlen(name) != 0) {
+				int retval = 0;
+
+				retval = callback(name, attributes);
+				if (retval) {
+					free(entry_cpy);
+					return retval;
+				}
+			}
+		}
+
+		free(entry_cpy);
+		entry = entry_end + 1;
+	} while (entry_end != NULL);
+
+	return 0;
+}
+
+/*
+ * Search for the last matching string in another string with the option to
+ * start looking at a certain point (i.e. ignore anything beyond that point).
+ */
+static char *reverse_strstr(const char *searched, const char *search_for,
+	const char *searched_start)
+{
+	char *result = NULL;
+
+	if (*search_for == '\0')
+		return (char *)searched;
+
+	for (;;) {
+		char *match = strstr(searched, search_for);
+
+		/*
+		 * Stop looking if no new match is found or looking past the
+		 * searched_start pointer
+		 */
+		if (match == NULL || (searched_start != NULL &&
+		    match + strlen(search_for) > searched_start))
+			break;
+
+		result = match;
+		searched = match + 1;
+	}
+
+	return result;
+}
+
+/*
+ * Retrieve the attributes string associated with a single name in the list
+ * There is no protection on attributes being too small for the value
+ */
+int env_attr_lookup(const char *attr_list, const char *name, char *attributes)
+{
+	const char *entry = NULL;
+
+	if (!attributes)
+		/* bad parameter */
+		return -1;
+	if (!attr_list)
+		/* list not found */
+		return 1;
+
+	entry = reverse_strstr(attr_list, name, NULL);
+	while (entry != NULL) {
+		const char *prevch = entry - 1;
+		const char *nextch = entry + strlen(name);
+
+		/* Skip spaces */
+		while (*prevch == ' ')
+			prevch--;
+		while (*nextch == ' ')
+			nextch++;
+
+		/* check for an exact match */
+		if ((entry == attr_list ||
+		     *prevch == ENV_ATTR_LIST_DELIM) &&
+		    (*nextch == ENV_ATTR_SEP ||
+		     *nextch == ENV_ATTR_LIST_DELIM ||
+		     *nextch == '\0'))
+			break;
+
+		entry = reverse_strstr(attr_list, name, entry);
+	}
+	if (entry != NULL) {
+		int len;
+
+		/* skip the name */
+		entry += strlen(name);
+		/* skip spaces */
+		while (*entry == ' ')
+			entry++;
+		if (*entry != ENV_ATTR_SEP)
+			len = 0;
+		else {
+			const char *delim;
+			static const char delims[] = {
+				ENV_ATTR_LIST_DELIM, ' ', '\0'};
+
+			/* skip the attr sep */
+			entry += 1;
+			/* skip spaces */
+			while (*entry == ' ')
+				entry++;
+
+			delim = strpbrk(entry, delims);
+			if (delim == NULL)
+				len = strlen(entry);
+			else
+				len = delim - entry;
+			memcpy(attributes, entry, len);
+		}
+		attributes[len] = '\0';
+
+		/* success */
+		return 0;
+	}
+
+	/* not found in list */
+	return 2;
+}
diff --git a/common/env_callback.c b/common/env_callback.c
new file mode 100644
index 0000000..b3989c6
--- /dev/null
+++ b/common/env_callback.c
@@ -0,0 +1,132 @@ 
+/*
+ * (C) Copyright 2012
+ * Joe Hershberger, National Instruments, joe.hershberger@ni.com
+ *
+ * See file CREDITS for list of people who contributed to this
+ * project.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ * MA 02111-1307 USA
+ */
+
+#include <common.h>
+#include <environment.h>
+
+/*
+ * Look up a callback function pointer by name
+ */
+struct env_clbk_tbl *find_env_callback(const char *name)
+{
+	struct env_clbk_tbl *clbkp;
+	int i;
+	int num_callbacks = ll_entry_count(struct env_clbk_tbl, env_clbk);
+
+	if (name == NULL)
+		return NULL;
+
+	/* look up the callback in the linker-list */
+	for (i = 0, clbkp = ll_entry_start(struct env_clbk_tbl, env_clbk);
+	     i < num_callbacks;
+	     i++, clbkp++) {
+		if (strcmp(name, clbkp->name) == 0)
+			return clbkp;
+	}
+
+	return NULL;
+}
+
+/*
+ * Look for a possible callback for a newly added variable
+ * This is called specifically when the variable did not exist in the hash
+ * previously, so the blanket update did not find this variable.
+ */
+void env_callback_init(ENTRY *var_entry)
+{
+	const char *var_name = var_entry->key;
+	const char *callback_list = getenv(ENV_CALLBACK_VAR);
+	char callback_name[256] = "";
+	struct env_clbk_tbl *clbkp;
+	int ret = 1;
+
+	/* look in the ".callbacks" var for a reference to this variable */
+	if (callback_list != NULL)
+		ret = env_attr_lookup(callback_list, var_name, callback_name);
+
+	/* only if not found there, look in the static list */
+	if (ret)
+		ret = env_attr_lookup(ENV_CALLBACK_LIST_STATIC, var_name,
+			callback_name);
+
+	/* if an association was found, set the callback pointer */
+	if (!ret && strlen(callback_name)) {
+		clbkp = find_env_callback(callback_name);
+		if (clbkp != NULL)
+			var_entry->callback = clbkp->callback;
+	}
+}
+
+/*
+ * Called on each existing env var prior to the blanket update since removing
+ * a callback association should remove its callback.
+ */
+static int clear_callback(ENTRY *entry)
+{
+	entry->callback = NULL;
+
+	return 0;
+}
+
+/*
+ * Call for each element in the list that associates variables to callbacks
+ */
+static int set_callback(const char *name, const char *value)
+{
+	ENTRY e, *ep;
+	struct env_clbk_tbl *clbkp;
+
+	e.key	= name;
+	e.data	= NULL;
+	hsearch_r(e, FIND, &ep, &env_htab, 0);
+
+	/* does the env variable actually exist? */
+	if (ep != NULL) {
+		/* the assocaition delares no callback, so remove the pointer */
+		if (value == NULL || strlen(value) == 0)
+			ep->callback = NULL;
+		else {
+			/* assign the requested callback */
+			clbkp = find_env_callback(value);
+			if (clbkp != NULL)
+				ep->callback = clbkp->callback;
+		}
+	}
+
+	return 0;
+}
+
+static int on_callbacks(const char *name, const char *value, enum env_op op,
+	int flags)
+{
+	/* remove all callbacks */
+	hwalk_r(&env_htab, clear_callback);
+
+	/* configure any static callback bindings */
+	env_attr_walk(ENV_CALLBACK_LIST_STATIC, set_callback);
+	/* configure any dynamic callback bindings */
+	env_attr_walk(value, set_callback);
+
+	return 0;
+}
+U_BOOT_ENV_CALLBACK(callbacks, on_callbacks);
diff --git a/include/env_attr.h b/include/env_attr.h
new file mode 100644
index 0000000..6ef114f
--- /dev/null
+++ b/include/env_attr.h
@@ -0,0 +1,55 @@ 
+/*
+ * (C) Copyright 2012
+ * Joe Hershberger, National Instruments, joe.hershberger@ni.com
+ *
+ * See file CREDITS for list of people who contributed to this
+ * project.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ * MA 02111-1307 USA
+ */
+
+#ifndef __ENV_ATTR_H__
+#define __ENV_ATTR_H__
+
+#define ENV_ATTR_LIST_DELIM	','
+#define ENV_ATTR_SEP		':'
+
+/*
+ * env_attr_walk takes as input an "attr_list" that takes the form:
+ *	attributes = [^,:\s]*
+ *	entry = name[:attributes]
+ *	list = entry[,list]
+ * It will call the "callback" function with the "name" and attribute as "value"
+ * The callback may return a non-0 to abort the list walk.
+ * This return value will be passed through to the caller.
+ * 0 is returned on success.
+ */
+extern int env_attr_walk(const char *attr_list,
+	int (*callback)(const char *name, const char *value));
+
+/*
+ * env_attr_lookup takes as input an "attr_list" with the same form as above.
+ * It also takes as input a "name" to look for.
+ * If the name is found in the list, it's value is copied into "attributes".
+ * There is no protection on attributes being too small for the value.
+ * It returns -1 if attributes is NULL, 1 if "name" is not found, 2 if
+ * "attr_list" is NULL.
+ * Returns 0 on success.
+ */
+extern int env_attr_lookup(const char *attr_list, const char *name,
+	char *attributes);
+
+#endif /* __ENV_ATTR_H__ */
diff --git a/include/env_callback.h b/include/env_callback.h
new file mode 100644
index 0000000..1a25e3f
--- /dev/null
+++ b/include/env_callback.h
@@ -0,0 +1,62 @@ 
+/*
+ * (C) Copyright 2012
+ * Joe Hershberger, National Instruments, joe.hershberger@ni.com
+ *
+ * See file CREDITS for list of people who contributed to this
+ * project.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ * MA 02111-1307 USA
+ */
+
+#ifndef __ENV_CALLBACK_H__
+#define __ENV_CALLBACK_H__
+
+#include <linker_lists.h>
+#include <search.h>
+
+#define ENV_CALLBACK_VAR ".callbacks"
+
+/* Board configs can define additional static callback bindings */
+#ifndef CONFIG_ENV_CALLBACK_LIST_STATIC
+#define CONFIG_ENV_CALLBACK_LIST_STATIC
+#endif
+
+/*
+ * This list of callback bindings is static, but may be overridden by defining
+ * a new assogiation in the ".callbacks" environment variable.
+ */
+#define ENV_CALLBACK_LIST_STATIC ENV_CALLBACK_VAR ":callbacks," \
+	CONFIG_ENV_CALLBACK_LIST_STATIC
+
+struct env_clbk_tbl {
+	const char *name;		/* Callback name */
+	int (*callback)(const char *name, const char *value, enum env_op op,
+		int flags);
+};
+
+struct env_clbk_tbl *find_env_callback(const char *);
+void env_callback_init(ENTRY *var_entry);
+
+/*
+ * Define a callback that can be associated with variables.
+ * when associated through the ".callbacks" environment variable, the callback
+ * will be executed any time the variable is inserted, overwritten, or deleted.
+ */
+#define U_BOOT_ENV_CALLBACK(name, callback) \
+	ll_entry_declare(struct env_clbk_tbl, name, env_clbk, env_clbk) = \
+	{#name, callback}
+
+#endif /* __ENV_CALLBACK_H__ */
diff --git a/include/env_default.h b/include/env_default.h
index 375e5ca..cff378d 100644
--- a/include/env_default.h
+++ b/include/env_default.h
@@ -24,6 +24,8 @@ 
  * MA 02111-1307 USA
  */
 
+#include <env_callback.h>
+
 #ifdef DEFAULT_ENV_INSTANCE_EMBEDDED
 env_t environment __PPCENV__ = {
 	ENV_CRC,	/* CRC Sum */
@@ -36,6 +38,9 @@  static char default_environment[] = {
 #else
 const uchar default_environment[] = {
 #endif
+#ifdef	CONFIG_ENV_CALLBACK_LIST_DEFAULT
+	ENV_CALLBACK_VAR "=" CONFIG_ENV_CALLBACK_LIST_DEFAULT "\0"
+#endif
 #ifdef	CONFIG_BOOTARGS
 	"bootargs="	CONFIG_BOOTARGS			"\0"
 #endif
diff --git a/include/environment.h b/include/environment.h
index 4b19f32..6c30215 100644
--- a/include/environment.h
+++ b/include/environment.h
@@ -164,6 +164,8 @@  extern void env_reloc(void);
 
 #ifndef DO_DEPS_ONLY
 
+#include <env_attr.h>
+#include <env_callback.h>
 #include <search.h>
 
 extern struct hsearch_data env_htab;
diff --git a/include/search.h b/include/search.h
index 1e48deb..d68e24a 100644
--- a/include/search.h
+++ b/include/search.h
@@ -47,6 +47,8 @@  typedef enum {
 typedef struct entry {
 	const char *key;
 	char *data;
+	int (*callback)(const char *name, const char *value, enum env_op op,
+		int flags);
 } ENTRY;
 
 /* Opaque type for internal use.  */
@@ -120,6 +122,9 @@  extern int himport_r(struct hsearch_data *__htab,
 		     const char *__env, size_t __size, const char __sep,
 		     int __flag, int nvars, char * const vars[]);
 
+/* Walk the whole table calling the callback on each element */
+extern int hwalk_r(struct hsearch_data *__htab, int (*callback)(ENTRY *));
+
 /* Flags for himport_r(), hexport_r(), hdelete_r(), and hsearch_r() */
 #define H_NOCLEAR	(1 << 0) /* do not clear hash table before importing */
 #define H_FORCE		(1 << 1) /* overwrite read-only/write-once variables */
diff --git a/lib/hashtable.c b/lib/hashtable.c
index 7c6b96c..e922666 100644
--- a/lib/hashtable.c
+++ b/lib/hashtable.c
@@ -54,7 +54,8 @@ 
 #define	CONFIG_ENV_MAX_ENTRIES 512
 #endif
 
-#include "search.h"
+#include <env_callback.h>
+#include <search.h>
 
 /*
  * [Aho,Sethi,Ullman] Compilers: Principles, Techniques and Tools, 1986
@@ -274,6 +275,17 @@  static inline int _compare_and_overwrite_entry(ENTRY item, ACTION action,
 				return 0;
 			}
 
+			/* If there is a callback, call it */
+			if (htab->table[idx].entry.callback &&
+			    htab->table[idx].entry.callback(item.key,
+			    item.data, env_op_overwrite, flag)) {
+				debug("callback() rejected setting variable "
+					"%s, skipping it!\n", item.key);
+				__set_errno(EINVAL);
+				*retval = NULL;
+				return 0;
+			}
+
 			free(htab->table[idx].entry.data);
 			htab->table[idx].entry.data = strdup(item.data);
 			if (!htab->table[idx].entry.data) {
@@ -398,6 +410,9 @@  int hsearch_r(ENTRY item, ACTION action, ENTRY ** retval,
 
 		++htab->filled;
 
+		/* This is a new entry, so look up a possible callback */
+		env_callback_init(&htab->table[idx].entry);
+
 		/* check for permission */
 		if (htab->change_ok != NULL && htab->change_ok(
 		    &htab->table[idx].entry, item.data, env_op_create, flag)) {
@@ -409,6 +424,18 @@  int hsearch_r(ENTRY item, ACTION action, ENTRY ** retval,
 			return 0;
 		}
 
+		/* If there is a callback, call it */
+		if (htab->table[idx].entry.callback &&
+		    htab->table[idx].entry.callback(item.key, item.data,
+		    env_op_create, flag)) {
+			debug("callback() rejected setting variable "
+				"%s, skipping it!\n", item.key);
+			_hdelete(item.key, htab, &htab->table[idx].entry, idx);
+			__set_errno(EINVAL);
+			*retval = NULL;
+			return 0;
+		}
+
 		/* return new entry */
 		*retval = &htab->table[idx].entry;
 		return 1;
@@ -437,6 +464,7 @@  static void _hdelete(const char *key, struct hsearch_data *htab, ENTRY *ep,
 	debug("hdelete: DELETING key \"%s\"\n", key);
 	free((void *)ep->key);
 	free(ep->data);
+	ep->callback = NULL;
 	htab->table[idx].used = -1;
 
 	--htab->filled;
@@ -466,6 +494,15 @@  int hdelete_r(const char *key, struct hsearch_data *htab, int flag)
 		return 0;
 	}
 
+	/* If there is a callback, call it */
+	if (htab->table[idx].entry.callback &&
+	    htab->table[idx].entry.callback(key, NULL, env_op_delete, flag)) {
+		debug("callback() rejected deleting variable "
+			"%s, skipping it!\n", key);
+		__set_errno(EINVAL);
+		return 0;
+	}
+
 	_hdelete(key, htab, ep, idx);
 
 	return 1;
@@ -838,11 +875,9 @@  int himport_r(struct hsearch_data *htab,
 		e.data = value;
 
 		hsearch_r(e, ENTER, &rv, htab, flag);
-		if (rv == NULL) {
+		if (rv == NULL)
 			printf("himport_r: can't insert \"%s=%s\" into hash table\n",
 				name, value);
-			return 0;
-		}
 
 		debug("INSERT: table %p, filled %d/%d rv %p ==> name=\"%s\" value=\"%s\"\n",
 			htab, htab->filled, htab->size,
@@ -873,3 +908,27 @@  int himport_r(struct hsearch_data *htab,
 	debug("INSERT: done\n");
 	return 1;		/* everything OK */
 }
+
+/*
+ * hwalk_r()
+ */
+
+/*
+ * Walk all of the entries in the hash, calling the callback for each one.
+ * this allows some generic operation to be performed on each element.
+ */
+int hwalk_r(struct hsearch_data *htab, int (*callback)(ENTRY *))
+{
+	int i;
+	int retval;
+
+	for (i = 1; i <= htab->size; ++i) {
+		if (htab->table[i].used > 0) {
+			retval = callback(&htab->table[i].entry);
+			if (retval)
+				return retval;
+		}
+	}
+
+	return 0;
+}