--- a/targets/Linux/plc_Linux_main.c	Fri Jun 01 15:46:17 2018 +0300
+++ b/targets/Linux/plc_Linux_main.c	Thu Jun 07 13:45:35 2018 +0300
@@ -82,6 +82,10 @@
 
 int PLC_shutdown = 0;
 
+int ForceSaveRetainReq(void) {
+    return PLC_shutdown;
+}
+
 void PLC_thread_proc(void *arg)
 {
     while (!PLC_shutdown) {
@@ -231,34 +235,3 @@
 {
     pthread_mutex_lock(&python_mutex);
 }
-
-#ifndef HAVE_RETAIN
-void InitRetain(void)
-{
-}
-
-void CleanupRetain(void)
-{
-}
-
-int CheckRetainBuffer(void)
-{
-	return 1;
-}
-
-void ValidateRetainBuffer(void)
-{
-}
-
-void InValidateRetainBuffer(void)
-{
-}
-
-void Retain(unsigned int offset, unsigned int count, void *p)
-{
-}
-
-void Remind(unsigned int offset, unsigned int count, void *p)
-{
-}
-#endif // !HAVE_RETAIN

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/targets/Linux/plc_Linux_main_retain.c	Thu Jun 07 13:45:35 2018 +0300
@@ -0,0 +1,300 @@
+#ifndef HAVE_RETAIN
+#include <stdio.h>
+#include <stdint.h>
+#include <unistd.h>
+#include "iec_types.h"
+
+int GetRetainSize();
+
+/* Retain buffer.  */
+FILE *retain_buffer;
+const char rb_file[]      = "retain_buffer_file";
+const char rb_file_bckp[] = "retain_buffer_file.bak";
+
+
+/* Retain header struct.  */
+struct retain_info_t {
+	uint32_t retain_size;
+	uint32_t hash_size;
+	uint8_t* hash;
+	uint32_t header_offset;
+	uint32_t header_crc;
+};
+
+/* Init retain info structure.  */
+struct retain_info_t retain_info;
+
+/* CRC lookup table and initial state.  */
+uint32_t crc32_table[256];
+uint32_t retain_crc;
+
+
+/* Generate CRC32 lookup table.  */
+void GenerateCRC32Table(void)
+{
+	unsigned int i, j;
+	/* Use CRC-32-IEEE 802.3 polynomial 0x04C11DB7 (bit reflected).  */
+	uint32_t poly = 0xEDB88320;
+
+	for (i = 0; i <= 0xFF; i++)
+	{
+		uint32_t c = i;
+		for (j = 0 ; j < 8 ; j++)
+			c = (c & 1) ? (c >> 1 ) ^ poly : (c >> 1);
+		crc32_table[i] = c;
+	}
+}
+
+
+/* Calculate CRC32 for len bytes from pointer buf with init starting value.  */
+uint32_t GenerateCRC32Sum(const void* buf, unsigned int len, uint32_t init)
+{
+	uint32_t crc = ~init;
+	unsigned char* current = (unsigned char*) buf;
+	while (len--)
+		crc = crc32_table[(crc ^ *current++) & 0xFF] ^ (crc >> 8);
+	return ~crc;
+}
+
+/* Calc CRC32 for retain file byte by byte.  */
+int CheckFileCRC(FILE* file_buffer)
+{
+	/* Set the magic constant for one-pass CRC calc according to ZIP CRC32.  */
+	const uint32_t magic_number = 0x2144df1c;
+
+	/* CRC initial state.  */
+	uint32_t calc_crc32 = 0;
+	char data_block = 0;
+
+	while(!feof(file_buffer)){
+		if (fread(&data_block, sizeof(data_block), 1, file_buffer))
+			calc_crc32 = GenerateCRC32Sum(&data_block, sizeof(char), calc_crc32);
+	}
+
+	/* Compare crc result with a magic number.  */
+	return (calc_crc32 == magic_number) ? 1 : 0;
+}
+
+/* Compare current hash with hash from file byte by byte.  */
+int CheckFilehash(void)
+{
+	int k;
+	int offset = sizeof(retain_info.retain_size);
+
+	rewind(retain_buffer);
+	fseek(retain_buffer, offset , SEEK_SET);
+
+	uint32_t size;
+	fread(&size, sizeof(size), 1, retain_buffer);
+	if (size != retain_info.hash_size)
+		return 0;
+
+	for(k = 0; k < retain_info.hash_size; k++){
+		uint8_t file_digit;
+		fread(&file_digit, sizeof(char), 1, retain_buffer);
+		if (file_digit != *(retain_info.hash+k))
+			return 0;
+	}
+
+	return 1;
+}
+
+void InitRetain(void)
+{
+	int i;
+
+	/* Generate CRC32 lookup table.  */
+	GenerateCRC32Table();
+
+	/* Get retain size in bytes */
+	retain_info.retain_size = GetRetainSize();
+
+	/* Hash stored in retain file as array of char in hex digits
+	   (that's why we divide strlen in two).  */
+	retain_info.hash_size = PLC_ID ? strlen(PLC_ID)/2 : 0;
+	//retain_info.hash_size = 0;
+	retain_info.hash = malloc(retain_info.hash_size);
+
+	/* Transform hash string into byte sequence.  */
+	for (i = 0; i < retain_info.hash_size; i++) {
+		int byte = 0;
+		sscanf((PLC_ID + i*2), "%02X", &byte);
+		retain_info.hash[i] = byte;
+	}
+
+	/* Calc header offset.  */
+	retain_info.header_offset = sizeof(retain_info.retain_size) + \
+		sizeof(retain_info.hash_size) + \
+		retain_info.hash_size;
+
+	/*  Set header CRC initial state.  */
+	retain_info.header_crc = 0;
+
+	/* Calc crc for header.  */
+	retain_info.header_crc = GenerateCRC32Sum(
+		&retain_info.retain_size,
+		sizeof(retain_info.retain_size),
+		retain_info.header_crc);
+
+	retain_info.header_crc = GenerateCRC32Sum(
+		&retain_info.hash_size,
+		sizeof(retain_info.hash_size),
+		retain_info.header_crc);
+
+	retain_info.header_crc = GenerateCRC32Sum(
+		retain_info.hash,
+		retain_info.hash_size,
+		retain_info.header_crc);
+}
+
+void CleanupRetain(void)
+{
+	/* Free hash memory.  */
+	free(retain_info.hash);
+}
+
+int CheckRetainFile(const char * file)
+{
+	retain_buffer = fopen(file, "rb");
+	if (retain_buffer) {
+		/* Check CRC32 and hash.  */
+		if (CheckFileCRC(retain_buffer))
+			if (CheckFilehash())
+				return 1;
+		fclose(retain_buffer);
+		retain_buffer = NULL;
+	}
+	return 0;
+}
+
+int CheckRetainBuffer(void)
+{
+	retain_buffer = NULL;
+	if (!retain_info.retain_size)
+		return 1;
+
+	/* Check latest retain file.  */
+	if (CheckRetainFile(rb_file))
+		return 1;
+
+	/* Check if we have backup.  */
+	if (CheckRetainFile(rb_file_bckp))
+		return 1;
+
+	/* We don't have any valid retain buffer - nothing to remind.  */
+	return 0;
+}
+
+#ifndef FILE_RETAIN_SAVE_PERIOD_S
+#define FILE_RETAIN_SAVE_PERIOD_S 1.0
+#endif
+
+static double CalcDiffSeconds(IEC_TIME* t1, IEC_TIME *t2)
+{
+	IEC_TIME dt ={
+		t1->tv_sec  - t2->tv_sec,
+		t1->tv_nsec - t2->tv_nsec
+	};
+
+	if ((dt.tv_nsec < -1000000000) || ((dt.tv_sec > 0) && (dt.tv_nsec < 0))){
+		dt.tv_sec--;
+		dt.tv_nsec += 1000000000;
+	}
+	if ((dt.tv_nsec > +1000000000) || ((dt.tv_sec < 0) && (dt.tv_nsec > 0))){
+		dt.tv_sec++;
+		dt.tv_nsec -= 1000000000;
+	}
+	return dt.tv_sec + 1e-9*dt.tv_nsec;
+}
+
+
+int RetainSaveNeeded(void)
+{
+	int ret = 0;
+	static IEC_TIME last_save;
+	IEC_TIME now;
+	double diff_s;
+
+	/* no retain */
+	if (!retain_info.retain_size)
+		return 0;
+
+	/* periodic retain flush to avoid high I/O load */
+	PLC_GetTime(&now);
+
+	diff_s = CalcDiffSeconds(&now, &last_save);
+
+	if ((diff_s > FILE_RETAIN_SAVE_PERIOD_S) || ForceSaveRetainReq()) {
+		ret = 1;
+		last_save = now;
+	}
+	return ret;
+}
+
+void ValidateRetainBuffer(void)
+{
+	if (!retain_buffer)
+		return;
+
+	/* Add retain data CRC to the end of buffer file.  */
+	fseek(retain_buffer, 0, SEEK_END);
+	fwrite(&retain_crc, sizeof(uint32_t), 1, retain_buffer);
+
+	/* Sync file buffer and close file.  */
+#ifdef __WIN32
+	fflush(retain_buffer);
+#else
+	fsync(fileno(retain_buffer));
+#endif
+
+	fclose(retain_buffer);
+	retain_buffer = NULL;
+}
+
+void InValidateRetainBuffer(void)
+{
+	if (!RetainSaveNeeded())
+		return;
+
+	/* Rename old retain file into *.bak if it exists.  */
+	rename(rb_file, rb_file_bckp);
+
+	/* Set file CRC initial value.  */
+	retain_crc = retain_info.header_crc;
+
+	/* Create new retain file.  */
+	retain_buffer = fopen(rb_file, "wb+");
+	if (!retain_buffer) {
+		fprintf(stderr, "Failed to create retain file : %s\n", rb_file);
+		return;
+	}
+
+	/* Write header to the new file.  */
+	fwrite(&retain_info.retain_size,
+		sizeof(retain_info.retain_size), 1, retain_buffer);
+	fwrite(&retain_info.hash_size,
+		sizeof(retain_info.hash_size),   1, retain_buffer);
+	fwrite(retain_info.hash ,
+		sizeof(char), retain_info.hash_size, retain_buffer);
+}
+
+void Retain(unsigned int offset, unsigned int count, void *p)
+{
+	if (!retain_buffer)
+		return;
+
+	/* Generate CRC 32 for each data block.  */
+	retain_crc = GenerateCRC32Sum(p, count, retain_crc);
+
+	/* Save current var in file.  */
+	fseek(retain_buffer, retain_info.header_offset+offset, SEEK_SET);
+	fwrite(p, count, 1, retain_buffer);
+}
+
+void Remind(unsigned int offset, unsigned int count, void *p)
+{
+	/* Remind variable from file.  */
+	fseek(retain_buffer, retain_info.header_offset+offset, SEEK_SET);
+	fread((void *)p, count, 1, retain_buffer);
+}
+#endif // !HAVE_RETAIN