SQLite: How embed the memvfs extension to Amalgamation? - sqlite

Question

I need to load/save SQLite database in memory buffer. For this, I want embed the memvfs extension into sqlite3 code and compile it wholly as sqlite3.dll.
How do it?
Update1:
I want use the memvfs as temp memory buffer. My program load data from net to buffer, connect to this memory buffer and restore data into empty in-memory db. I thoutgh that inclusion of memvfs to sqlite amalgamation would improve perfomance.
Update2:
If you want to use memvfs extension pay attention to bug in readme comment in source. Use "PRAGMA journal_mode=OFF" instead "journal_mode=NONE"
Update3:
Another bug in memvfs.c - use 'max' instead 'maxsz' for maxsz param in URI.
The sqlite developers carefully set a rakes :(

Answer 1

Test program to demonstrate using memvfs:
#include <fcntl.h>
#include <sqlite3.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
int main(void) {
sqlite3 *db;
char *err;
// Open an in-memory database to use as a handle for loading the memvfs extension
if (sqlite3_open(":memory:", &db) != SQLITE_OK) {
fprintf(stderr, "open :memory: %s\n", sqlite3_errmsg(db));
return EXIT_FAILURE;
}
sqlite3_enable_load_extension(db, 1);
if (sqlite3_load_extension(db, "./memvfs", NULL, &err) != SQLITE_OK) {
fprintf(stderr, "load extension: %s\n", err);
return EXIT_FAILURE;
}
// Done with this database
sqlite3_close(db);
// Read the real database into memory
int fd = open("foo.db", O_RDONLY);
if (fd < 0) {
perror("open");
return EXIT_FAILURE;
}
struct stat s;
if (fstat(fd, &s) < 0) {
perror("fstat");
return EXIT_FAILURE;
}
void *memdb = sqlite3_malloc64(s.st_size);
if (read(fd, memdb, s.st_size) != s.st_size) {
perror("read");
return EXIT_FAILURE;
}
close(fd);
// And open that memory with memvfs now that it holds a valid database
char *memuri = sqlite3_mprintf("file:whatever?ptr=0x%p&sz=%lld&freeonclose=1",
memdb, (long long)s.st_size);
printf("Trying to open '%s'\n", memuri);
if (sqlite3_open_v2(memuri, &db, SQLITE_OPEN_READWRITE | SQLITE_OPEN_URI,
"memvfs") != SQLITE_OK) {
fprintf(stderr, "open memvfs: %s\n", sqlite3_errmsg(db));
return EXIT_FAILURE;
}
sqlite3_free(memuri);
// Try querying the database to show it works.
sqlite3_stmt *stmt;
if (sqlite3_prepare_v2(db, "SELECT b FROM test", -1, &stmt, NULL) !=
SQLITE_OK) {
fprintf(stderr, "prepare: %s\n", sqlite3_errmsg(db));
sqlite3_close(db);
return EXIT_FAILURE;
}
for (int rc = sqlite3_step(stmt); rc == SQLITE_ROW; rc = sqlite3_step(stmt)) {
printf("%d\n", sqlite3_column_int(stmt, 0));
}
sqlite3_finalize(stmt);
sqlite3_close(db);
return 0;
}
Usage:
# Create a test database to use with memvfs
$ sqlite3 foo.db
sqlite> CREATE TABLE test(b INTEGER);
sqlite> INSERT INTO test VALUES (1), (2);
sqlite> .quit
# Compile the memvfs module and test program
$ gcc -O -fPIC -shared -o memvfs.so memvfs.c
$ gcc -O -Wall -Wextra testmem.c -lsqlite3
# And run it.
$ ./a.out
Trying to open 'file:whatever?ptr=0x56653FE2B940&sz=8192&freeonclose=1'
1
2
Same workflow if you compile it directly into your program instead of using a loadable module; you just have to call sqlite3_memvfs_init() with the right arguments instead of using sqlite3_load_extension().