reading junk out of a blob in sqlite - c

Question

I have a snippet of code which is somehow messing up the reading of bytes of a sqlite database. I've experienced this multiple times: first with a PNG where the last three bytes would differ, second with the king james bible and thirdly with a very simple test case. I'm stumped as to where I'm messing up. In all these cases, the command line sqlite tool can see the data inside the database correctly (both when viewed manually and when using writefile).
So the insertion is definitely working correctly, it's just that somehow my extraction is erroneous somehow. I am quite a novice at C so I expect I may have misallocated memory in someway, I just don't understand how.
#include <stdio.h>
#include <sqlite3.h>
#include <stdlib.h>
static char const dummy_content[] =
"hello world more \n\n\n hello worlds \nlalalala";
int
main(int argc, char **argv) {
sqlite3 *db;
sqlite3_stmt *stmt;
int rc, file_len, sqlite_len;
const char *file_contents;
const char *sqlite_contents;
rc = sqlite3_open("blah.db", &db);
if ( rc ) {
fprintf(stderr, "couldn't open db\n");
return 1;
}
/* initialise the schema */
rc = sqlite3_exec(db,
"BEGIN TRANSACTION;"
"DROP TABLE IF EXISTS blobs;"
"CREATE TABLE blobs(id TEXT NOT NULL, value BLOB, PRIMARY KEY(id));"
"COMMIT;",
NULL, NULL, NULL);
if ( rc ) {
fprintf(stderr, "couldn't initialise schema");
return 1;
}
file_contents = dummy_content;
file_len = sizeof(dummy_content);
/* insert the bytes */
rc = sqlite3_prepare_v2(db,
"INSERT INTO blobs VALUES(?, ?);",
-1,
&stmt, NULL);
if ( rc ) {
fprintf(stderr, "error preparing stmt: %s", sqlite3_errmsg(db));
return 1;
}
printf("prepared stmt\n");
rc = sqlite3_bind_text(stmt, 1, "boring", -1, NULL);
rc = rc | sqlite3_bind_blob(stmt, 2, file_contents, file_len, NULL);
if ( rc ) {
fprintf(stderr, "error binding to sql stmt");
return 1;
}
if ( (rc = sqlite3_step(stmt)) != SQLITE_DONE) {
fprintf(stderr, "something went wrong with execution");
}
sqlite3_finalize(stmt);
if ( rc != SQLITE_DONE ) return 1;
printf("loaded db\n");
/* load the bytes */
rc = sqlite3_prepare_v2(db,
"SELECT value FROM blobs WHERE id = ?;",
-1, &stmt, NULL);
if ( rc ) {
fprintf(stderr, "error preparing stmt: %s", sqlite3_errmsg(db));
return 1;
}
rc = sqlite3_bind_text(stmt, 1, "boring", -1, NULL);
if ( rc ) { fprintf(stderr, "error binding"); return 1; }
sqlite_len = -1;
while ( (rc = sqlite3_step(stmt)) != SQLITE_DONE ) {
if ( rc == SQLITE_ERROR ) {
fprintf(stderr, "error executing sql");
sqlite3_finalize(stmt);
return 1;
} else if ( rc == SQLITE_ROW ) {
sqlite_contents = sqlite3_column_blob(stmt, 0);
sqlite_len = sqlite3_column_bytes(stmt, 0);
}
}
if ( sqlite_len < 0 ) {
fprintf(stderr, "no value found in db");
sqlite3_finalize(stmt);
return 1;
}
printf("sqlite_len: %i file_len: %i\n", sqlite_len, file_len);
printf("from file string: %s\n", file_contents);
printf("from sqlite string: %s\n", sqlite_contents);
/* this frees the memory for the sqlite_contents */
sqlite3_finalize(stmt);
return 0;
}

Answer 1

You can only use the value returned by sqlite3_column_blob() until the next time you call sqlite3_step(). Your code isn't obeying that restriction (You're trying to print out the blob after another call to sqlite3_step() returns SQLITE_DONE)
From the documentation:
The pointers returned are valid until a type conversion occurs as described above, or until sqlite3_step() or sqlite3_reset() or sqlite3_finalize() is called.