[Valgrind-users] problem debugging mysql code...

[Diego T. <dtg...@te...>]

ehlo!

i've got the following issue when using valgrind. Maybe i'm doing something
wrong or i don't know how to use valgrind. Another point is that maybe this
isn't the correct place to ask about my problem... i'll see :)

the following function:

>----
MYSQL_ROW db_read(const char *q, ...) {
MYSQL_ROW row;
MYSQL_RES * res;

[...]
if ((res = mysql_store_result(&g.mysql_dbh)) != NULL) {
    row = mysql_fetch_row(res);
    mysql_free_result(res);
    return row;
}
[...]
>----

and when debugging with valgrind, i got the following:

==23479== Invalid read of size 1
==23479==    at 0x4028AD24: __strtol_internal (in /lib/libc-2.3.1.so)
==23479==    by 0x40288848: atoi (in /lib/libc-2.3.1.so)
==23479==    by 0x412F874D: mod_load (in
/home/dtorres/prog/develop/rb/mod/db.so)
==23479==    by 0x804CC32: module_init (src/modules.c:43)
==23479==    Address 0x410C8294 is 32 bytes inside a block of size 8164
free'd
==23479==    at 0x4015D6A4: free (vg_clientfuncs.c:185)
==23479==    by 0x40213838: my_no_flags_free (in
/usr/lib/libmysqlclient.so.10.0.0)
==23479==    by 0x402165F4: free_root (in /usr/lib/libmysqlclient.so.10.0.0)
==23479==    by 0x4020F5E8: mysql_free_result (in
/usr/lib/libmysqlclient.so.10.0.0)
==23479==

the atoi is performed over the returned row: atoi(row[0]). So i understand
that when i'm releasing with mysql_free_result, row is contained inside the
res pointer. Checking the documentation from the mysql api doesn't clarify
this situation, it doesn't state if its safe to access rows returned from
mysql_fetch_row. So lets fix it:


MYSQL_ROW db_read(const char *q, ...) {
MYSQL_ROW row;
MYSQL_ROW row2;
MYSQL_RES * res;

[...]
if ((res = mysql_store_result(&g.mysql_dbh)) != NULL) {
    row = mysql_fetch_row(res);
    memcpy(&row2, &row, sizeof(MYSQL_ROW));
    mysql_free_result(res);
    return row2;
}
[...]

now i'm copying the row content to another variable, but got the same
result.

==24120== Invalid read of size 1
==24120==    at 0x4028AA6E: __strtol_internal (in /lib/libc-2.3.1.so)
==24120==    by 0x40288848: atoi (in /lib/libc-2.3.1.so)
==24120==    by 0x412F86F9: mod_load (in
/home/dtorres/prog/develop/rb/mod/db.so)
==24120==    by 0x804CC32: module_init (src/modules.c:43)
==24120==    Address 0x410C18FC is 32 bytes inside a block of size 8164
free'd
==24120==    at 0x4015D6A4: free (vg_clientfuncs.c:185)
==24120==    by 0x40213838: my_no_flags_free (in
/usr/lib/libmysqlclient.so.10.0.0)
==24120==    by 0x402165F4: free_root (in /usr/lib/libmysqlclient.so.10.0.0)
==24120==    by 0x4020F5E8: mysql_free_result (in
/usr/lib/libmysqlclient.so.10.0.0)


The strange thing is that the program is working without problems, and data
from the returned row is always valid. coincidence? If someone has any
idea... please, help! :)