Segfault on g_strdupv function
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21-06-2021 - |
Question
I am getting segfault on this function.
/**
* Excutes the passed query and returs the the first row as an array of
* strings. You must free this array by calling g_strfreev()
*/
static gchar** mysql_single_row(MYSQL *mysql_handle, char* query){
my_ulonglong num_rows=0;
MYSQL_RES *result = NULL;
gchar ** single_row = NULL;
GString *q = g_string_new(query);
MYSQL_ROW row={0};
int query_status = mysql_real_query(mysql_handle, q->str, q->len);
if(query_status!=0){
g_string_free(q, TRUE);
return NULL;
}
fprintf(stderr, "Storing mysql result!\n");
result = mysql_store_result(mysql_handle);
if(result==NULL){
/// it was not a query that returns statemnet (e.g. INSERT, DELETE)
g_string_free(q, TRUE);
return NULL;
}
num_rows = mysql_num_rows(result);
fprintf(stderr, "Total rows = %Ld\n", num_rows);
if(num_rows>0){
/// We only fetch the first row
row = mysql_fetch_row(result);
fprintf(stderr, "Copy single rows\n");
single_row = g_strdupv(row); // <------------- SIGSEGV happens here
fprintf(stderr, "Copy single rows done\n");
}else{
mysql_free_result(result);
g_string_free(q, TRUE);
return NULL;
}
/// clean up
g_string_free(q, TRUE);
mysql_free_result(result);
return single_row;
}
basically what I want to do is to execute some 'SELECT' query and return the first row as an array of strings. According to the manual g_strdupv
should copy the returned char **
and makes a new one. I return this. Later I clean this up using g_strfreev
which is the recommended method.
But why I am getting segfaults here. I ran it with valgrind. Output and the corresponding code can be found here
Solution
g_strdupv()
copies a NULL
-terminated array of C strings (which each must be NUL-terminated). The MySQL documentation on the C API Data Structures states that MYSQL_ROW
is an array of byte strings which are not necessarily NUL-terminated "if field values may contain binary data". Thus a MYSQL_ROW
is neither guaranteed to be a NULL
-terminated array nor an array of C strings.
The segfault is likely occurring because g_strdupv()
keeps looking for the NULL
terminator, but doesn't find one until it attempts a read of non-process memory.