MySQL 5.6 C API Developer Guide
unsigned int mysql_num_fields(MYSQL_RES *result)
To pass a MYSQL*
argument instead, use
unsigned int mysql_field_count(MYSQL *mysql)
.
Returns the number of columns in a result set.
You can get the number of columns either from a pointer to a
result set or to a connection handler. You would use the
connection handler if
mysql_store_result()
or
mysql_use_result()
returned
NULL
(and thus you have no result set
pointer). In this case, you can call
mysql_field_count()
to
determine whether
mysql_store_result()
should
have produced a nonempty result. This enables the client
program to take proper action without knowing whether the
query was a SELECT
(or
SELECT
-like) statement. The
example shown here illustrates how this may be done.
See Section 21.1, “Why mysql_store_result() Sometimes Returns NULL After mysql_query() Returns Success”.
MYSQL_RES *result; unsigned int num_fields; unsigned int num_rows; if (mysql_query(&mysql,query_string)) { // error } else // query succeeded, process any data returned by it { result = mysql_store_result(&mysql); if (result) // there are rows { num_fields = mysql_num_fields(result); // retrieve rows, then call mysql_free_result(result) } else // mysql_store_result() returned nothing; should it have? { if (mysql_errno(&mysql)) { fprintf(stderr, "Error: %s\n", mysql_error(&mysql)); } else if (mysql_field_count(&mysql) == 0) { // query does not return data // (it was not a SELECT) num_rows = mysql_affected_rows(&mysql); } } }
An alternative (if you know that your query should have
returned a result set) is to replace the
mysql_errno(&mysql)
call
with a check whether
mysql_field_count(&mysql)
returns 0. This happens only if something went wrong.