CODEWITHSUNDEEP
mysqlcgtk2
user3829969
user3829969

Reputation:

Segmentation fault with mysql_num_fields

I am trying to integrate mysql with a gtk2 gui. It is a db client for a Mysql database. I put down the code to open the database, then try to read the records from the table and print them out in the terminal. It compiles but when I try to run it, I get a segmentation fault at the mysql_num_fields instruction.

I tried to run it and got a segmentation fault.

void openDatabase(){

 MYSQL *con;
 MYSQL_RES *res;
 MYSQL_ROW row;

 char *server = "localhost";
 char *user = "root";
 char *password = "Excalibur&1"; /* set me first */
 char *database = "people";

 con = mysql_init(NULL);

 /* Connect to database */
 if (!mysql_real_connect(con, server, user, password,
                                  database, 0, NULL, 0)) {
     fprintf(stderr, "%s\n", mysql_error(con));
     exit(1);
 }

 if (mysql_query(con, "SELECT * FROM cars"))
 {
     fprintf("Success!");
 }

 res = mysql_store_result(con);

 if (res == NULL)
 {
     fprintf("Success!");
 }

 int num_fields = mysql_num_fields(res);



 while ((row = mysql_fetch_row(res)))
 {
     for(int i = 0; i < num_fields; i++)
     {
          printf("%s ", row[i] ? row[i] : "NULL");
     }

     printf("\n");
 }

 mysql_free_result(res);
 mysql_close(con);

 }

Upvotes: 1

Views: 103

Answers (1)

Jens
Jens

Reputation: 72737

The segfault is due to fprintf("Success"); the first arg must be a ptr-to-FILE, like stderr.

Also, from the mysql_store_result() documentation:

Return Values

A pointer to a MYSQL_RES result structure with the results. NULL if the statement did not return a result set or an error occurred. To determine whether an error occurred, check whether mysql_error() returns a nonempty string, mysql_errno() returns nonzero, or mysql_field_count() returns zero.

You should rewrite the code to this:

if (res == NULL)
{
     fprintf(stderr, "Failed\n");
     exit(1);
}

In general it is useful to

  • turn on all warnings of your compiler
  • learn how to run a program under a debugger

Good compilers can catch printf format mismatches like this. Debuggers can show you a backtrace after a segfault and point to fprintf as the problematic call.

Upvotes: 0

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