C program: Segmentation fault when selecting from MySQL with field that contains NULL

Question

It all seems to work, but if there is a MySQL field with NULL in it I get a Segmentation Fault.

Here's the code:

int 
selectDB(char * cmd)
{
    printf("Cmd: %s\n", cmd);
    MYSQL *conn;
    MYSQL_RES *result;
    MYSQL_ROW row;
    int i;

    conn = mysql_init(NULL);

    if (conn == NULL) {
        printf("Error %u: %s\n", mysql_errno(conn), mysql_error(conn));
        exit(1);
    }

        if (mysql_real_connect(conn, "localhost", "root",
    "myPassword", "myDB", 0, NULL, 0) == NULL)
    {
        printf("Error %u: %s\n", mysql_errno(conn), mysql_error(conn));
        exit(1);    
    }

    if (mysql_query(conn, cmd)) 
    {
        printf("Error %u: %s\n", mysql_errno(conn), mysql_error(conn));
        exit(1);
    }

    if (!(result = mysql_store_result(conn)))
   {
        printf("Error %u: %s\n", mysql_errno(conn), mysql_error(conn));
        exit(1);
    }

    while((row = mysql_fetch_row(result))) {
        for (i=0 ; i < mysql_num_fields(result); i++) 
            printf("%s\n", row[i]);
    }

    if (!mysql_eof(result))
    {
        printf("Error %u: %s\n", mysql_errno(conn), mysql_error(conn));
        exit(1);
    }

   mysql_free_result(result);   
    mysql_close(conn);
    return 0;
}

I'm calling it with

char cmd[1000];
snprintf(cmd, 999, "SELECT * FROM users WHERE id = %s", valueptr);
selectDB(cmd);

i wouldn't say a dupe, necessarily... - it seems the OP didn't know that mysql can put NULL pointers in the row. — Claudiu
– Claudiu, Commented May 3, 2011 at 18:46
@Claudio- you are right. this might help him, but not a dup. sorry. — MByD
– MByD, Commented May 3, 2011 at 18:47

user2100815 · Accepted Answer · 2011-05-03 18:45:19Z

2

From the MySQL docs:

NULL values in the row are indicated by NULL pointers.

You need something like:

 for (i=0 ; i < mysql_num_fields(result); i++) 
        printf("%s\n", row[i] ? row[i] : "NULL" );

answered May 3, 2011 at 18:45

user2100815

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1 Comment

Frank Vilea Over a year ago

Thanks, that just confirmed my assumption.

Claudiu · Accepted Answer · 2011-05-03 18:44:54Z

1

My only guess:

When you do:

printf("%s\n", row[i]);

it expects a pointer to a string. When you give it NULL (not a pointer to a string containing NULL, but NULL), it tries to print what's at memory location 0x00 and gives a segmentation fault.

Try checking for row[i] being a valid pointer (non-NULL) before printing it.

Also if your row has any integers in it or anything besides strings you will also get a segfault.

answered May 3, 2011 at 18:44

Claudiu

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