Home > Articles > Data > MySQL

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

Client 2—Adding Error Checking

Our second client will be like the first one, but it will be modified to take into account the possibility of errors occurring. It seems to be fairly common in programming texts to say "Error checking is left as an exercise for the reader," probably because checking for errors is—let's face it—such a bore. Nevertheless, I prefer to promote the view that MySQL client programs should test for error conditions and respond to them appropriately. The client library calls that return status values do so for a reason, and you ignore them at your peril. You end up trying to track down obscure problems that occur in your programs due to failure to check for errors, users of your programs wonder why those programs behave erratically, or both.

Consider our program, client1. How do you know whether or not it really connected to the server? You could find out by looking in the server log for Connect and Quit events corresponding to the time at which you ran the program:

990516 21:52:14   20 Connect  paul@localhost on
                  20 Quit

Alternatively, you might see an Access denied message instead:

990516 22:01:47   21 Connect  Access denied for user: 'paul@localhost'
                  (Using password: NO)

This message indicates that no connection was established at all. Unfortunately, client1 doesn't tell us which of these outcomes occurred. In fact, it can't. It doesn't perform any error checking, so it doesn't even know itself what happened. In any case, you certainly shouldn't have to look in the log to find out whether or not you were able to connect to the server! Let's fix that right away.

Routines in the MySQL client library that return a value generally indicate success or failure in one of two ways:

  • Pointer-valued functions return a non-NULL pointer for success and NULL for failure. (NULL in this context means "a C NULL pointer," not "a MySQL NULL column value.")

  • Of the client library routines we've used so far, mysql_init() and mysql_real_connect() both return a pointer to the connection handler to indicate success and NULL to indicate failure.

  • Integer-valued functions commonly return 0 for success and non-zero for failure. It's important not to test for specific non-zero values, such as -1. There is no guarantee that a client library function returns any particular value when it fails. On occasion, you may see older code that tests a return value incorrectly like this:

  • if (mysql_XXX() == -1)    /* this test is incorrect */
       fprintf (stderr, "something bad happened\n");

    This test might work, and it might not. The MySQL API doesn't specify that any non-zero error return will be a particular value, other than that it (obviously) isn't zero. The test should be written either like this:

    if (mysql_XXX())        /* this test is correct */
       fprintf (stderr, "something bad happened\n");

    or like this:

    if (mysql_XXX() != 0)    /* this test is correct */
       fprintf (stderr, "something bad happened\n");

The two tests are equivalent. If you look through the source code for MySQL itself, you'll find that generally it uses the first form of the test, which is shorter to write.

Not every API call returns a value. The other client routine we've used, mysql_close(), is one that does not. (How could it fail? And if it did, so what? You were done with the connection, anyway.)

When a client library call fails and you need more information about the failure, two calls in the API are useful. mysql_error() returns a string containing an error message, and mysql_errno() returns a numeric error code. You should call them right after an error occurs because if you issue another API call that returns a status, any error information you get from mysql_error() or mysql_errno() will apply to the later call instead.

Generally, the user of a program will find the error string more enlightening than the error code. If you report only one of the two, I suggest it be the string. For completeness, the examples in this chapter report both values.

Taking the preceding discussion into account, we'll write our second client, client2. It is similar to client1, but with proper error-checking code added. The source file, client2.c, looks like this:

/* client2.c */

#include <stdio.h>
#include <mysql.h>

#define def_host_name  NULL /* host to connect to (default = localhost) */
#define def_user_name  NULL /* user name (default = your login name) */
#define def_password   NULL /* password (default = none) */
#define def_db_name    NULL /* database to use (default = none) */

MYSQL  *conn;        /* pointer to connection handler */

main (int argc, char *argv[])
  conn = mysql_init (NULL);
  if (conn == NULL)
    fprintf (stderr, "mysql_init() failed (probably out of memory)\n");
    exit (1);
  if (mysql_real_connect (
        conn,          /* pointer to connection handler */
        def_host_name, /* host to connect to */
        def_user_name, /* user name */
        def_password,  /* password */
        def_db_name,   /* database to use */
        0,             /* port (use default) */
        NULL,          /* socket (use default) */
        0)             /* flags (none) */
      == NULL)
    fprintf (stderr, "mysql_real_connect() failed:\nError %u (%s)\n",
              mysql_errno (conn), mysql_error (conn));
    exit (1);
  mysql_close (conn);
  exit (0);

The error-checking logic is based on the fact that both mysql_init() and mysql_real_connect() return NULL if they fail. Note that although the program checks the return value of mysql_init(), no error-reporting function is called if it fails. That's because the connection handler cannot be assumed to contain any meaningful information when mysql_init() fails. By contrast, if mysql_real_connect() fails, the connection handler doesn't reflect a valid connection, but does contain error information that can be passed to the error-reporting functions. (Don't pass the handler to any other client routines, though! Because they generally assume a valid connection, your program may crash.)

Compile and link client2, and then try running it:

% client2

If client2 produces no output (as just shown), it connected successfully. On the other hand, you might see something like this:

% client2
mysql_real_connect() failed:
Error 1045 (Access denied for user: 'paul@localhost' (Using password: NO))

This output indicates no connection was established, and lets you know why. It also means that our first program, client1, never successfully connected to the server, either! (After all, client1 used the same connection parameters.) We didn't know it then because client1 didn't bother to check for errors. client2 does check, so it can tell us when something goes wrong. That's why you should always test API function return values.

Answers to questions on the MySQL mailing list often have to do with error checking. Typical questions are "Why does my program crash when it issues this query?" or "How come my query doesn't return anything?" In many cases, the program in question didn't check whether or not the connection was established successfully before issuing the query or didn't check to make sure the server successfully executed the query before trying to retrieve the results. Don't make the mistake of assuming that every client library call succeeds.

The rest of the examples in this chapter perform error checking, and you should, too. It might seem like more work, but in the long run it's really less because you spend less time tracking down subtle problems. I'll also take this approach of checking for errors in Chapters 7, "The Perl DBI API," and 8, "The PHP API."

Now, suppose you did see an Access denied message when you ran the client2 program. How can you fix the problem? One possibility is to change the #define lines for the hostname, username, and password to values that allow you to access your server. That might be beneficial, in the sense that at least you'd be able to make a connection. But the values would still be hardcoded into your program. I recommend against that approach, especially for the password value. You might think that the password becomes hidden when you compile your program into binary form, but it's not hidden at all if someone can run strings on the program. (Not to mention the fact that anyone with read access to your source file can get the password with no work at all.)

We'll deal with the access problem in the section "Client 4—Getting Connection Parameters at Runtime." First, I want to show some other ways of writing your connection code.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020