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This chapter is from the book

2.6. Secure C Programming

We mentioned The CERT C Secure Coding Standard in the Preface and indicated that we would follow certain guidelines that will help you avoid programming practices that open systems to attacks.

Avoid Single-Argument printfs

One such guideline is to avoid using printf with a single string argument. If you need to display a string that terminates with a newline, use the puts function, which displays its string argument followed by a newline character. For example, in Fig. 2.1, line 8

printf( "Welcome to C!\n" );                  

should be written as:

puts( "Welcome to C!" );                      

We did not include \n in the preceding string because puts adds it automatically.

If you need to display a string without a terminating newline character, use printf with two arguments—a "%s" format control string and the string to display. The %s conversion specifier is for displaying a string. For example, in Fig. 2.3, line 8

printf( "Welcome " );                        

should be written as:

printf( "%s", "Welcome " );                   

Although the printfs in this chapter as written are actually not insecure, these changes are responsible coding practices that will eliminate certain security vulnerabilities as we get deeper into C—we’ll explain the rationale later in the book. From this point forward, we use these practices in the chapter examples and you should use them in your own code.

For more information on this issue, see CERT C Secure Coding rule FIO30-C

www.securecoding.cert.org/confluence/display/seccode/
   FIO30-C.+Exclude+user+input+from+format+strings   

In Chapter 6’s Secure C Programming section, we’ll explain the notion of user input as referred to by this CERT guideline.

scanf and printf, scanf_s and printf_s

We introduced scanf and printf in this chapter. We’ll be saying more about these in subsequent Secure C Coding Guidelines sections. We’ll also discuss scanf_s and printf_s, which were introduced in C11.

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