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The incorrect_password() function calculates the size of the message, allocates dynamic storage, and then constructs the message in the allocated memory using the snprintf() function. The addition operations are not checked for integer overflow because the string referenced by user is known to have a length of 256 or less. Because the %s characters are replaced by the string referenced by user in the call to snprintf(), the resulting string needs 1 less byte is required to store the resulting string and terminating null-byte character. This idiom than is allocated. Using snprintf() to construct a message is commonly used for displaying the same message in multiple locations or when the message is difficult to build. The resulting code contains a format-string vulnerability, however, because the msg includes untrusted user input and is passed as the format-string argument in the call to fprintf().
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