Page History

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• SPACE—the space (" ") character
• N/E—no effect
• NTBS—char* argument pointing to a null-terminated byte string
• NTWS—wchar_t* argument pointing to a null-terminated wide character string
• XSI—ISO/IEC 9945-2003 XSI extension

Noncompliant Code Example

Mismatches between arguments and conversion specifications may result in undefined behavior.  Many compilers can diagnose type mismatches in formatted output function invocations.  In the following noncompliant code example, the error_type argument to printf() is incorrectly matched with the %s specifier (should be %d), and the error_msg argument is incorrectly matched with the %d specifier (should be %s).  One possible result of this invocation is that printf() will interpret the error_type argument as a pointer, and try to read a string from the address that error_type contains (likely this will result in an access violation):

const char *error_msg = "Resource not available to user.";
int error_type = 3;
/* ... */
printf("Error (type %s): %d\n", error_type, error_msg);

Compliant Solution

This compliant solution ensures that the format arguments match their respective format specifications:

const char *error_msg = "Resource not available to user.";
int error_type = 3;
/* ... */
printf("Error (type %d): %s\n", error_type, error_msg);

Noncompliant Code Example

The width and precision arguments to printf() format directives must be of type int. Subclause 7.21.6.1 of the C Standard [ISO/IEC 9899:2011] states:

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int print_int(int i, size_t width, size_t prec) {
  int n;

  n = printf("%*.*d", width, prec, i);

  return n;
}

Compliant Solution

In this compliant solution, the field width and precision arguments to printf() format directives are of type int:

int print_int(int i, int width, int prec) {
  int n;

  n = printf("%*.*d", width, prec, i);

  return n;
}

Risk Assessment

In most cases, incorrectly specified format strings will result in abnormal program termination.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

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