DCL11-C. Understand the type issues associated with variadic functions

The variable parameters of a variadic function—that is, those that correspond with the position of the ellipsis—are interpreted by the va_arg() macro. The va_arg() macro is used to extract the next argument from an initialized argument list within the body of a variadic function implementation. The size of each parameter is determined by the specified type. If the type is inconsistent with the corresponding argument, the behavior is undefined and may result in misinterpreted data or an alignment error (see EXP36-C. Do not cast pointers into more strictly aligned pointer types).

The variable arguments to a variadic function are not checked for type by the compiler. As a result, the programmer is responsible for ensuring that they are compatible with the corresponding parameter after the default argument promotions:

• Integer arguments of types ranked lower than int are promoted to int if int can hold all the values of that type; otherwise, they are promoted to unsigned int (the integer promotions).
• Arguments of type float are promoted to double.

Noncompliant Code Example (Type Interpretation Error)

The C printf() function is implemented as a variadic function. This noncompliant code example swaps its null-terminated byte string and integer parameters with respect to how they are specified in the format string. Consequently, the integer is interpreted as a pointer to a null-terminated byte string and dereferenced, which will likely cause the program to abnormally terminate. Note that the error_message pointer is likewise interpreted as an integer.

const char *error_msg = "Error occurred";
/* ... */
printf("%s:%d", 15, error_msg);

Compliant Solution (Type Interpretation Error)

This compliant solution modifies the format string so that the conversion specifiers correspond to the arguments:

const char *error_msg = "Error occurred";
/* ... */
printf("%d:%s", 15, error_msg);

As shown, care must be taken to ensure that the arguments passed to a format string function match up with the supplied format string.

Noncompliant Code Example (Type Alignment Error)

In this noncompliant code example, a type long long integer is incorrectly parsed by the printf() function with a %d specifier. This code may result in data truncation or misrepresentation when the value is extracted from the argument list.

long long a = 1;
const char msg[] = "Default message";
/* ... */
printf("%d %s", a, msg);

Because a long long was not interpreted, if the long long uses more bytes for storage, the subsequent format specifier %s is unexpectedly offset, causing unknown data to be used instead of the pointer to the message.

Compliant Solution (Type Alignment Error)

This compliant solution adds the length modifier ll to the %d format specifier so that the variadic function parser for printf() extracts the correct number of bytes from the variable argument list for the long long argument:

long long a = 1;
const char msg[] = "Default message";
/* ... */
printf("%lld %s", a, msg);

Noncompliant Code Example (NULL)

The C Standard allows NULL to be either an integer constant or a pointer constant. While passing NULL as an argument to a function with a fixed number of arguments will cause NULL to be cast to the appropriate pointer type, when it is passed as a variadic argument, this will not happen if sizeof(NULL) != sizeof(void *). This is possible for several reasons:

• Pointers and ints may have different sizes on a platform where NULL is an integer constant
• The platform may have different pointer types with different sizes on a platform. In that case, if NULL is a void pointer, it is the same size as a pointer to char (C11 section 6.2.5, paragraph 28), which might be sized differently than the required pointer type.

On either such platform, the following code will have undefined behavior:

char* string = NULL;
printf("%s %d\n", string, 1);

On a system with 32-bit int and 64-bit pointers, printf() may interpret the NULL as high-order bits of the pointer and the third argument 1 as the low-order bits of the pointer. In this case, printf() will print a pointer with the value 0x00000001 and then attempt to read an additional argument for the %d conversion specifier, which was not provided.

Compliant Solution (NULL)

This compliant solution avoids sending NULL to printf():

char* string = NULL;
printf("%s %d\n", (string ? string : "null"), 1);

Risk Assessment

Inconsistent typing in variadic functions can result in abnormal program termination or unintended information disclosure.

Automated Detection

Related Vulnerabilities

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

Related Guidelines