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Variadic functions accept a variable number of arguments but are problematic. Variadic functions define an implicit contract between the function writer and the function user that allows the function to determine the number of arguments passed in any particular invocation. Failure to exercise care when invoking a variadic function to ensure that it knows when to stop processing arguments enforce this contract may result in undefined behavior. See undefined behavior 141 of Appendix J of the C Standard.

Argument Processing

Wiki MarkupIn the following code example, the variadic function {{average()}} is used to determine the average value of its passed positive integer arguments \[[Seacord 05c|AA. C References#Seacord 05c]\]. The function processes arguments until it finds one with a value of {{va_eol}} ({{\-1}} calculates the average value of the positive integer arguments passed to the function [Seacord 2013]. The function processes arguments until it encounters an argument with the value of va_eol (-1).

enum { va_eol = -1 };

unsigned int average(int first, ...) {
  unsigned int count = 0;
  unsigned int sum = 0;
  int i = first;
  va_list args;

  va_start(args, first);

  while (i != va_eol) {
    sum += i;
    count++;
    i = va_arg(args, int);
  }

  va_end(args);
  return(count ? (sum / count) : 0);
}

Note that va_start() must be called to initialize the argument list and that va_end() must always be called when finished with a variable argument list.

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Noncompliant Code Example

In this non-compliant noncompliant code example, the average() function above is called as follows:

int avg = average(1, 4, 6, 4, 1);

The omission of the va_eol terminating value means that the function will continue to process values from the stack until it encounters a va_eol by coincidence or an error occurs.

Compliant Solution

The following call maintains This compliant solution enforces the contract by adding a va_eol as the last final argument.:

int avg = average(1, 4, 6, 4, 1, va_eol);

...

Noncompliant Code Example

Another common mistake is to use more format conversion specifiers than supplied arguments. This results in undefined behavior, for example, extracting non-existent arguments off the stack and unintentionally exposing data. The following example illustrates this, as shown in this noncompliant code example:

const char
char const *error_msg = "Resource not available to user.";
/* ... */
printf("Error (%s): %s", error_msg);

This code results in nonexistent arguments being processed by the function, potentially leaking information about the process.

Compliant Solution

The following code This compliant solution matches the number of format specifiers with the number of variable arguments.:

const 
char const *error_msg = "Resource not available to user.";
/* ... */
printf("Error: %s", error_msg);

Argument List Caveats

C99 C functions that accept the variadic primitive va_list as an argument pose an additional risk. Calls to vfprintf(), vfscanf(), vprintf(), vscanf(), vsnprintf(), vsprintf(), and vsscanf() use the va_arg() macro, invalidating the parameterized va_list. Consequently, once a va_list is passed as an argument to any of these functions, it cannot be used again without a call to va_end() followed by a call to va_start().

Risk Assessment

Incorrectly using a variadic function can result in abnormal program termination or unintended information disclosure.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.15, "Variable arguments," and Section 7.19.6.8, "The {{vfprintf}} function"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "OTR Subprogram Signature Mismatch"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 16.1
\[[Seacord 05c|AA. C References#Seacord 05c]\]

Automated Detection

Related Guidelines

Bibliography

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Image Added Image Added Image AddedDCL09-A. Declare functions that return an errno error code with a return type of errno_t      02. Declarations and Initialization (DCL)       DCL11-A. Understand the type issues associated with variadic functions