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Do not call a function with the wrong number or type of arguments. 

The C Standard identifies five distinct situations in which undefined behavior may arise as a result of invoking a function using a declaration that is incompatible with its definition or with incorrect types or numbers of arguments:

Functions that are appropriately declared (as in DCL40-C. Do not create incompatible declarations of the same function or object) will typically generate a compiler error if they are supplied with the wrong number or types of arguments. However, there are cases in which supplying the incorrect arguments to a function will, at best, generate compiler warnings. Although such warnings should be resolved, they do not prevent program compilation. (See MSC00-C. Compile cleanly at high warning levels.)

Noncompliant Code Example

The header tgmath.h provides type-generic macros for math functions. Although most functions from the math.h header have a complex counterpart in complex.h, several functions do not. Calling any of the following type-generic functions with complex values results in undefined behavior.

Functions That Should Not Be Called with Complex Values

This noncompliant code example attempts to take the base-2 logarithm of a complex number, resulting in undefined behavior:

#include <complex.h>
#include <tgmath.h>
 
void func(void) {
  double complex c = 2.0 + 4.0 * I;
  double complex result = log2(c);
}

Compliant Solution (Complex Number)

If the clog2() function is not available for your implementation, you can take the base-2 logarithm of a complex number using log() instead of log2(), as log() can be used on complex arguments, as shown in this compliant solution:

#include <complex.h>
#include <tgmath.h>
 
void func(void) {
  double complex c = 2.0 + 4.0 * I;
  double complex result = log(c)/log(2);
}

Compliant Solution (Real Number)

This compliant solution can be used if the programmer's intent was to take the base-2 logarithm of the real part of the complex number. 

#include <complex.h>
#include <tgmath.h>
 
void func(void) {
  double complex c = 2.0 + 4.0 * I;
  double complex result = log2(creal(c));
}

Noncompliant Code Example

In this noncompliant example, the C standard library function strchr() is called through the function pointer fp with incorrectly typed arguments. According to the C Standard, subclause 6.3.2.3, paragraph 8 [ISO/IEC 9899:2011],

A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer. If a converted pointer is used to call a function whose type is not compatible with the referenced type, the behavior is undefined.

See undefined behavior 26.

#include <stdio.h>
#include <string.h>

char *(*fp)();

int main(void) {
  const char *c;
  fp = strchr;
  c = fp('e', "Hello");
  printf("%s\n", c);
  return 0;
}

Noncompliant Code Example

In this noncompliant example, the pointer to the C standard library function strchr() is declared with the correct arguments. This code still exhibits the same undefined behavior, but most compilers will warn that the arguments passed to fp do not match its declaration.

#include <stdio.h>
#include <string.h>

char *(*fp)(const char *, int);
int main(void) {
  const char *c;
  fp = strchr;
  c = fp('e', "Hello");
  printf("%s\n", c);
  return 0;
}

Compliant Solution

In this compliant solution, the function pointer fp is invoked with the correct number and type of arguments:

#include <stdio.h>
#include <string.h>

char *(*fp)(const char *, int);

int main(void) {
  const char *c;
  fp = strchr;
  c = fp("Hello",'e');
  printf("%s\n", c);
  return 0;
}

Noncompliant Code Example

In this noncompliant example, the function f() is defined to take an argument of type long but f() is called from another file with an argument of type int:

/* In another source file */
long f(long x) {
  return x < 0 ? -x : x;
}

/* In this source file, no f prototype in scope */
long f();
 
long g(int x) {
  return f(x);
}

Compliant Solution

In this compliant solution, the prototype for the function f() is included in the source file in the scope of where it is called, and the function f() is correctly called with an argument of type long:

/* In another source file */
 
long f(long x) {
  return x < 0 ? -x : x;
}

/* f prototype in scope in this source file */

long f(long x); 

long g(int x) {
  return f((long)x);  
}

Noncompliant Code Example (POSIX)

The POSIX function open() [IEEE Std 1003.1:2013] is a variadic function with the following prototype:

int open(const char *path, int oflag, ... );

The open() function accepts a third argument to determine a newly created file's access mode. If open() is used to create a new file, and the third argument is omitted, the file may be created with unintended access permissions. (See FIO06-C. Create files with appropriate access permissions.)

In this noncompliant code example from a vulnerability in the useradd() function of the shadow-utils package CVE-2006-1174, the third argument to open() has been accidentally omitted:

fd = open(ms, O_CREAT|O_EXCL|O_WRONLY|O_TRUNC);

Note that, technically, it is also incorrect to pass a third argument to open() when not creating a new file (that is, with the O_CREAT flag not set). A POSIX implementation could, if it wished, return an EINVAL error in this case. However, in practice, it is unlikely to cause a problem.

Compliant Solution (POSIX)

To correct this example, a third argument is specified in the call to open():

#include <fcntl.h>
 
void func(const char *ms, mode_t perms) {
  /* ... */
  int fd;
  fd = open(ms, O_CREAT|O_EXCL|O_WRONLY|O_TRUNC, perms);
  if (fd == -1) {
    /* Handle error */
  }
}

Risk Assessment

Calling a function with incorrect arguments can result in unexpected or unintended program behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography