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

Functions that are appropriately declared (as in DCL07DCL40-C. Include the appropriate type information in function declarators) will typically generate a compiler error if Do not create incompatible declarations of the same function or object) will typically generate a compiler diagnostic message 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. While Although such warnings should be resolved, they do not prevent program compilation. (See MSC00-C. Compile cleanly at high warning levels.)

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 also undefined behavior 26 in Annex J of the C Standard.)

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

char *(*fp)();

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

Compliant Solution

In this compliant solution, the function pointer fp points to a function returning char * with the correct number and type of arguments:

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

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

int main 
void func(void) {
  constdouble charcomplex *c;
  fp = strchrc = 2.0 + 4.0 * I;
  cdouble = fp("Hello",'e');
  printf("%s\n", c);
  return 0complex result = log2(c);
}

Noncompliant Code Example

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Compliant Solution (Complex Number)

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

/* In another source file */
#include <string<tgmath.h>
void copy(char *dst, const char *src, size_t size) {
void  if (!strncpy(dst, src, size))func(void) {
  double complex /*c Report= error */
  }
}
 
/* In this source file, no copy prototype in scope */
void copy();
 
void g(const char *s) {
  enum { BUFFERSIZE = 20 };
  char buf[BUFFERSIZE];
  copy(buf, s2.0 + 4.0 * I;
  double complex result = log(c)/log(2);
}

Compliant Solution

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(Real Number)

The programmer can use In this compliant solution , the prototype for the copy() function is included in the scope in the source file where it is used, and the copy() function is passed the correct number and type of argumentsif the intent is to take the base-2 logarithm of the real part of the complex number:

/* In another source file */
#include <string<tgmath.h>
 
void copy(char *dst, const char *src, size_t sizefunc(void) {
  if (strncpy(dst, src, size) == 0) {
    /* Report error */
  }
}
 
/* Copy prototype in scope in this source file */
void copy(char *dst, const char *src, size_t size);
 
void g(const char *s) {
  enum { BUFFERSIZE = 20 };
  char buf[BUFFERSIZE];
  copy(buf, s, BUFFERSIZE); 
}
double complex c = 2.0 + 4.0 * I;
  double complex result = log2(creal(c));
}

Noncompliant Code Example

In this noncompliant example, the

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C standard library function strchr() is called through the function pointer fp declared with a prototype with incorrectly typed arguments. According to the C Standard, 6.3.2.3, paragraph 8 [ISO/IEC 9899:2024]

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 25.

/* In another source file */
void buginf(const char *fmt, ...#include <stdio.h>
#include <string.h>

char *(*fp)();

int main(void) {
   /* ... */
}
 
/* In this source file, no buginf prototype in scope */
void buginf();
 
void h(void) {
  buginf("bug in function %s, line %d\n", "h", __LINE__);
}

While this code appears to be well-defined due to the prototype-less declaration of buginf(), this code exhibits undefined behavior per subclause 6.5.2.2 paragraph 6 [ISO/IEC 9899:2011]:

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

Compliant Solution

In this compliant solution, the prototype for the function buginffunction pointer fp, which points to the C standard library function strchr(), is included in the scope in the source file where it is useddeclared with the correct parameters and is invoked with the correct number and type of arguments:

/* In another source file */
void buginf#include <stdio.h>
#include <string.h>

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

int main(void) {
   /* ...const char */
}

/* buginf prototype in scope in this source file */
void buginf(const char *fmt, ...);
 
void h(void) {
  buginf("bug in function %s, line %d\n", "h", __LINE__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 scope source file in the source file scope of where it is usedcalled, and the function f() is correctly called with an argument of type long:

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The POSIX function open() [Open Group 2004IEEE Std 1003.1:2013] is a variadic function with the following prototype:

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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 is accidentally omitted:

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

Note that, technicallyTechnically, 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 In this examplecompliant solution, 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

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-685 and EXP37-C

EXP37-C = Union( CWE-685, CWE-686) Intersection( CWE-685, CWE-686) = Ø

CWE-686 and EXP37-C

Intersection( EXP37-C, FIO47-C) =

• Invalid argument types passed to format I/O function

EXP37-C – FIO47-C =

• Invalid argument types passed to non-format I/O function

FIO47-C – EXP37-C =

• Invalid format string, but correctly matches arguments in number and type

EXP37-C = Union( CWE-685, CWE-686)

Intersection( CWE-685, CWE-686) = Ø

CWE-628 and EXP37-C

CWE-628 = Union( EXP37-C, list) where list =

• Improper ordering of function arguments (that does not violate argument types)

• Wrong argument values or references

Bibliography

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