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

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

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Compliant Solution (Real Number)

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

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

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

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#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;
}

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Compliant Solution

In this noncompliant examplecompliant solution, the function pointer fp, which points 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 parameters and 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:

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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 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, 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 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 */
  }
}

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Bibliography

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