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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 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 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 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

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 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 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 205.

Functions That Should Not Be Called with Complex Values

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In this noncompliant example, the 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 9899:20112024]

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 the referenced type, the behavior is undefined.

See undefined behavior 2625.

#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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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.

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• Wrong argument values or references

Bibliography

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