CertC-DCL31ΒΆ

Declare identifiers before using them

Required inputs: IR

The C11 Standard requires type specifiers and forbids implicit function declarations. The C90 Standard allows implicit typing of variables and functions. Consequently, some existing legacy code uses implicit typing. Some C compilers still support legacy code by allowing implicit typing, but it should not be used for new code. Such an implementation may choose to assume an implicit declaration and continue translation to support existing programs that used this feature.

Noncompliant Code Example (Implicit int)

C no longer allows the absence of type specifiers in a declaration. The C Standard, 6.7.2 [ ISO/IEC 9899:2011 ], states

At least one type specifier shall be given in the declaration specifiers in each declaration, and in the specifier-qualifier list in each struct declaration and type name.

This noncompliant code example omits the type specifier:

extern foo;

Some C implementations do not issue a diagnostic for the violation of this constraint. These nonconforming C translators continue to treat such declarations as implying the type int.

Compliant Solution (Implicit int)

This compliant solution explicitly includes a type specifier:

extern int foo;
Noncompliant Code Example (Implicit Function Declaration)

Implicit declaration of functions is not allowed; every function must be explicitly declared before it can be called. In C90, if a function is called without an explicit prototype, the compiler provides an implicit declaration.

The C90 Standard [ ISO/IEC 9899:1990] includes this requirement:

If the expression that precedes the parenthesized argument list in a function call consists solely of an identifier, and if no declaration is visible for this identifier, the identifier is implicitly declared exactly as if, in the innermost block containing the function call, the declaration extern int identifier(); appeared.

If a function declaration is not visible at the point at which a call to the function is made, C90-compliant platforms assume an implicit declaration of extern int identifier();.

This declaration implies that the function may take any number and type of arguments and return an int. However, to conform to the current C Standard, programmers must explicitly prototype every function before invoking it. An implementation that conforms to the C Standard may or may not perform implicit function declarations, but C does require a conforming implementation to issue a diagnostic if it encounters an undeclared function being used.

In this noncompliant code example, if malloc() is not declared, either explicitly or by including stdlib.h, a compiler that conforms only to C90 may implicitly declare  malloc() as  int malloc(). If the platform's size of  int is 32 bits, but the size of pointers is 64 bits, the resulting pointer would likely be truncated as a result of the implicit declaration of  malloc(), returning a 32-bit integer. 

#include <stddef.h>
/* #include <stdlib.h> is missing */

int main(void) {
  for (size_t i = 0; i < 100; ++i) {
    /* int malloc() assumed */
    char *ptr = (char *)malloc(0x10000000);
    *ptr = 'a';
  }
  return 0;
}
Implementation Details

When compiled with Microsoft Visual Studio 2013 for a 64-bit platform, this noncompliant code example will eventually cause an access violation when dereferencing  ptr in the loop.

Compliant Solution (Implicit Function Declaration)

This compliant solution declares  malloc() by including the appropriate header file: 

#include <stdlib.h>

int main(void) {
  for (size_t i = 0; i < 100; ++i) {
    char *ptr = (char *)malloc(0x10000000);
    *ptr = 'a';
  }
  return 0;
}

For more information on function declarations, see  DCL07-C. Include the appropriate type information in function declarators.

Noncompliant Code Example (Implicit Return Type)

Do not declare a function with an implicit return type. For example, if a function returns a meaningful integer value, declare it as returning int. If it returns no meaningful value, declare it as returning void. 

#include <limits.h>
#include <stdio.h>
 
foo(void) {
  return UINT_MAX;
}

int main(void) {
  long long int c = foo();
  printf("%lld\n", c);
  return 0;
}

Because the compiler assumes that foo() returns a value of type int for this noncompliant code example, UINT_MAX is incorrectly converted to -1.

Compliant Solution (Implicit Return Type)

This compliant solution explicitly defines the return type of foo() as unsigned int. As a result, the function correctly returns UINT_MAX . 

#include <limits.h>
#include <stdio.h>

unsigned int foo(void) {
  return UINT_MAX;
}

int main(void) {
  long long int c = foo();
  printf("%lld\n", c);
  return 0;
}
Risk Assessment

Because implicit declarations lead to less stringent type checking, they can introduce unexpected and erroneous behavior. Occurrences of an omitted type specifier in existing code are rare, and the consequences are generally minor, perhaps resulting in abnormal program termination.

Rule Severity Likelihood Remediation Cost Priority Level
DCL31-C Low Unlikely Low P3 L3
Related Guidelines
Taxonomy Taxonomy item Relationship
CERT C Secure Coding Standard DCL07-C. Include the appropriate type information in function declarators Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013 Subprogram Signature Mismatch [OTR] Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012 Rule 8.1 (required) Prior to 2018-01-12: CERT: Unspecified Relationship
Bibliography
[ ISO/IEC 9899:1990]
[ ISO/IEC 9899:2011] Subclause 6.7.2, "Type Specifiers"
[ Jones 2008]
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/declarations-and-initialization-dcl/dcl31-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

implicit_int

Type shall be explicitly stated

None

False

reference_to_missing_function_prototype

Referenced function needs prototype declaration

None

False

reference_to_undeclared_function

Referenced function needs a declaration

None

False

Options