CertC-DCL37¶

Do not declare or define a reserved identifier

Required inputs: IR

According to the C Standard, 7.1.3 [ ISO/IEC 9899:2011],

All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.

All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included, unless explicitly stated otherwise.

All identifiers with external linkage (including future library directions) and errno are always reserved for use as identifiers with external linkage.

Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.

Additionally, subclause 7.31 defines many other reserved identifiers for future library directions.

No other identifiers are reserved. (The POSIX standard extends the set of identifiers reserved by the C Standard to include an open-ended set of its own. See Portable Operating System Interface [POSIX^®], Base Specifications, Issue 7, Section 2.2, "The Compilation Environment" [ IEEE Std 1003.1-2013].) The behavior of a program that declares or defines an identifier in a context in which it is reserved or that defines a reserved identifier as a macro name is undefined. (See undefined behavior 106.)

Noncompliant Code Example (Include Guard)

A common, but noncompliant, practice is to choose a reserved name for a macro used in a preprocessor conditional guarding against multiple inclusions of a header file. (See also PRE06-C. Enclose header files in an include guard.) The name may clash with reserved names defined by the implementation of the C standard library in its headers or with reserved names implicitly predefined by the compiler even when no C standard library header is included.

#ifndef _MY_HEADER_H_
#define _MY_HEADER_H_

/* Contents of <my_header.h> */

#endif /* _MY_HEADER_H_ */

Compliant Solution (Include Guard)

This compliant solution avoids using leading underscores in the macro name of the include guard:

#ifndef MY_HEADER_H
#define MY_HEADER_H

/* Contents of <my_header.h> */

#endif /* MY_HEADER_H */

Noncompliant Code Example (File Scope Objects)

In this noncompliant code example, the names of the file scope objects _max_limit and _limit both begin with an underscore. Because _max_limit is static, this declaration might seem to be impervious to clashes with names defined by the implementation. However, because the header <stddef.h> is included to define size_t, a potential for a name clash exists. (Note, however, that a conforming compiler may implicitly declare reserved names regardless of whether any C standard library header is explicitly included.)

In addition, because _limit has external linkage, it may clash with a symbol of the same name defined in the language runtime library even if such a symbol is not declared in any header. Consequently, it is not safe to start the name of any file scope identifier with an underscore even if its linkage limits its visibility to a single translation unit.

#include <stddef.h>

static const size_t _max_limit = 1024;
size_t _limit = 100;

unsigned int getValue(unsigned int count) {
  return count < _limit ? count : _limit;
}

Compliant Solution (File Scope Objects)

In this compliant solution, names of file scope objects do not begin with an underscore:

#include <stddef.h>

static const size_t max_limit = 1024;
size_t limit = 100;

unsigned int getValue(unsigned int count) {
  return count < limit ? count : limit;
}

Noncompliant Code Example (Reserved Macros)

In this noncompliant code example, because the C standard library header <inttypes.h> is specified to include <stdint.h>, the name SIZE_MAX conflicts with a standard macro of the same name, which is used to denote the upper limit of size_t. In addition, although the name INTFAST16_LIMIT_MAX is not defined by the C standard library, it is a reserved identifier because it begins with the INT prefix and ends with the _MAX suffix. (See the C Standard, 7.31.10.)

#include <inttypes.h>
#include <stdio.h>

static const int_fast16_t INTFAST16_LIMIT_MAX = 12000;

void print_fast16(int_fast16_t val) {
  enum { SIZE_MAX = 80 };
  char buf[SIZE_MAX];
  if (INTFAST16_LIMIT_MAX < val) {
    sprintf(buf, "The value is too large");
  } else {
    snprintf(buf, SIZE_MAX, "The value is %" PRIdFAST16, val);
  }
}

Compliant Solution (Reserved Macros)

This compliant solution avoids redefining reserved names or using reserved prefixes and suffixes:

#include <inttypes.h>
#include <stdio.h>
 
static const int_fast16_t MY_INTFAST16_UPPER_LIMIT = 12000;

void print_fast16(int_fast16_t val) {
  enum { BUFSIZE = 80 };
  char buf[BUFSIZE];
  if (MY_INTFAST16_UPPER_LIMIT < val) {
    sprintf(buf, "The value is too large");
  } else {
    snprintf(buf, BUFSIZE, "The value is %" PRIdFAST16, val);
  }
}

Noncompliant Code Example (Identifiers with External Linkage)

This noncompliant example provides definitions for the C standard library functions malloc() and free(). Although this practice is permitted by many traditional implementations of UNIX (for example, the Dmalloc library), it is undefined behavior according to the C Standard. Even on systems that allow replacing malloc(), doing so without also replacing aligned_alloc(), calloc(), and realloc() is likely to cause problems.

#include <stddef.h>
 
void *malloc(size_t nbytes) {
  void *ptr;
  /* Allocate storage from own pool and set ptr */
  return ptr;
}

void free(void *ptr) {
  /* Return storage to own pool */
}

Compliant Solution (Identifiers with External Linkage)

The compliant, portable solution avoids redefining any C standard library identifiers with external linkage. In addition, it provides definitions for all memory allocation functions:

#include <stddef.h>

void *my_malloc(size_t nbytes) {
  void *ptr;
  /* Allocate storage from own pool and set ptr */
  return ptr;
}

void *my_aligned_alloc(size_t alignment, size_t size) {
  void *ptr;
  /* Allocate storage from own pool, align properly, set ptr */
  return ptr;
}

void *my_calloc(size_t nelems, size_t elsize) {
  void *ptr;
  /* Allocate storage from own pool, zero memory, and set ptr */
  return ptr;
}

void *my_realloc(void *ptr, size_t nbytes) {
  /* Reallocate storage from own pool and set ptr */
  return ptr;
}

void my_free(void *ptr) {
  /* Return storage to own pool */
}

Noncompliant Code Example ( `errno`)

In addition to symbols defined as functions in each C standard library header, identifiers with external linkage include errno and math_errhandling. According to the C Standard, 7.5, paragraph 2 [ ISO/IEC 9899:2011], the behavior of a program is undefined when

A macro definition of errno is suppressed in order to access an actual object, or the program defines an identifier with the name errno.

See undefined behavior 114.

The errno identifier expands to a modifiable lvalue that has type int but is not necessarily the identifier of an object. It might expand to a modifiable lvalue resulting from a function call, such as *errno(). It is unspecified whether errno is a macro or an identifier declared with external linkage. If a macro definition is suppressed to access an actual object, or if a program defines an identifier with the name errno, the behavior is undefined.

Legacy code is apt to include an incorrect declaration, such as the following:

extern int errno;

Compliant Solution ( `errno`)

The correct way to declare errno is to include the header <errno.h>:

#include <errno.h>

Implementations conforming to C are required to declare errno in <errno.h>, although some historic implementations failed to do so.

Exceptions

DCL37-C-EX1: Provided that a library function can be declared without reference to any type defined in a header, it is permissible to declare that function without including its header provided that declaration is compatible with the standard declaration.

/* Not including stdlib.h */
void free(void *);
 
void func(void *ptr) {
  free(ptr);
}

Such code is compliant because the declaration matches what stdlib.h would provide and does not redefine the reserved identifier. However, it would not be acceptable to provide a definition for the free() function in this example.

DCL37-C-EX2: For compatibility with other compiler vendors or language standard modes, it is acceptable to create a macro identifier that is the same as a reserved identifier so long as the behavior is idempotent, as in this example:

/* Sometimes generated by configuration tools such as autoconf */
#define const const

/* Allowed compilers with semantically equivalent extension behavior */
#define inline __inline

DCL37-C-EX3: As a compiler vendor or standard library developer, it is acceptable to use identifiers reserved for your implementation. Reserved identifiers may be defined by the compiler, in standard library headers or headers included by a standard library header, as in this example declaration from the glibc standard C library implementation:

/*
  The following declarations of reserved identifiers exist in the glibc implementation of
  <stdio.h>. The original source code may be found at:
  https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=include/stdio.h;hb=HEAD
*/

#  define __need_size_t
#  include <stddef.h>
/* Generate a unique file name (and possibly open it).  */
extern int __path_search (char *__tmpl, size_t __tmpl_len,
                       const char *__dir, const char *__pfx,
                       int __try_tempdir);

Risk Assessment

Using reserved identifiers can lead to incorrect program operation.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
DCL37-C	Low	Unlikely	Low	P3	L3

Related Guidelines

Taxonomy	Taxonomy item	Relationship
CERT C Secure Coding Standard	PRE00-C. Prefer inline or static functions to function-like macros	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C Secure Coding Standard	PRE06-C. Enclose header files in an include guard	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C Secure Coding Standard	PRE31-C. Avoid side effects in arguments to unsafe macros	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	DCL51-CPP. Do not declare or define a reserved identifier	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961	Using identifiers that are reserved for the implementation [resident]	Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012	Rule 21.1 (required)	Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012	Rule 21.2 (required)	Prior to 2018-01-12: CERT: Unspecified Relationship

Bibliography

[ IEEE Std 1003.1-2013]	Section 2.2, "The Compilation Environment"
[ ISO/IEC 9899:2011]	7.1.3, "Reserved Identifiers" 7.31.10, "Integer Types `<stdint.h>`"

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/dcl37-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
decl_using_library_macro	The names of standard library macros, objects and functions, as well as reserved identifiers, shall not be reused.	None	False
enumerator_having_libname	The names of standard library macros, objects and functions, as well as reserved identifiers, shall not be reused.	None	False
field_having_libname	The names of standard library macros, objects and functions shall not be reused.	None	False
macro_having_libname	The names of standard library macros, objects and functions shall not be reused.	None	False
macro_having_reserved_name	Definition of reserved identifier or standard library element	None	False
routine_having_libname	The names of standard library macros, objects and functions, as well as reserved identifiers, shall not be reused.	None	False
type_having_libname	The names of standard library macros, objects and functions shall not be reused.	None	False
undef_of_reserved_name	#undef of reserved identifier or standard library element	None	False
variable_having_libname	The names of standard library macros, objects and functions, as well as reserved identifiers, shall not be reused.	None	False

Options¶

This rule shares the following common options: exclude_in_macros, exclude_messages_in_system_headers, excludes, extend_exclude_to_macro_invocations, includes, justification_checker, languages, post_processing, provider, report_at, severity
The following places define options that affect this rule: Stylechecks, Analysis-GlobalOptions

additional_reserved_identifiers¶

additional_reserved_identifiers : set[str] = {'assert', 'defined', 'errno'}

Names listed here are seen as violations as well.

allow_function_declarations¶

allow_function_declarations : bool = True

Whether nondefining function declarations with library names are allowed

allow_identifiers¶

allow_identifiers : set[bauhaus.analysis.config.GlobPattern] = {'_CRT_*_NO_WARNINGS'}

Name patterns listed here are not seen as violations.

allow_reserved_identifier_as_include_guard¶

allow_reserved_identifier_as_include_guard : bool = False

If True, macros used for include guards are not checked for being a reserved identifier.

check_for_keyword¶

check_for_keyword : bool = False

Whether macro names being keywords should be reported as well.

check_locals¶

check_locals : bool = True

Whether parameters and local variables should also be checked.

check_reserved_enum_identifier¶

check_reserved_enum_identifier : bool = True

Whether enumerator names should be checked to use a reserved identifier

check_reserved_function_identifier¶

check_reserved_function_identifier : bool = True

Whether function names should be checked to use a reserved identifier

check_reserved_macro_identifier¶

check_reserved_macro_identifier : bool = False

Whether to report reserved names (e.g. names starting with underscore).

check_reserved_type_identifier¶

check_reserved_type_identifier : bool = False

Whether type names should be checked to use a reserved identifier.

check_reserved_variable_identifier¶

check_reserved_variable_identifier : bool = True

Whether variable names should be checked to use a reserved identifier

report_fields¶

report_fields : bool = True

Whether fields using a library name should be reported.

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Navigation

CertC-DCL37¶

Noncompliant Code Example (Include Guard)

Compliant Solution (Include Guard)

Noncompliant Code Example (File Scope Objects)

Compliant Solution (File Scope Objects)

Noncompliant Code Example (Reserved Macros)

Compliant Solution (Reserved Macros)

Noncompliant Code Example (Identifiers with External Linkage)

Compliant Solution (Identifiers with External Linkage)

Noncompliant Code Example ( `errno`)

Compliant Solution ( `errno`)

Exceptions

Risk Assessment

Related Guidelines

Bibliography

Options¶

additional_reserved_identifiers¶

allow_function_declarations¶

allow_identifiers¶

allow_reserved_identifier_as_include_guard¶

check_for_keyword¶

check_locals¶

check_reserved_enum_identifier¶

check_reserved_function_identifier¶

check_reserved_macro_identifier¶

check_reserved_type_identifier¶

check_reserved_variable_identifier¶

report_fields¶

CertC-DCL37¶

Noncompliant Code Example (Include Guard)

Compliant Solution (Include Guard)

Noncompliant Code Example (File Scope Objects)

Compliant Solution (File Scope Objects)

Noncompliant Code Example (Reserved Macros)

Compliant Solution (Reserved Macros)

Noncompliant Code Example (Identifiers with External Linkage)

Compliant Solution (Identifiers with External Linkage)

Noncompliant Code Example ( errno)

Compliant Solution ( errno)

Exceptions

Risk Assessment

Related Guidelines

Bibliography

Options¶

additional_reserved_identifiers¶

allow_function_declarations¶

allow_identifiers¶

allow_reserved_identifier_as_include_guard¶

check_for_keyword¶

check_locals¶

check_reserved_enum_identifier¶

check_reserved_function_identifier¶

check_reserved_macro_identifier¶

check_reserved_type_identifier¶

check_reserved_variable_identifier¶

report_fields¶

Noncompliant Code Example ( `errno`)

Compliant Solution ( `errno`)