Every declaration should be for a single variable, on its own line, with an explanatory comment about the role of the variable. Declaring multiple variables in a single declaration can cause confusion regarding the types of the variables and their initial values. If more than one variable is declared in a declaration, care must be taken that the type and initialized value of the variable are handled correctly.

Noncompliant Code Example

In this noncompliant code example, a programmer or code reviewer might mistakenly believe that the two variables src and c are declared as char *. In fact, src has a type of char *, whereas c has a type of char.

char *src = 0, c = 0;

Compliant Solution

In this compliant solution, each variable is declared on a separate line:

char *src;    /* Source string */
char c;       /* Character being tested */

Although this change has no effect on compilation, the programmer's intent is clearer.

Noncompliant Code Example

In this noncompliant code example, a programmer or code reviewer might mistakenly believe that both i and j have been initialized to 1. In fact, only j has been initialized, and i remains uninitialized.

int i, j = 1;

Compliant Solution

In this compliant solution, it is readily apparent that both i and j have been initialized to 1:

int i = 1;
int j = 1;

Exceptions

DCL04-C-EX1: Multiple loop control variables can be declared in the same for statement, as shown in the following function:

#include <limits.h>  /* For CHAR_BIT */
#include <stddef.h>  /* For size_t   */
 
extern size_t popcount(uintmax_t);
#define PRECISION(umax_value) popcount(umax_value) 

 size_t bitcount(size_t n) {
  const size_t limit = PRECISION(SIZE_MAX);
  size_t count = 0;
  for (size_t i = 0, j = 1; i < limit; ++i, j <<= 1) {
    if (n & j)
      ++count;
  }
  return count;
}

The PRECISION() macro provides the correct precision for any integer type and is defined in INT35-C. Use correct integer precisions—see that rule for more information.

DCL04-C-EX2: Multiple, simple variable declarations can be declared on the same line given that there are no initializations. A simple variable declaration is one that is not a pointer or array.

int i, j, k;

Risk Assessment

Declaring no more than one variable per declaration can make code easier to read and eliminate confusion.

Automated Detection

Tool	Version	Checker	Description
Axivion Bauhaus Suite		CertC-DCL04
CodeSonar		LANG.STRUCT.DECL.ML	Multiple Declarations on Line
ECLAIR		CC2.DCL04	Fully implemented
LDRA tool suite		579 S	Fully implemented
Parasoft C/C++test		CERT_C-DCL04-a	Each variable should be declared in a separate declaration statement
PC-lint Plus		9146	Partially supported: exceptions not supported
SonarQube C/C++ Plugin		SingleDeclarationPerStatement

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines