DCL04-C. Do not declare more than one variable per declaration

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

Related Vulnerabilities

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

Related Guidelines