MISRA C:2012 Rule 12.3

The comma operator should not be used

Description

Rule Definition

The comma operator should not be used.

Rationale

The comma operator can be detrimental to readability. You can often write the same code in another form.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

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Comma Usage in C Code

typedef signed int abc, xyz, jkl;
static void func1 ( abc, xyz, jkl );       /* Compliant - case 1 */
int foo(void)
{
    volatile int rd = 1;                   /* Compliant - case 2*/
    int var=0, foo=0, k=0, n=2, p, t[10];  /* Compliant - case 3*/
    int abc = 0, xyz = abc + 1;            /* Compliant - case 4*/
    int jkl = ( abc + xyz, abc + xyz );    /* Noncompliant - case 1*/
    var = 1, foo += var, n = 3;          /* Noncompliant - case 2*/ 
    var = (n = 1, foo = 2);              /* Noncompliant - case 3*/
    for ( int *ptr = &t[ 0 ],var = 0 ;
          var < n; ++var, ++ptr){}    /* Noncompliant - case 4*/
    if ((abc,xyz)<0) { return 1; }         /* Noncompliant - case 5*/
}

In this example, the code shows various uses of commas in C code.

Noncompliant Cases

Case	Reason for noncompliance
1	When reading the code, it is not immediately obvious what `jkl` is initialized to. For example, you could infer that `jkl` has a value `abc+xyz`, `(abc+xyz)*(abc+xyz)`, `f((abc+xyz),(abc+xyz))`, and so on.
2	When reading the code, it is not immediately obvious whether `foo` has a value 0 or 1 after the statement.
3	When reading the code, it is not immediately obvious what value is assigned to `var`.
4	When reading the code, it is not immediately obvious which values control the `for` loop.
5	When reading the code, it is not immediately obvious whether the `if` statement depends on `abc`, `xyz`, or both.

Compliant Cases

Case	Reason for compliance
1	Using commas to call functions with variables is allowed.
2	Comma operator is not used.
3 & 4	When using the comma for initialization, the variables and their values are immediately obvious.

Check Information

Group: Expressions

Category: Advisory

AGC Category: Advisory