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A switch statement can be mixed with a block of code by starting the block in one case label, then having another case label within the block. The block can be pictured as spanning more than one case statement.

Section Subclause 6.8.4.2, paragraph 2, of the C Standard [ISO/IEC 9899:2011] says,

If a switch statement has an associated case or default label within the scope of an identifier with a variably modified type, the entire switch statement shall be within the scope of that identifier.¹⁵⁴

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The examples here fall under the exception MSC17-C-EX2 in MSC17-C. Finish every set of statements associated with a case label with a break statement.

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int f(int i) {
  int j=0;
  switch (i) {
    case 1:
      for(j=0;j<10;j++) {
      //* No break,; process case 2 as well */
    case 2: //* switch jumps inside the for block */
        j++;
      //* No break,; process case 3 as well */
    case 3:
        j++;
      }
      break;
  default:
    //* Default action */
    break;
  }
  return j;
}

Implementation Details

When i = 1, the entire for loop is executed. When i = 2, two increments to j are made before the loop starts. When i = 3, one increment to j is made before the loop starts. The default case is no loop. Consequently, the function has the following behavior:

Compliant

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Solution

The compliant example solution separates the switch and for blocks:

int f(int i) {
  int j=0;
  switch (i) {
    case 1:
      //* No break,; process case 2 as well */
    case 2:
      j++;
      //* No break,; process case 3 as well */
    case 3:
      j++;
      break;
    default:
      //* Default action */
      return j;
  }
  for(j++;j<10;j++) {
    j+=2;
  }
  return j;
}

Noncompliant Code Example (Duff's Device)

Duff's device is a curious optimization applied to code intended to perform a serial copy. That is, it copies a series of bytes into one memory output in turn. A simple code to do this would be as follows:

size_t count; /* Must be nonzero */
char *to;     /* Output destination */
char *from;   /* Points to count bytes to copy */

do {
  *to = *from++;     /*
                  /    * Note that the "to" pointer 
                      * is NOT incremented.
                      */
} while (--count > 0);

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The code is widely considered to be legal valid C and C++ and is supported by all compliant compilers. When describing Duff's device, the creator [Duff 1988] noted,

Many people . . . have said that the worst feature of C is that switches don't break automatically before each case label. This code forms some sort of argument in that debate, but I'm not sure whether it's for or against.

Compliant

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Solution (Duff's Device)

This is an alternative implementation of Duff's device, which separates the switch statement and loop:

int n = (count + 7) / 8;
switch (count % 8) {
  case 0: *to = *from++; /* fallFall through */
  case 7: *to = *from++; /* fallFall through */
  case 6: *to = *from++; /* fallFall through */
  case 5: *to = *from++; /* fallFall through */
  case 4: *to = *from++; /* fallFall through */
  case 3: *to = *from++; /* fallFall through */
  case 2: *to = *from++; /* fallFall through */
  case 1: *to = *from++; /* fallFall through */
}
while (--n > 0) {
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
  *to = *from++;
}

Risk Assessment

Automated Detection

Related Guidelines

Bibliography

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