According to the C Standard, 6.8.4.2, paragraph 4 [ISO/IEC 9899:2011],
A switch statement causes control to jump to, into, or past the statement that is the switch body, depending on the value of a controlling expression, and on the presence of a default label and the values of any case labels on or in the switch body.
If a programmer declares variables, initializes them before the first case statement, and then tries to use them inside any of the case statements, those variables will have scope inside the switch
block but will not be initialized and will consequently contain indeterminate values. Reading such values also violates EXP33-C. Do not read uninitialized memory.
This noncompliant code example declares variables and contains executable statements before the first case label within the switch
statement:
#include <stdio.h> extern void f(int i); void func(int expr) { switch (expr) { int i = 4; f(i); case 0: i = 17; /* Falls through into default code */ default: printf("%d\n", i); } } |
When the preceding example is executed on GCC 4.8.1, the variable i
is instantiated with automatic storage duration within the block, but it is not initialized. Consequently, if the controlling expression expr
has a nonzero value, the call to printf()
will access an indeterminate value of i
. Similarly, the call to f()
is not executed.
Value of |
|
---|---|
0 | 17 |
Nonzero | Indeterminate |
In this compliant solution, the statements before the first case label occur before the switch
statement:
#include <stdio.h> extern void f(int i); int func(int expr) { /* * Move the code outside the switch block; now the statements * will get executed. */ int i = 4; f(i); switch (expr) { case 0: i = 17; /* Falls through into default code */ default: printf("%d\n", i); } return 0; } |
Using test conditions or initializing variables before the first case statement in a switch
block can result in unexpected behavior and undefined behavior.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
DCL41-C | Medium | Unlikely | Medium | P4 | L3 |
Tool | Version | Checker | Description |
Astrée | switch-skipped-code | Fully checked | |
Axivion Bauhaus Suite | CertC-DCL41 | Fully implemented | |
Clang | -Wsometimes-uninitialized | ||
CodeSonar | LANG.STRUCT.SW.BAD | Malformed switch Statement | |
Coverity | MISRA C 2004 Rule 15.0 MISRA C 2012 Rule 16.1 | Implemented | |
Helix QAC | C2008, C2882, C3234 | Fully implemented | |
Klocwork | CERT.DCL.SWITCH.VAR_BEFORE_CASE | Fully implemented | |
LDRA tool suite | 385 S | Fully implemented | |
Parasoft C/C++test | CERT_C-DCL41-a | A switch statement shall only contain switch labels and switch clauses, and no other code | |
PC-lint Plus | 527 | Assistance provided | |
Polyspace Bug Finder | CERT C: Rule DCL41-C | Checks for ill-formed switch statements (rule partially covered) | |
PVS-Studio | V622 | ||
RuleChecker | switch-skipped-code | Fully checked | |
TrustInSoft Analyzer | initialisation | Exhaustively detects undefined behavior (see the compliant and the non-compliant example). |
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Key here (explains table format and definitions)
Taxonomy | Taxonomy item | Relationship |
---|---|---|
MISRA C:2012 | Rule 16.1 (required) | Prior to 2018-01-12: CERT: Unspecified Relationship |
[ISO/IEC 9899:2011] | 6.8.4.2, "The switch Statement" |