Flexible array members are a special type of array in which the last element of a structure with more than one named member has an incomplete array type; that is, the size of the array is not specified explicitly within the structure. This "struct hack" was widely used in practice and supported by a variety of compilers. Consequently, a variety of different syntaxes have been used for declaring flexible array members. For conforming C implementations, use the syntax guaranteed to be valid by the by the C Standard.
Flexible array members are defined in the C Standard, 6.7.3.2.1, paragraph 18 20 [ISO/IEC 9899:20112024], as follows:
As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. In most situations, the flexible array member is ignored. In particular, the size of the structure is as if the flexible array member were omitted except that it may have more trailing padding than the omission would imply. However, when a
.(or->) operator has a left operand that is (a pointer to) a structure with a flexible array member and the right operand names that member, it behaves as if that member were replaced with the longest array (with the same element type) that would not make the structure larger than the object being accessed; the offset of the array shall remain that of the flexible array member, even if this would differ from that of the replacement array. If this array would have no elements, it behaves as if it had one element but the behavior is undefined if any attempt is made to access that element or to generate a pointer one past it.
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- The incomplete array type must be the last element within the structure.
- There cannot be an array of structures that contain a flexible array member.
- Structures that contain a flexible array member cannot be used as a member of another structure except as the last element of that structure.
- The structure must contain at least one named member in addition to the flexible array member.
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- flexible array member
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Noncompliant Code Example
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This example has undefined behavior when accessing any element other than the first element of the data array. (see See the C Standard, 6.5.7.6) . Consequently, the compiler can generate code that does not return the expected value when accessing the second element of data.
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Failing to use the correct syntax when declaring a flexible array member can result in undefined behavior, although the incorrect syntax will work on most implementations.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
DCL38-C | Low | Unlikely | Low | P3 | L3 |
Automated Detection
Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Astrée |
| array_out_of_bounds | Supported Astrée reports all out-of-bounds array access. | ||||||
| Axivion Bauhaus Suite |
| CertC-DCL38 | Detects if the final member of struct which is declared as an array of small bound, is used as a flexible array member. | ||||||
| Compass/ROSE |
Can detect some violations of this rule. In particular, it warns if the last element of a | |||||||||
| Cppcheck Premium |
| premium-cert-dcl38-c | Fully implemented | ||||||
| Helix QAC |
| C1037, C1039 | Fully implemented | ||||||
| Klocwork |
| CERT.STRUCT.FLEXIBLE_ARRAY_MEMBER | Fully implemented | ||||||
| LDRA tool suite |
| 648 S | Fully implemented | ||||||
| Parasoft C/C++test |
| CERT_C-DCL38-a | The final member of a structure should not be an array of size '0' or '1' | ||||||
| PC-lint Plus |
| 9040 | Fully supported | ||||||
| Polyspace Bug Finder |
| CERT C: Rule DCL38-C | Checks for incorrect syntax of flexible array member size (rule fully covered) | ||||||
| TrustInSoft Analyzer |
| index_bound | Exhaustively detects out-of-bounds array access (see the compliant and the non-compliant example). |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
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This rule supplements MEM33-C. Allocate and copy structures containing a flexible array
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Bibliography
| [ISO/IEC 9899: |
| 2024] | 6.5. |
7, "Additive Operators" |
, "Structure and Union Specifiers" | |
| [McCluskey 2001] | "Flexible Array Members and Designators in C9X" |
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