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If a dynamically sized structure is needed, flexible array members should be used.
Non-Compliant Code Example
In the following non-compliant code, an array of size 1 is declared, but when the struct itself is instantiated, the size computed for malloc() is modified to take into account the full size of the dynamic array.
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However, in the above code, the only member that is guaranteed to be valid, by strict C99 definition, is {{flexArrayStructP\[0\]}}. Thus, for all {{i > 0}}, the results of the assignment are undefined. |
Compliant Solution
This compliant solution uses the flexible array member to achieve a dynamically sized structure.
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However, some restrictions do apply. The incomplete array type must be the last element within the struct. You also cannot have an array of structs if the struct contains flexible array members, and structs that contain a flexible array member cannot be used as a member in the middle of another struct.
Risk Assessment
Although the non-compliant method results in undefined behavior, it will work under most architectures.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
MEM38-C | 1 (low) | 1 (unlikely) | 3 (low) | P3 | L3 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[McCluskey 01|AA. C References#McCluskey 01]\] ;login:, July 2001, Volume 26, Number 4 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.2.1, "Structure and union specifiers" |