{"record":{"id":"9a68bf7ac3c218ae","repo":"java-native-access/jna","slug":"structure-exceeds-provided-memory-bounds","errorCode":null,"errorMessage":"Structure exceeds provided memory bounds","messagePattern":"Structure exceeds provided memory bounds","errorType":"exception","errorClass":"IllegalArgumentException","httpStatus":null,"severity":"error","filePath":"src/com/sun/jna/Structure.java","lineNumber":376,"sourceCode":"                this.memory.write(0, buf, 0, buf.length);\n            }\n            else {\n                if (size == CALCULATE_SIZE) {\n                    size = calculateSize(false);\n                }\n                if (size != CALCULATE_SIZE) {\n                    this.memory = m.share(offset, size);\n                } else {\n                    // Ensure our memory pointer is initialized, even if we can't\n                    // yet figure out a proper size/layout\n                    this.memory = m.share(offset);\n                }\n            }\n            this.array = null;\n            this.readCalled = false;\n        }\n        catch(IndexOutOfBoundsException e) {\n            throw new IllegalArgumentException(\"Structure exceeds provided memory bounds\", e);\n        }\n    }\n\n    /** Ensure this memory has its size and layout calculated and its\n        memory allocated. */\n    protected void ensureAllocated() {\n        ensureAllocated(false);\n    }\n\n    /** Ensure this memory has its size and layout calculated and its\n        memory allocated.\n        @param avoidFFIType used when computing FFI type information\n        to avoid recursion\n    */\n    private void ensureAllocated(boolean avoidFFIType) {\n        if (memory == null) {\n            allocateMemory(avoidFFIType);\n        }","sourceCodeStart":358,"sourceCodeEnd":394,"githubUrl":"https://github.com/java-native-access/jna/blob/d036ad9781adad4b66693e8fa7098e4ac665e0a3/src/com/sun/jna/Structure.java#L358-L394","documentation":"Structure.useMemory(Pointer) wraps an IndexOutOfBoundsException from the underlying memory access and rethrows it as this IllegalArgumentException. It means the externally supplied Pointer is smaller than the Structure's calculated size, so reading/writing the struct would run past the end of that memory block.","triggerScenarios":"Calling structure.useMemory(ptr) (directly or via the constructor Structure(Pointer)) where the pointed-to block is shorter than calculateSize(); also triggered through read/write/toArray paths that call useMemory, and from ensureAllocated when sharing external memory.","commonSituations":"Mapping a Structure onto memory allocated by native code (or another smaller JNA Memory/struct) whose size was computed for a different/older struct definition; adding fields to a Java Structure without updating the native-side buffer size; reusing one small struct's memory for a larger struct via toArray-style sharing.","solutions":["Ensure the supplied Pointer backs at least structure.size() bytes: allocate with new Memory(structure.size()) before useMemory.","Call structure.size()/calculateSize(true) first and size the buffer from that value, especially after modifying struct fields.","If the memory comes from native code, verify the native allocation matches the current Java field layout (field order, type sizes, alignment).","Catch IllegalArgumentException only to produce a better diagnostic; the fix is always providing a large-enough buffer."],"exampleFix":"// before\nMyStruct s = new MyStruct();\ns.useMemory(smallBuffer); // IllegalArgumentException if smallBuffer.size() < s.size()\n\n// after\nMyStruct s = new MyStruct();\nMemory m = new Memory(s.size());\ns.useMemory(m);","handlingStrategy":"validation","validationCode":"MyStruct s = new MyStruct();\nint needed = s.size();\nif (ptr instanceof Memory && ((Memory) ptr).size() < needed) {\n    throw new IllegalArgumentException(\"buffer too small: \" + ((Memory) ptr).size() + \" < \" + needed);\n}","typeGuard":"static boolean fits(Pointer p, Structure s) { return !(p instanceof Memory) || ((Memory) p).size() >= s.size(); }","tryCatchPattern":"try {\n    structure.useMemory(ptr);\n} catch (IllegalArgumentException e) {\n    // backing block smaller than structure.size(); reallocate and retry\n    structure.useMemory(new Memory(structure.size()));\n}","preventionTips":["Always size external buffers from structure.size() at the point of use, not a hard-coded constant.","Re-run calculateSize(true) after editing Structure fields and resize buffers accordingly.","Prefer JNA auto-allocation unless overlaying genuinely external native memory.","When overlaying native memory, assert the native allocation size matches the Java struct layout."],"tags":["jna","structure","memory-bounds","buffer-overflow"],"backgroundTag":"value-out-of-range","analyzedSha":"d036ad9781adad4b66693e8fa7098e4ac665e0a3","analyzedAt":"2026-09-12T06:50:59.239Z","contentChangedAt":"2026-09-12T06:50:59.239Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}