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Since the introduction of TJFLAG_NOREALLOC in libjpeg-turbo 1.2.x, the TurboJPEG C API documentation has (confusingly) stated that: - if the JPEG buffer pointer points to a pre-allocated buffer, then the JPEG buffer size must be specified, and - the JPEG buffer size should be specified if the JPEG buffer is pre-allocated to an arbitrary size. The documentation never explicitly stated that the JPEG buffer size should be specified if the JPEG buffer is pre-allocated to a worst-case size, but since focus does not imply exclusion, it also never explicitly stated the reverse. Furthermore, the documentation never stated that this was contingent upon TJPARAM_NOREALLOC/TJFLAG_NOREALLOC. However, effectively the compression and lossless transformation functions ignored the JPEG buffer size(s) passed to them, and assumed that the JPEG buffer(s) had been allocated to a worst-case size, if TJPARAM_NOREALLOC/TJFLAG_NOREALLOC was set. This behavior was an accidental and undocumented throwback to libjpeg-turbo 1.1.x, in which the tjCompress() function provided no way to specify the JPEG buffer size. It was always a bad idea for applications to rely upon that behavior (although our own TJBench application unfortunately did.) However, if such applications exist in the wild, the new behavior would constitute a breaking change, so it has been introduced only into libjpeg-turbo 3.1.x and only into TurboJPEG 3 API functions. The previous behavior has been retained when calling functions from the TurboJPEG 2.1.x API and prior versions. Did I mention that APIs are hard?