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POConsistency of two C FILE* streams on a single file
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copied!<p>I need to implement a simple "spill to disk" layer for large volume of data coming off a network socket. I was hoping to have two C <code>FILE*</code> streams, one used by a background thread writing to the file, one used by a front end thread reading it.</p> <p>The two streams are so one thread can be writing at one offset, while the other is reading elsewhere - without taking a lock and blocking the other thread.</p> <p>There will be a paging mechanism so the reads/writes are at random access locations - not necessarily sequential.</p> <p>One more caveat, this needs to work on Windows and Linux.</p> <p>The question: after the fwrite to the first stream has returned, is that written data guaranteed to be immediately visible to an fread on the second stream?</p> <p>If not, what other options might I consider?</p> <hr> <p>So Posix pread/pwrite functions turned out to be what I needed. Here's a version for Win32:</p> <pre><code>size_t pread64(int fd, void* buf, size_t nbytes, __int64 offset) { OVERLAPPED ovl; memset(&ovl, 0, sizeof(ovl)); *((__int64*)&ovl.Offset)=offset; DWORD nBytesRead; if (!ReadFile((HANDLE)_get_osfhandle(fd), buf, nbytes, &nBytesRead, &ovl)) return -1; return nBytesRead; } size_t pwrite64(int fd, void* buf, size_t nbytes, __int64 offset) { OVERLAPPED ovl; memset(&ovl, 0, sizeof(ovl)); *((__int64*)&ovl.Offset)=offset; DWORD nBytesWritten; if (!WriteFile((HANDLE)_get_osfhandle(fd), buf, nbytes, &nBytesWritten, &ovl)) return -1; return nBytesWritten; } </code></pre> <p>(And thank you everyone for input on this - much appreciated).</p>

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