Function AllocateFileRange

src/commons/util/util.cpp:781–820

this function tries to make a particular range of a file allocated (corresponding to disk space) it is advisory, and the range specified in the arguments will never contain live data

// this function tries to make a particular range of a file allocated (corresponding to disk space)
// it is advisory, and the range specified in the arguments will never contain live data
void AllocateFileRange(FILE* file, unsigned int offset, unsigned int length) {
#if defined(WIN32)
    // Windows-specific version
    HANDLE hFile = (HANDLE)_get_osfhandle(_fileno(file));
    LARGE_INTEGER nFileSize;
    int64_t nEndPos      = (int64_t)offset + length;
    nFileSize.u.LowPart  = nEndPos & 0xFFFFFFFF;
    nFileSize.u.HighPart = nEndPos >> 32;
    SetFilePointerEx(hFile, nFileSize, 0, FILE_BEGIN);
    SetEndOfFile(hFile);
#elif defined(MAC_OSX)
    // OSX specific version
    fstore_t fst;
    fst.fst_flags      = F_ALLOCATECONTIG;
    fst.fst_posmode    = F_PEOFPOSMODE;
    fst.fst_offset     = 0;
    fst.fst_length     = (off_t)offset + length;
    fst.fst_bytesalloc = 0;
    if (fcntl(fileno(file), F_PREALLOCATE, &fst) == -1) {
        fst.fst_flags = F_ALLOCATEALL;
        fcntl(fileno(file), F_PREALLOCATE, &fst);
    }
    ftruncate(fileno(file), fst.fst_length);
#elif defined(__linux__)
    // Version using posix_fallocate
    off_t nEndPos = (off_t)offset + length;
    posix_fallocate(fileno(file), 0, nEndPos);
#else
    // Fallback version
    // TODO: just write one byte per block
    static const char buf[65536] = {};
    fseek(file, offset, SEEK_SET);
    while (length > 0) {
        unsigned int now = 65536;
        if (length < now) now = length;
        fwrite(buf, 1, now, file);  // allowed to fail; this function is advisory anyway
        length -= now;
    }
#endif
}

void ShrinkDebugFile() {
    // Scroll debug.log if it's getting too big