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[io] Add ROOT::Internal::DumpBin(TMemFile&) for debugging purposes #20628

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[io] Add ROOT::Internal::DumpBin(TMemFile&) for debugging purposes #20628

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10 changes: 10 additions & 0 deletions io/io/inc/TMemFile.h
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Original file line number Diff line number Diff line change
Expand Up @@ -16,7 +16,17 @@
#include <vector>
#include <memory>

class TMemFile;

namespace ROOT::Internal {

void DumpBin(const TMemFile &file, FILE *out);

}

class TMemFile : public TFile {
friend void ROOT::Internal::DumpBin(const TMemFile &file, FILE *out);

public:
using ExternalDataPtr_t = std::shared_ptr<const std::vector<char>>;
/// A read-only memory range which we do not control.
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20 changes: 20 additions & 0 deletions io/io/src/TMemFile.cxx
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Expand Up @@ -312,6 +312,26 @@ void TMemFile::Print(Option_t *option /* = "" */) const
}
}

////////////////////////////////////////////////////////////////////////////////
/// Writes the contents of the TMemFile to the given file. This is meant to be
/// used mostly for debugging, as it dumps the current file's content as-is with
/// no massaging (meaning the content might not be a valid ROOT file): for regular use cases, use Cp().
/// Example usage:
/// ~~~ {.cpp}
/// FILE *out = fopen("memfile_dump.root", "wb");
/// ROOT::Internal::DumpBin(memFile, out);
/// fclose(out);
Comment on lines +321 to +323
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@pcanal pcanal Dec 3, 2025

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What is the advantage of requiring the 3 steps instead of folding them into the function?

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@silverweed silverweed Dec 4, 2025

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Mostly the ability to pass stdout or similar, which would be more awkward to do with a string argument.
Also one may potentially want to dump multiple MemFiles to the same output, though that's probably rare.

/// ~~~
void ROOT::Internal::DumpBin(const TMemFile &file, FILE *out)
{
const auto *cur = &file.fBlockList;
while (cur && cur->fSize) {
fwrite(cur->fBuffer, cur->fSize, 1, out);
cur = cur->fNext;
}
fflush(out);
}

////////////////////////////////////////////////////////////////////////////////
/// Wipe all the data from the permanent buffer but keep, the in-memory object
/// alive.
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