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Fix flush
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@vitaut
vitaut committed Jan 20, 2024
1 parent 4c5b4af commit 11ba127
Showing 1 changed file with 2 additions and 1 deletion.
3 changes: 2 additions & 1 deletion include/fmt/format-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1503,7 +1503,7 @@ template <typename F> class apple_file : public file_base<F> {
auto get_write_buffer() const -> span<char> {
if (!this->file_->_p || offset() == this->file_->_bf._size) {
// Force buffer initialization by placing and removing a char in a buffer.
fputc(0, this->file_);
putc_unlocked(0, this->file_);
--this->file_->_p;
++this->file_->_w;
}
Expand Down Expand Up @@ -1571,6 +1571,7 @@ class file_print_buffer : public buffer<char> {
static void grow(buffer<char>& buf, size_t) {
auto& self = static_cast<file_print_buffer&>(buf);
self.set_buffer();
self.clear();
}

public:
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4 comments on commit 11ba127

@b0ae989c
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It seems this fix does not address the flushing issue in parallel code (MPI and UPC++ specifically). Maybe we should add a comment in docs to remind users to invoke stream flush explicitly?

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@vitaut vitaut commented on 11ba127 Jan 21, 2024

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@b0ae989c what issue are you referring to?

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Starting at commit 6b68dff, the behavior of fmt::print changes when it's used in a parallel context that involves multiple computing nodes. Even with this commit fix, the following program does not output anything until the very end of the simulation

auto main() -> int {
  fmt::print("test begins\n");
  // Other work. This part of the program needs a few days to finish.
  fmt::print("test ends\n");
  return 0;
}

The old behavior is that "test begins" is written to the output logs before the time consuming work starts. Right now, this line is not written to logs until the entire program finishes (after several days).

We manage to fix this by adding std::cout << std::flush; after every fmt::print calls. Note that this issue does not appear in sequential code.

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@vitaut vitaut commented on 11ba127 Jan 27, 2024

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@b0ae989c, should be fixed in 6435b16 which adds line buffering support.

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