P2539R0
Should the output of std::print to a terminal be synchronized with the underlying stream?

1. Discussion

To prevent mojibake std::print may use a native Unicode API when writing to a terminal bypassing the stream buffer. During the review of [P2093] "Formatted output" Tim Song suggested that synchronizing std::print with the underlying stream may be beneficial for gradual adoption. This paper briefly discusses this option.

Consider the following example:

printf("first\n");
std::print("second\n");

This will produce the expected output:

first
second

because stdout is line buffered by default.

However, this may reorder the output:

printf("first");
std::print("second");

because of buffering in printf but not std::print.

It is possible to prevent reordering by flushing the buffer before writing to a terminal in std::print making this case less surprising. This will incur additional cost but only for the terminal case and when transcoding is needed.

Neither {fmt} ([FMT]) nor Rust ([RUST-STDIO]) do such synchronization in their implementations of print.

2. References