How windows handles \r and \n characters in XXXprintf() functions. 

The following program simulates what done when UL writes to a log file.

- First LogTagSet::vwrite writes the output to a buffer using os::vsnprintf()

- Then, LogFileStreamOutput::write writes the buffer to a text file using fprintf()

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#define  _CRT_SECURE_NO_WARNINGS 1 // Build as a console app with Visual Studio 2019
#include <stdio.h>

void dump(const char* what, const char* s) {
	printf("const char %s[] = {", what);
	while (*s) {
		printf("0x%02x, ", *s); s++;
	}
	printf("}\n");
}
int main()
{
	char buffer[100];
	char s[] = { 'a', '\r', '\n', 'b', 0 };
	dump("s", s);
	snprintf(buffer, 100, "aa\n%s\nbb", s);
	dump("buffer", buffer);
	FILE* f = fopen("c:\\temp\\foo.txt", "w+");
	fprintf(f, "%s", buffer);
	fclose(f);
}

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$ ./test.exe
const char s[] = {0x61, 0x0d, 0x0a, 0x62, }
const char buffer[] = {0x61, 0x61, 0x0a, 0x61, 0x0d, 0x0a, 0x62, 0x0a, 0x62, 0x62, }

$ hexdump -ve '1/1 "0x%.2x, "' foo.txt
0x61, 0x61, 0x0d, 0x0a, 0x61, 0x0d, 0x0d, 0x0a, 0x62, 0x0d, 0x0a, 0x62, 0x62,
opc@hotspot-windows /cygdrive/c/temp
----------------------------------------------------------------------

Observation:

[1] sprintf() does not perform \n -> \r\n conversion. The characters substituted for the %s are
    exactly the same chars as in s[].

const char buffer[] = {0x61, 0x61, 0x0a, 0x61, 0x0d, 0x0a, 0x62, 0x0a, 0x62, 0x62, }
                                         <-        %s         ->

[2] fprintf performs \n -> \r\n conversion (if it configured to write into a text file). See
    the bytes marked by vvvvvv

            vvvvvvvvvv              vvvvvvvvvv        vvvvvvvvvv
0x61, 0x61, 0x0a,       0x61, 0x0d, 0x0a,       0x62, 0x0a,       0x62, 0x62   <- input to fprintf
0x61, 0x61, 0x0d, 0x0a, 0x61, 0x0d, 0x0d, 0x0a, 0x62, 0x0d, 0x0a, 0x62, 0x62,  <- output written by fprintf into foo.txt
                              ^^^^^^^^^^^^^^^^

    Note that fprintf does NOT process the \r characters. It leaves them alone. See the bytes marked by ^^^^^^^^
    E.g., the input bytes \r\n becomes \r\r\n in foo.txt

Conclusion:

Windows C library performs newline translation only on the \n characters. Since our proposal will transform
all \n characters to {0x5c, 0x6e}, the Windows C library will write the data as is into the output file,
without any further translation.