Date: Fri, 30 Nov 2018 21:51:39 +1100 From: Xan Phung <xan.phung@...il.com> To: musl@...ts.openwall.com Subject: stdio glitch & questions Hi, A few questions about stdio: (1) I notice __toread.c uses angular quotes for <stdio_impl.h> whereas all other source files use "stdio_impl.h". I assume the latter is correct and __toread.c's use of angular quotes was a glitch & it should really be double quotes... is that correct? (2) I notice vfprintf first tries to call printf_core with f=0 (line 667) then calls printf_core again with f set to the actual file to receive output (line 682). Why is printf_core called twice? I struggle to understand the purpose of the first call with f=0. (3) When I do a step thru the __fwritex function to understand how printf works, I notice the resulting writev system calls pass on the output data as a two element iovec array, with the 1st element comprising all line buffered text up to & including the last variable data item, and then the 2nd element comprising the residual format string trailing the last variable data item (more often than not just a single '\n'). For example, printf("error: %s\n", msg) would put all text up to & including %s text in first iovec and the second iovec contains only '\n'. I understand the rationale of this is to avoid copying the final '\n' to the buffer at f->wpos. (There is actually guaranteed space in the buffer itself due to a check at line 10 of fwrite.c). The use the array of 2x iovec's presumably then relies on Linux kernel scatter-gather I/O to then optimally handle the iovec array, ie: that the writev() of 2x iovec is more efficient than avoiding the copy of a few additional bytes (often a single '\n' byte) into f->wpos, and then using a single write() syscall. Isn't this a big assumption? With Linux itself, can we really know that Linux device drivers are smart enough to do writev() optimally? Also, there is a lot of interest in porting musl to non-Linux os's, many of which do not have writev(). (I am porting musl to WebAssembly and to Plan 9). I can prepare a patch of a version using write() instead of writev() if there is interest in this... regards Xan Phung Content of type "text/html" skipped
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