The routine skip_empty_lines_and_read_char() is an optimization to skip over blocks of comment lines. When it reads an unescaped '#' it uses the helper routine skip_to_end_of_line(). But skip_to_end_of_line() doesn't fold lines as it should and like its parent caller does. (See patch at end of message.)
I found this bug perusing the BearSSL source code. 25 # The lines below are a horrible hack that nonetheless works. On a 26 # "make" utility compatible with Single Unix v4 (this includes GNU and 27 # BSD make), the '\' at the end of a command line counts as an escape 28 # for the newline character, so the next line is still a comment. 29 # However, Microsoft's nmake.exe (that comes with Visual Studio) does 30 # not interpret the final '\' that way in a comment. The end result is 31 # that when using nmake.exe, this will include "mk/Win.mk", whereas 32 # GNU/BSD make will include "mk/Unix.mk". 33 34 # \ 35 !ifndef 0 # \ 36 !include mk/NMake.mk # \ 37 !else 38 .POSIX: 39 include mk/SingleUnix.mk 40 # Extra hack for OpenBSD make. 41 ifndef: all 42 0: all 43 endif: all 44 # \ 45 !endif -- https://bearssl.org/gitweb/?p=BearSSL;a=blob;f=Makefile;hb=9dc62112 It's definitely a bug. POSIX says, There are three kinds of comments: blank lines, empty lines, and a <number-sign> ( '#' ) and all following characters up to the first unescaped <newline> character. and When an escaped <newline> (one preceded by a <backslash>) is found anywhere in the makefile except in a command line, an include line, or a line immediately preceding an include line, it shall be replaced, along with any leading white space on the following line, with a single <space>. Fortunately POSIX leaves unspecified what happens to an escaped newline preceding an include line. My patch should be the end of the matter unless there's some other bug lurking. Here's a test case, # \ broken:;@echo broken fixed:;@echo fixed and the patch, Index: lowparse.c =================================================================== RCS file: /cvs/src/usr.bin/make/lowparse.c,v retrieving revision 1.35 diff -u -r1.35 lowparse.c --- lowparse.c 21 Oct 2016 16:12:38 -0000 1.35 +++ lowparse.c 23 Feb 2018 20:02:59 -0000 @@ -247,20 +247,21 @@ static int skip_to_end_of_line(void) { - if (current->F) { - if (current->end - current->ptr > 1) - current->ptr = current->end - 1; - if (*current->ptr == '\n') - return *current->ptr++; - return EOF; - } else { - int c; + int escaped = 0, c; - do { - c = read_char(); - } while (c != '\n' && c != EOF); - return c; + while (EOF != (c = read_char())) { + if (escaped) { + if (c == '\n') + current->origin.lineno++; + escaped = 0; + } else if (c == '\\') { + escaped = 1; + } else if (c == '\n') { + break; + } } + + return c; }
