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according to posix, readv "shall be equivalent to read(), except..."
that it places the data into the buffers specified by the iov array.
however on linux, when reading from a terminal, each iov element
behaves almost like a separate read. this means that if the first iov
exactly satisfied the request (e.g. a length-one read of '\n') and the
second iov is nonzero length, the syscall will block again after
getting the blank line from the terminal until another line is read.
simply put, entering a single blank line becomes impossible.
the solution, fortunately, is simple. whenever the buffer size is
nonzero, reduce the length of the requested read by one byte and let
the last byte go through the buffer. this way, readv will already be
in the second (and last) iov, and won't re-block on the second iov.
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since vfprintf will provide a temporary buffer in the case where the
target FILE has a zero buffer size, don't bother setting up a real
buffer for vdprintf. this also allows us to skip the call to fflush
since we know everything will be written out before vfprintf returns.
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this change makes it so most calls to fprintf(stderr, ...) will result
in a single writev syscall, as opposed to roughly 2*N syscalls (and
possibly more) where N is the number of format specifiers. in
principle we could use a much larger buffer, but it's best not to
increase the stack requirements too much. most messages are under 80
chars.
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this could cause problems if the application uses dup2(fd,fileno(f))
to redirect, and the old fd was not seekable but the new fd is.
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the biggest change in this commit is that stdio now uses readv to fill
the caller's buffer and the FILE buffer with a single syscall, and
likewise writev to flush the FILE buffer and write out the caller's
buffer in a single syscall.
making this change required fundamental architectural changes to
stdio, so i also made a number of other improvements in the process:
- the implementation no longer assumes that further io will fail
following errors, and no longer blocks io when the error flag is set
(though the latter could easily be changed back if desired)
- unbuffered mode is no longer implemented as a one-byte buffer. as a
consequence, scanf unreading has to use ungetc, to the unget buffer
has been enlarged to hold at least 2 wide characters.
- the FILE structure has been rearranged to maintain the locations of
the fields that might be used in glibc getc/putc type macros, while
shrinking the structure to save some space.
- error cases for fflush, fseek, etc. should be more correct.
- library-internal macros are used for getc_unlocked and putc_unlocked
now, eliminating some ugly code duplication. __uflow and __overflow
are no longer used anywhere but these macros. switch to read or
write mode is also separated so the code can be better shared, e.g.
with ungetc.
- lots of other small things.
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sadly the C language does not specify any such implicit conversion, so
this is not a matter of just fixing warnings (as gcc treats it) but
actual errors. i would like to revisit a number of these changes and
possibly revise the types used to reduce the number of casts required.
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this implementation is extremely ugly and inefficient, but it avoids a
good deal of code duplication and bloat. it may be cleaned up later to
eliminate the remaining code duplication and some of the warts, but i
don't really care about its performance.
note that swprintf is not yet implemented.
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this change is in preparation for upcoming PIC/shared library support.
the intent is to avoid going through the GOT, mainly so that dprintf
is operable immediately, prior to processing of relocations. having
dprintf accessible from the dynamic linker will make writing and
debugging the dynamic linker much easier.
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- hide all the legacy xxxxxx32 name cruft in syscall.h so the actual
source files can be clean and uniform across all archs.
- cleanup llseek/lseek and mmap2/mmap handling for 32/64 bit systems
- alternate implementation for nice if the target lacks nice syscall
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