Backend for newlib's "scanf" family of functions
Scott L. Brookes
Wed Jul 23 21:07:00 GMT 2014
I am new to the newlib mailing list, but I hope that you will be able to help me
understand how scanf and related functions work.
I am trying to implement piping in a shell on a custom operating system that
uses newlib as its C library. I have an implementation of pipes in the operating
system - open, read, write, and close all work as defined in the POSIX standard
when called in relation to a pipe/FIFO.
For piping the output of one command to the input of another, the common process
is to open a pipe, then fork both processes and use the dup2 call to set stdout
for the first process to the write end of the pipe and stdin for the second to
the read end of the pipe. This works just fine and calling read and write (to
file descriptors 0 and 1, respectively) works as I would expect. However,
replacing these with printf and scanf does not work. The printf side works just
fine (data is written to the buffer), but the scanf side is not working.
How is scanf repackaged into read calls? Calling with a string format specifier,
there is no way to know how many bytes to ask for in the read. Expirementation
saw a read request for 1024 bytes, which would lead me to believe that it is up
to my pipe implementation to give only chars up to a whitespace, but how does
this jive with the POSIX definition of a read on a pipe?
Coincidentally, we found out when trying to tackle this problem that scanf never
worked for reading from stdin, either... but I think that this case raises the
Thanks so much! Any info will help!
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