Rerouting stdin and stdout from C

Question

I want to reopen the stdin and stdout  and perhaps stderr while I m at it  filehandles  so that future calls to printf   or putchar   or puts   will go to a file  and future calls to getc   and such will come from a file   1  I don t want to permanently lose standard input output error  I may want to reuse them later in the program   2  I don t want to open new filehandles because these filehandles would have to be either passed around a lot or global  shudder    3  I don t want to use any open   or fork   or other system-dependent functions if I can t help it   So basically  does it work to do this   stdin   fopen  newin    r      And  if it does  how can I get the original value of stdin back  Do I have to store it in a FILE   and just get it back later

User · Answer

I think you re looking for something like freopen

User · Answer

Why use freopen    The C89 specification has the answer in one of the endnotes for the section on  lt stdio h gt       116  The primary use of the freopen function is to change the file associated with a standard   text stream  stderr    stdin  or stdout   as those identifiers need not be   modifiable lvalues to which the value   returned by the fopen function   may be assigned    freopen is commonly misused  e g  stdin   freopen  newin    r   stdin    This is no more portable than fclose stdin   stdin   fopen  newin    r     Both expressions attempt to assign to stdin  which is not guaranteed to be assignable    The right way to use freopen is to omit the assignment  freopen  newin    r   stdin

User · Answer

This is a modified version of Tim Post s method  I used  dev tty instead of  dev stdout  I don t know why it doesn t work with stdout  which is a link to  proc self fd 1    freopen  log txt   w  stdout           freopen   dev tty   w  stdout     By using  dev tty the output is redirected to the terminal from where the app was launched   Hope this info is useful

User · Answer

freopen solves the easy part   Keeping old stdin around is not hard if you haven t read anything and if you re willing to use POSIX system calls like dup or dup2   If you re started to read from it  all bets are off   Maybe you can tell us the context in which this problem occurs   I d encourage you to stick to situations where you re willing to abandon old stdin and stdout and can therefore use freopen

User · Answer

freopen   my newstdin    r   stdin   freopen   my newstdout    w   stdout   freopen   my newstderr    w   stderr        do your stuff  freopen   dev stdin    r   stdin             This peaks the needle on my round-peg-square-hole-o-meter  what are you trying to accomplish   Edit   Remember that stdin  stdout and stderr are file descriptors 0  1 and 2 for every newly created process  freopen   should keep the same fd s  just assign new streams to them   So  a good way to ensure that this is actually doing what you want it to do would be   printf  Stdout is descriptor  d n   fileno stdout    freopen   tmp newstdout    w   stdout   printf  Stdout is now  tmp newstdout and hopefully still fd  d n      fileno stdout    freopen   dev stdout    w   stdout   printf  Now we put it back  hopefully its still fd  d n      fileno stdout      I believe this is the expected behavior of freopen    as you can see  you re still only using three file descriptors  and associated streams    This would override any shell redirection  as there would be nothing for the shell to redirect  However  its probably going to break pipes  You might want to be sure to set up a handler for SIGPIPE  in case your program finds itself on the blocking end of a pipe  not FIFO  pipe    So    your program --stdout  tmp stdout txt --stderr  tmp stderr txt should be easily accomplished with freopen   and keeping the same actual file descriptors  What I don t understand is why you d need to put them back once changing them  Surely  if someone passed either option  they would want it to persist until the program terminated

User · Answer

This is the most readily available  handy and useful way to do  freopen  dir   r  stdin

User · Answer

And in the meantime  there s a C source code library that will do all this for you  redirecting stdout or stderr   But the cool part is that it lets you assign as many callback functions as you want to the intercepted streams  allowing you then to very easily send a single message to multiple destinations  a DB  a text file  etc   On top of that  it makes it trivial to create new streams that look and behave the same as stdout and stderr  where you can redirect these new streams to multiple locations as well   look for U-Streams C library on  oogle

User · Answer

The os function dup2   should provide what you need  if not references to exactly what you need   More specifically  you can dup2   the stdin file descriptor to another file descriptor  do other stuff with stdin  and then copy it back when you want   The dup   function duplicates an open file descriptor  Specifically  it provides an alternate interface to the service provided by the fcntl   function using the F DUPFD constant command value  with 0 for its third argument  The duplicated file descriptor shares any locks with the original  On success  dup   returns a new file descriptor that has the following in common with the original   Same open file  or pipe  Same file pointer  both file descriptors share one file pointer  Same access mode  read  write  or read write

[c] Rerouting stdin and stdout from C

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