launch other app and send input to other app

[peterve]

Hi,

Assume I have the following basic application :
(removed all code that is irrelevant)

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

int main()
{
	char buf[1024];
	gets(buf);
        printf("Buffer : %s",buf);
}

What do I want to do ? I want to call this application from another C application (which in itself is not a problem - execl does the trick just fine); but I also need to provide the input to 'buf' and send a return, so it displays my input on the screen

How can I do this ?

thanks


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[mseth]

POSIX capable systems should have a version of popen/_popen which can allow for reading/writing to another application.

POPEN(3)            Linux Programmer's Manual            POPEN(3)



NAME
       popen, pclose - process I/O

SYNOPSIS
       #include <stdio.h>

       FILE *popen(const char *command, const char *type);

       int pclose(FILE *stream);

DESCRIPTION
       The  popen()  function opens a process by creating a pipe,
       forking, and invoking the shell.  Since a pipe is by defi­
       nition  unidirectional, the type argument may specify only
       reading or writing, not both; the resulting stream is cor­
       respondingly read-only or write-only.

       The  command  argument  is  a pointer to a null-terminated
       string containing a shell command line.  This  command  is
       passed  to  /bin/sh  using the -c flag; interpretation, if
       any, is performed by the shell.  The type  argument  is  a
       pointer  to  a null-terminated string which must be either
       `r' for reading or `w' for writing.

       The return value from popen() is  a  normal  standard  I/O
       stream  in  all  respects save that it must be closed with
       pclose() rather than fclose().  Writing to such  a  stream
       writes to the standard input of the command; the command's
       standard output is the same as that of  the  process  that
       called  popen(),  unless  this  is  altered by the command
       itself.  Conversely, reading  from  a  ``popened''  stream
       reads  the  command's  standard  output, and the command's
       standard input is the same as that  of  the  process  that
       called popen.

       Note  that  output  popen  streams  are  fully buffered by
       default.

       The pclose function waits for the  associated  process  to
       terminate  and  returns  the exit status of the command as
       returned by wait4.

RETURN VALUE
       The popen function returns NULL if the fork(2) or  pipe(2)
       calls fail, or if it cannot allocate memory.

       The  pclose function returns -1 if wait4 returns an error,
       or some other error is detected.

ERRORS
       The popen function does not set errno if memory allocation
       fails.  If the underlying fork() or pipe() fails, errno is
       set appropriately.  If the type argument is  invalid,  and
       this condition is detected, errno is set to EINVAL.

       If  pclose()  cannot obtain the child status, errno is set
       to ECHILD.

CONFORMING TO
       POSIX.2

BUGS
       Since the standard input of a command opened  for  reading
       shares  its  seek  offset  with  the  process  that called
       popen(), if the original process has done a buffered read,
       the command's input position may not be as expected.  Sim­
       ilarly, the output from a command opened for  writing  may
       become  intermingled  with  that  of the original process.
       The latter can be  avoided  by  calling  fflush(3)  before
       popen.

       Failure to execute the shell is indistinguishable from the
       shell's failure to execute command, or an  immediate  exit
       of the command.  The only hint is an exit status of 127.

HISTORY
       A  popen()  and  a pclose() function appeared in Version 7
       AT&T UNIX.

SEE ALSO
       fork(2), sh(1), pipe(2), wait4(2),  fflush(3),  fclose(3),
       fopen(3), stdio(3), system(3)



BSD MANPAGE                 1998-05-07                   POPEN(3)

 

[chipperMDW]

A more complex but more efficient way of doing the same thing is to:

- create a pipe
- fork

Parent:
- close the read end of pipe
- write your data to the pipe

Child:
- close the write end of the pipe
- set stdin to the pipe's fd
- exec the new process

The reason this method is more efficient is that using popen causes a shell to be spawned to interpret the command passed to it (much like the system command), and this executes the command directly.


If that's not a big deal, popen is easier and more intuitive to use.