wei@hpctdls.HP.COM (Bill Ives) (06/19/89)
To redirect a child's stdout to a file in the way you
described does not work because the child process does
not parse the ">" out of the command line. When the
redirection operators are used they are used on the DOS
command line --- where DOS ( namely command.com ) parses
them out and determines that redirection is to be done. The
same thing works for pipes "|". Since the command.com shell
does this parsing and redirection you may invoke it with the
child process name and args following like:
c:\command.com child.exe > outfile.dat
You should get the command.com specification out of your environment
by looking at the "COMSPEC=" environment variable.
You could also do the redirection yourself by using DOS functions
DOS_DUP_HANDLE, DOS_CLOSE .... The idea in using these is to do
the same thing the command shell does when it sees the ">" operator.
The basic algorithm for doing this is:
duplicate your stdout and save the duplicate in a temp variable
open the child's stdout file
close your stdout handle so the that it may be attached to the
file opened above.
use DOS_DUP again to attach the opened file to the stdout
your stdout is now the file that was opened.
invoke the child -- it will inherit the stdout that
is attached to the file.
restore your own stdout by closing stdout and duping the
temp handle saved in the first step. Your stdout will
then be restored.bga@bgalli.eds.com (Billy G. Allie) (06/20/89)
In article <8430@techunix.BITNET>, buzy@techunix.BITNET (boaz binnun) writes: > I need to redirect the output of a child process to a file, > I am working with Turbo C, what I did was : > > char *args[] = { "child.exe", "argument", ">", "filename", NULL }; > spawnl(P_WAIT,args[0],args[0],args[1],args[2],args[3],args[4]); > > and it didnt work,does anyone know why ? The redirection of the standard input and output files using the '>' and '<' characters is a function the shell (command.com in this case). Executing a program with spawn or exec with the redirection character(s) in the argument list will not cause redirection (unless the program is command.com or the prgram has been written to recognize the redirection characters as such). To redirect the output of a child process to a file you can do one of two things: 1. Use the system function, which will use command.com to execute the program and set up the redirection. For example: system("child argument >filename"); The disadvantage to this method is that another copy of command.com must be loaded into memory in addition to the child program. Also, there is a bug in command.com that prevents the exit code from the child process from returning correctly. The advantages are that it is easy to use and even pipes can be set up in this manner. 2. Set up the file redirection yourself before spawning the child process. This can be done as follows: int oldstdout, newstdout; ... oldstdout = dup(2); /* Save the original stdout handle */ newstdout = open("filename", O_CREAT|O_TRUNC|O_WRONLY); /* open output */ dup2(newstdout, 2); /* Make the new file the standard output */ close(newstdout); /* Close the (now) un-needed file handle */ spawnl(P_WAIT, "child.exe", "child", "argument"); /* Execute the child */ dup2(oldstdout, 2); /* Restore the original stdout handle */ close(oldstdout); /* Close the un-needed file handle */ Please note that I did not include any code to check the return values of the dup, dup2, close, open and spawnl function calls. Those checks will have to be added for the code sequence to be truly useful. The disadvantage of this method is that it is harder to set up. The advantages are it can be used to redirect any of the 5 standard devices used in the MS-DOS environment (stdin, stdout, stderr, stdaux and stdprn) and it does not require any additional processes to be loaded (i.e. command.com). I hope this helps you with your problem. -- ____ | Billy G. Allie | Internet..: bga@bgalli.eds.com | /| | 7436 Hartwell | UUCP......: uunet!{mcf|edsews}!bgalli!bga |-/-|----- | Dearborn, MI 48126 | Compuserve: 76337,2061 |/ |LLIE | (313) 582-1540 | Genie.....: BGALLIE
chris@mimsy.UUCP (Chris Torek) (06/21/89)
In various articles various people write about: fork exec spawn /bin/sh command.com sockets interrupt handlers device drivers and so forth. Unfortunately, they do it in comp.lang.c, although the only relation these things have to C is that they can be programmed in C, or called from C programs. But so can databases, operating systems, USENET news, UUCP, kermit, and so forth; and these are *not* discussed in comp.lang.c (or at least not anywhere near as often). The latter is how things should be. Please try to keep IBM-PC specific articles in IBM-PC specific newsgroups; and try to keep Unix-specific articles in Unix-specific newsgroups. Those of you who read this via INFO-C on the Internet, please use one of the other INFO-* lists. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris
buzy@techunix.BITNET (boaz binnun) (07/22/89)
I need to redirect the output of a child process to a file,
I am working with Turbo C,
what I did was :
char *args[] = { "child.exe",
"argument",
">",
"filename",
NULL };
and than :
spawnl(P_WAIT,args[0],args[0],args[1],args[2],args[3],args[4]);
and it didnt work,does anyone know why ?
Many thanks
Boaz Bin-nun buzy@techunix.bitnet or buzy@techunix.technion.ac.ildik@cwi.nl (Dik T. Winter) (07/22/89)
In article <8430@.........BITNET> ....%.........bitnet@jade.berkeley.edu (.... ......) writes: > I need to redirect the output of a child process to a file, > I am working with Turbo C, > Matt Crawford can say Aaaaarrrrrggggghhhhh again. -- dik t. winter, cwi, amsterdam, nederland INTERNET : dik@cwi.nl BITNET/EARN: dik@mcvax
wei@hpctdls.HP.COM (Bill Ives) (08/03/89)
The reason your redirection technic does not work is because
the '>' operator is a DOS command shell ( among other shells )
operator... Thus, your command works at the command prompt because
command.com parses it out and sets up the redirection before
starting the process ... As it stands now, your spawn command will
pass the > charater as a command line parameter to the process which
could look for it-- bad way to do this though. To do this without
the shell you will have to set up redirection yourself -- look up
Dos Dup Handle and Dos Close Handle for information about this...
If you have access to older ket notes this technic has been covered
in excessive detail-- I have covered it at least three times myself.
Email me if you can't figure it out and I'll send you an example..
Bill Ives
HP CTD
#include <std-disclaimer> /* I could never speak for HP */jamespw@mqcomp.oz (Warlow) (08/07/89)
In article <8430@techunix.BITNET> buzy%techunix.bitnet@jade.berkeley.edu (boaz binnun) writes: >I need to redirect the output of a child process to a file, ... >what I did was : > >char *args[] = { "child.exe", "argument", ">", "filename", NULL }; > >and than: > >spawnl(P_WAIT,args[0],args[0],args[1],args[2],args[3],args[4]); This won't work because the indirection operator > is interpreted by the shell, NOT by C. spawnl doesn't invoke the shell so the file child.exe sees the argument '>', which will probably frighten it. If you *really* want to use indirection, something like system("child.exe argument > filename"); will do the trick (but it's awful); otherwise, use fork to explicitly reset stdout yourself: if (fork() == 0) { /* child process */ stdout = freopen("filename", "w", stdout); execl(args[0],args[0],args[1],args[4]); } or some such.