jik@athena.mit.edu (Jonathan I. Kamens) (10/23/90)
I fail to see how this has anything to do with Unix internals. It is clearly a question about programming under Unix, which means that it clearly belongs in comp.unix.programmer. I've cross-posted to comp.unix.programmer and directed followups there. In article <24826@adm.BRL.MIL>, vilva@csvax.csc.lsu.edu (Vilva Natarajan) writes: |> I am trying to do system("man cat | wc -c") in my program, and i need |> to store the value returned by "wc" in a variable declared in my program. Use popen("man cat | wc -c", "r"), which will return a FILE *, and you can read the output of wc from the FILE *. See popen(3). -- Jonathan Kamens USnail: MIT Project Athena 11 Ashford Terrace jik@Athena.MIT.EDU Allston, MA 02134 Office: 617-253-8085 Home: 617-782-0710
6600bori@ucsbuxa.ucsb.edu (Boris Burtin) (02/22/91)
Can anyone tell me why I get a core dump
when I try to do :
system("last [username] > ~/filename");
Thanks...
- Boris Burtinjik@athena.mit.edu (Jonathan I. Kamens) (02/22/91)
In article <9321@hub.ucsb.edu>, 6600bori@ucsbuxa.ucsb.edu (Boris Burtin) writes: |> Can anyone tell me why I get a core dump |> when I try to do : |> |> system("last [username] > ~/filename"); I have no idea why you get a coredump, and there's no way really for us to tell. Have you attempted to analyze the coredump with a debugger to see what it's from? Is it your process coredumping, or is it the shell that's being started? I don't get a coredump on my machine. What type of machine are you using, running what version of Unix? In any case, I suspect that at least part of the problem is that the bourne shell, which is what system uses, doesn't understand "~". Change the "~" to "$HOME". Observe: 1 -> extern int system(char *); 2 -> system("last jik > ~/lastjik"); Linking from '/lib/libc.a' ... Linking completed. sh: ~/lastjik: cannot create (int) 256 3 -> system("last jik > $HOME/lastjik"); (int) 0 -- Jonathan Kamens USnail: MIT Project Athena 11 Ashford Terrace jik@Athena.MIT.EDU Allston, MA 02134 Office: 617-253-8085 Home: 617-782-0710
emmonsl@athena.ecs.csus.edu (L. Scott Emmons) (03/03/91)
In article <9321@hub.ucsb.edu> 6600bori@ucsbuxa.ucsb.edu (Boris Burtin) writes: >Can anyone tell me why I get a core dump >when I try to do : > > system("last [username] > ~/filename"); News has been stacked up here, so there's probably already been a reply to this (since it's been a week since your original post), but anyway... system() uses /bin/sh to execute the command, and sh doesn't know ~. Also, you are _much_ better off doing a fork() and then an exec() instead of a system()...system just does the equivalent of a fork() and an exec(), just _much_ less efficiently...If you need some help on using fork() and exec() please send me some mail and I'll be happy to give you a hand. (The reason that this is more efficient is because with exec() you don't fork a shell, you just execute the program directly). There is a story (well, a truth) around campus here. Someone needed to delete a file via a program. Instead of using unlink() they system("rm") the file... rm is just a program which calls unlink(), but with millions of times the overhead, because they used system() (ok, well not quite that much)...Anyway, avoid system() where exec() or a direct system call will do. Hope this offers you some help... L. Scott Emmons --------------- emmons@csus.csus.edu <or> ...[ucbvax]!ucdavis!csus!emmons Packet: kc6nfp@kg6xx.#nocal.ca.usa.na
emmonsl@athena.ecs.csus.edu (L. Scott Emmons) (03/04/91)
In article <1991Mar2.181423.13981@csusac.csus.edu> I wrote: >Also, you are _much_ better off doing a fork() and then an exec() >instead of a system()...system just does the equivalent of a fork() [...] Ack, I made a bit of an error in this post. You will also need to set up a pipe() to properly handle the redirection. I assume you were going to take the file then and read it back into the program? If so, a pipe() works well for this. Sorry for the mistake... L. Scott Emmons --------------- emmons@csus.csus.edu <or> ...[ucbvax]!ucdavis!csus!emmons Packet: kc6nfp@kg6xx.#nocal.ca.usa.na