creating c programs to accpet data from a pipe - '|'

schneidz · 11-20-2006, 10:57 AM

hi lq,
i have this c program:

Code:

schneidz@lq> cat columnate.c
#include <string.h>
#include <stdio.h>

main(int argc, char * argv[])
{
 printf("     %-20s %29s %30s\n", argv[1], argv[2], argv[3]);
}

it runs fine but when i try to run it after a pipe i get this weird output:

Code:

schneidz@lq> columnate.x hello world
     hello                                        world
schneidz@lq> echo hello world | columnate.x
                                          _=columnate.x MANPATH=/opt/vintela/vgp/man:/opt/vas/man:/usr/share/man:/usr/share/man/info/en_US/a_doc_lib/cmds:/usr/share/man/info:/usr/lpp/X11/Xamples/man:/usr/lpp/ssp/perl/man:/usr/lpp/ssp/perl5/lib/man:/usr/lpp/ssp/perl5/man:/usr/lpp/ssp/man:/usr/lpp/perfrep/man:/usr/share/man:/usr/dt/man:/usr/nsr/nsr_extract/man:/usr/OV/man

can somebody tell me how do i make my code accept input from a pipe ?

thanks much,
~schneidz

m0rg · 11-20-2006, 11:43 AM

I'm not sure right now, but I think that reading from a pipe is just reading from stdin. So I guess you just have to get you arguments from stdin instead of argv.

You can probably just test if there are any arguments provided on the command line, if not then read them from stdin and output like you already do.

Hope this helps

osor · 11-20-2006, 11:57 AM

Quote:

Originally Posted by schneidz

hi lq,
i have this c program:

Code:

schneidz@lq> cat columnate.c
#include <string.h>
#include <stdio.h>

main(int argc, char * argv[])
{
 printf("     %-20s %29s %30s\n", argv[1], argv[2], argv[3]);
}

it runs fine but when i try to run it after a pipe i get this weird output:

Code:

schneidz@lq> columnate.x hello world
     hello                                        world
schneidz@lq> echo hello world | columnate.x
                                          _=columnate.x MANPATH=/opt/vintela/vgp/man:/opt/vas/man:/usr/share/man:/usr/share/man/info/en_US/a_doc_lib/cmds:/usr/share/man/info:/usr/lpp/X11/Xamples/man:/usr/lpp/ssp/perl/man:/usr/lpp/ssp/perl5/lib/man:/usr/lpp/ssp/perl5/man:/usr/lpp/ssp/man:/usr/lpp/perfrep/man:/usr/share/man:/usr/dt/man:/usr/nsr/nsr_extract/man:/usr/OV/man

can somebody tell me how do i make my code accept input from a pipe ?

thanks much,
~schneidz

A pipe redirects output to stdin (fd 0). To get the desired effect (changing command-line-arguments) use xargs (i.e., echo "hello world" | xargs columnate.x).

matthewg42 · 11-20-2006, 01:11 PM

Firstly, your program is using command line arguments without first checking if they exist. This is why you get the junk output. When you use argv[n] you must check that you're not using more values in argv than it has in it, else you'll get pointers to something random, probably causing junk output like you get,m or maybe a segmentation fault (and your program crashing horribly).

If you want to print the command line arguments, you might do it like this:

Code:

#include <stdio.h>

int main(int argc, char **argv)
{
    int i;
    for (i = 1; i < argc; i++) {
        printf("%s ", argv[i]);
    }
    printf("\n");

    return 0;
}

Note that this has nothing to to with reading from a pipe! When you start a program from the shell like this:

Code:

wibble | wobble

...this means that the output of wibble (from the standard output file descriptor, 1) is connected to the input of wobble (the standard input file descriptor, 2). This has nothing to do with command line options.

In wobble, you would use the I/O function to read from the standard input file descriptor. There are many functions for doing this. If you want to get one character at a time, you might use fgetc, or one of it's friends, getc or getchar. There are several IO routines which may be suitable depending on the task. Here's an example wobble.c which changes all lower case letters to upper case letters and sends the output to standard output (which could then be piped into another process if you like).

Code:

#include <stdio.h>

int main(int argc, char **argv)
{
    int c;
    while ((c = getchar()) != EOF) {
        if (c >= 'a' && c <= 'z') {
            c -= 32;
        }
        putchar(c);
    }

    return (0);
}

schneidz · 11-20-2006, 02:00 PM

thanks guys, i understand now:

Code:

schneidz@lq> cat columnate-2.c
#include <string.h>
#include <stdio.h>

main(int argc, char * argv[])
{
 char arg1[50], arg2[50];
 scanf("%s", &arg1);
 scanf("%s", &arg2);
 printf("     %-20s %29s %30s\n", arg1, arg2);
}
schneidz@lq> echo hello world | columnate.x
     hello                                        world

i also tried xorgs and that is more an ideal solution (less re-coding/ compiling)
i love options... give me money

matthewg42 · 11-20-2006, 02:21 PM

Yes, you can use scanf, but beware! Your program has a security flaw! If someone passes an argument which is bigger than the sizes you have statically allocated to arg1 and arg2 (minus one character because a \0 is automatically appended by scanf), a buffer over-run will occur!

osor · 11-20-2006, 02:36 PM

Quote:

Originally Posted by matthewg42

...this means that the output of wibble (from the standard output file descriptor, 1) is connected to the input of wobble (the standard input file descriptor, 2). This has nothing to do with command line options.

It connects to file descriptor 0 (stdin).

Quote:

Originally Posted by schneidz

i also tried xorgs and that is more an ideal solution (less re-coding/ compiling)
i love options... give me money

WTF is xorgs?