Re: Help with some C

[Max Okumoto <okumoto@xxxxxxxx>]

PJ Hyett wrote:
> I'm trying my hand with a little unix programming, but my second
> execvp in main() below doesn't appear to be working correctly.
> A sample call to this would be ./rungenerator 20 1 and the output is as follows:
> parent: 27461
> child: 27462
> 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38
> child: 27463
>
> the output should be:
> parent: 27461
> child: 27462
> 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38
> child: 27463
> 1 3 5...etc
>
> #include <stdio.h>
> #include <sys/types.h>
> #include <unistd.h>
> #include <fcntl.h>
>
> int main(int argc, char* argv[]){

Style issue most people prefer if the function name
starts on the first column.  This way people can find
functions definitions by searching for "^main(".

int
main(int argc, char *argv[])
{

>   int fd[2];
>   pid_t child[2];
>   char *args[4];
>
>   if(argc != 3){

Style issue put space between {if, while, switch} and
the parenthesis.

>     write(2,"Invalid # of arguments\n",23);

Use printf() here.

>     return 1;
>   }
>
>   args[0]="./generate1";
>   args[2]=argv[1];
>   args[3]=argv[2];
>   args[4]="0";

"0" is not equal to NULL

The array args[] has a range from 0 to 3.  args[4] is not a
valid location to write/read anything.

>   printf("parent: %ld\n",(long) getpid());
>
>   if(child[0] = fork())
>     child[1] = fork();
>
>   if(!child[0]){
>     printf("child: %ld\n",(long) getpid());
>     args[1]="even";
>     execvp(args[0],args);
>   }
>   if(!child[1]){
>     printf("child: %ld\n",(long) getpid());
>     args[1]="odd";
>     execvp(args[0],args);
>   }
>
>   return 0;
> }
>
>
> here's the code to generate1 if you're curious:
> int main(int argc, char* argv[]){
>   int which, i, num, fd;
>   char buf[3];
>
>   if(argc != 4){
>     write(2,"Invalid # of arguments\n",23);
>     return 1;
>   }
>
>   if(!strcmp(argv[1],"odd"))
>     which=1;
>   else if(!strcmp(argv[1],"even"))
>     which=2;
>   else{
>     write(1,"Must be odd or even!\n",21);
>     return 2;
>   }
>
>   srand(which);
>
>   fd = atoi(argv[3]);
>
>   /* if we're counting 0 as even */
>   if(which==2)
>     write(fd,"0 ",2);
>
>   num = atoi(argv[2]) * 2;
>   for(i=which;i<num;i=i+2){

Style issue put spaces after the semicolons in
 for (i = xxx; i < yyy; i += 2) {
 }

>     sprintf(buf,"%d ",i);
>     write(fd,buf,3);
>   }
>   write(fd,"\n",1);
>
>   return 0;
> }
>
> Any help would be appreciated. Thanks, PJ.