[cs631apue] Question about execvp(3)

[kthompso]

Hey Bo,

Even using the -c option, I think the first argument passed to the 
command (via
the second parameter to execvp(3)) should be the program name.  So, at 
the minimum
you probably want to create a **char whose first element is the program 
name and
then pass it to execvp(3).

Just a side note -- the documentation for bash(1) looks to be incorrect. 
  It
reads:

   -c string If the -c option is present,  then  commands  are  read  
from
             string.   If  there  are arguments after the string, they 
are
             assigned to the positional parameters, starting with $0.

Seems to imply that the -c option doesn't pass the program name as the 
first
argument, since that's normally in position $0.  But a little testing 
reveals:

   $ bash -c "./printargs foo bar baz"
   argv[0]: ./printargs
   argv[1]: foo
   argv[2]: bar
   argv[3]: baz

ksh(1) and dash(1) display the same behavior, although the dash(1) 
documentation
is better:

   -c  Read commands from the command_string operand
       instead of from the standard input.  Special
       parameter 0 will be set from the command_name op
       -erand and the positional parameters ($1, $2, etc.)
       set from the remaining argument operands.

Hope that helps a little.

Best,
Kyle

On 12/16/2016 1:21 AM, bzhang41 wrote:
> Dear Prof.
> 
> I have a question about the final assignment.
> 
> When I tried to implement the -c options,
> I try to use fork(3) and then execvp(3),
> 
> But I can't get the result as I expected, first argument of the execvp 
> needs the
> real path of the executable file, and the second  argument of the 
> execvp needs
> some arguments with type char **.
> 
> But the command from -c options doesn't have any arguments
> how can I pass a empty char ** to execvp, and let it just executes the
> command without any arguments.
> 
> Or can I just call system(3) for -c options, I think that's a cheat 
> right ?
> 
> Thanks
> Bo
> 
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