arguments to main

[bsquared]

Ok, so I understand how arguments to a C program are handled in main; they show up as char array elements in a char array, of size 'argc'. What I can't figure out is how you can get at data passed into a program through the unix pipe, for instance, or redirected from a file.

For example, if from the command line I type

% echo "2+2" | yafu

where yafu is the C program receiving the string from echo, then "2+2" does not show up in **argv, and argc == 1. Same if I do

% yafu < in.txt

where in.txt contains "2+2"

Anyone here know how to get at data through arguments to main in these cases? A non-trivial amount of googling didn't get me anywhere... maybe I'm just a bad googler

- ben.

[bsquared]

10 seconds after posting the above, I figured something out, I think. The answer is you scan it in from stdin!

So, new question: how can I tell if what I got from scanf came from a pipe or a redirection rather than from the keyboard?

My problem is that I'm building in the ability for yafu to take a command line expression, evaluate it, and exit. In addition to the normal mode of operation which is open, wait for manual input, evaulate, wait for next input, etc. If I scanf in a string from a pipe, because I can't tell it came from a pipe, the program doesn't exit like I want it to, it just waits for the next input.

- ben.

[bsquared]

Ok, here's my hackish workaround. I'd still like to know how to do this right...

start = clock();
//wait for input from user
while (1)
{
   //get a new random seed after every transaction
 srand((unsignedint)time(&tt));
 
  //if there are command line arguments, don't wait for user input
 //the expression to be evaluated is already in 's'
 if (argc <= 1)
 {
    printf("\n");
    fflush(stdout); 
    scanf("%[^\n]",s); //read everything up to carriage return
    stop = clock();
    //hack. if no time has elapsed, this must have come from
    //the pipe or a redirect. treat the same as a command line expression.
    if (((double)(stop - start)/(double)CLOCKS_PER_SEC) < 0.01)
       do_once = 1; 
 }
 
.
.
.

[jasonp]

The msieve demo application uses a switch (-m) to determine whether to expect inputs from a pipe or from file. In principle you can assign a FILE * to either stdin or the result of fopen() and then proceed like normal, but IIRC when I tried that there were problems determining when a list of numbers was finished.

[akruppa]

There is some function to query open file descriptors that tells, among other things, whether the fd points to a tty or to a disk file. That's the usual way of determining whether stdio has been redirected, afaik. But for the life of me, I can't remember what the function is called... can someone help out here?

Btw, assigning something to stdin or stdout usually works, but is non-portable, strictly speaking... freopen() should be used instead.

Alex

[bsquared]

This is getting bizarre.

The following works for everything (windows, linux, gcc, MSVC, pipes, redirects, normal operation) *except* pipes in linux.

fseek(in,-1,SEEK_END);
if (argc > 1)
{
  //handle the case where we have command line arguments
  do_once = 1;
  strcpy(s,argv[1]);
}
else
{
  //on linux, gcc-3.2.3, this works for redirects but not for pipes
  //works fine for both on win, MSVC express edition 2008
  if (ftell(stdin) >= 0)
  {
     rewind(stdin);
     fgets(s,1024,in);
     do_once = 1;
  }
}

In that exceptional case, fseek says that there is nothing to read (returns -1), but if the very next thing I do is attempt to read stdin, it finds the string!! WTF!!

[akruppa]

Return value of -1 means an error. Does the global variable errno get set to something? According to "man 3 fseek", if the stream is not seekable (and I would expect a pipe not to be), errno should be set to EBADF.

Alex

[bsquared]

. yeah, -1 means an error... sorry.

So the bizarre thing is that windows is fine with seeking in a pipe. In windows I do

echo "2+2" | yafu

and get

fseek returned 0, errno = 0

while in linux I get

fseek returned -1, errno = 29

[akruppa]

I think in MS-DOS times, redirection didn't really do piping but created temporary files since the programs could not run concurrently (no multi tasking), so in MS-DOS an fseek() call on a pipe may have worked. I don't know if the temp file thing is still the case in multi-tasking capable Windows versions, but even if it isn't, MS may have decided to make fseek() not return an error in Windows if it didn't in MS-DOS. At any rate, I would expect fseek() on pipes not to be portable at all.

I'll try to find out again how to determine whether a file descriptor points to a tty.

Alex

[jasonp]

Alex,

Do you mean fstat() ? The man page seems to indicate it almost gets you what you need, but it works with unix file descriptors and not FILE structures.

BTW I meant that stdin and stdout are lvalues and can be used as the RHS in an assignment (they are not compile-time constants though)

[bsquared]

I could also make it work if there were such a thing as a non-blocking read of a character from a stream. I'm showcasing my C ignorance here, but is there such a function? If so then I could just test a read, and handle the error if there was nothing there, rather than have the program sit there waiting for input. No beans for fgetc, getc, fgets, scanf...