Re: fgets/strtok and LINE_MAX

[Don Cragun <dcragun@xxxxxxxxx>]

The subject line on this used to be: "Re: [1003.1(2008)/Issue 7
0000177]: mismatch between example description and code".  I have
changed it back to "Re: fgets/strtok and LINE_MAX" because Bug #177
is about a mismatch between the header of the example in strtok()
(where it says "The following example uses strtok( ) to break a line
into two character strings separated by any combination of <space>,
<tab>, or <newline> characters." and the following code that includes:
	key = strtok(line, " \n");
	data = strtok(NULL, " \n");
which does not look for <tab> characters.

This discussion and the recent comments added to Bug #177 have nothing  
to do with strtok().

Please find further comments in-line below...

Cheers,
Don

On Nov 5, 2009, at 8:24 AM, shwaresyst@xxxxxx wrote:

 ... ... ...
> 2. {LINE_MAX} is in the class of variables that aren't allowed to  
> return -1 from sysconf(). In the absence of a larger value set at  
> runtime it is required to return the static LINE_MAX from limits.h,  
> which in turn is required to be >= _POSIX2_LINE_MAX, so the test on  
> sysconf()'s return is superfluous.
This is not true.  It is true that LINE_MAX has to be defined in
<limits.h> and may have a value that is less than the maximum line
length supported by the system when an application is running.  But,
as stated on P2063, L65328-65331 in the RETURN VALUE section of the
description of sysconf() ("If the variable corresponding to name is
described in <limits.h> as a maximum or minimum value and the
variable has no limit, sysconf( ) shall return −1 without changing
the value of errno.  Note that indefinite limits do not imply infinite
limits; see <limits.h>."),  sysconf(_SC_LINE_MAX) may indeed return
-1 without setting errno to indicate that the standard utilities
support lines of arbitrary length.

> As it's simple enough, I believe below adequately addresses being  
> reasonably robust and shows the NULL is accounted for. Adding the  
> partial line processing I believe would overly obfuscate the call  
> setup requirements the example was intended to show, so replacing  
> the ellipsis in the while loop with code of that nature is more a  
> subject for a separate tutorial example and I've inserted  
> descriptive comments in their place of possible caveats. fp is made  
> an argument so the open/close call logic can be relegated to the  
> calling code and the non-void return type is for reporting error  
> codes.
I will work off-line to come up with a better example for fgets() before
the conference call next Thursday.  I don't know yet if I will try to
make it a tutorial on how to process arbitrary length lines or if I will
just add comments noting that lines can be arbitrarily long and state
that the example has limitations by ignoring this possibility.

> Mark
>
> <pre>
> #include <limits.h>
> #include <unistd.h>
> #include <stdlib.h>
> #include <stdio.h>
>
> int f(FILE *fp)
> {
>   long line_max; /* used to avoid multiple sysconf() calls */
>   char *line;      /* the buffer */
> /* declare other variables function needs */
>
> /* get implementation instance's max expectable line length and  
> account for fgets() appending a NULL char */
>   line_max = sysconf(_SC_LINE_MAX) + 1;
As noted above, this can set line_max to zero.

>   line = malloc(line_max);               /* get the buffer space */
And, malloc(0) is allowed to return a pointer to a newly allocated
buffer that
is zero bytes long...

>    if (line=NULL) return ENOMEM;     /* error check required by  
> malloc() */
I assume you meant "line == NULL" instead of "line = NULL".  But the
idea of having an example always fail on a system that doesn't limit
line lengths doesn't seem to be setting a good example of how to
write portable code.

> /* set up other declared variables needed for the loop's processing */
>
>   while (fgets(line, line_max, fp) != NULL) {
> /* process returned full or partial line, accounting for there may  
> be embedded NULL chars
>     along with the appended one; assumes fgets result = line when  
> non-NULL */
>   };
> /* process possible fgets error values returned in errno and handle  
> normal EOF condition */
>
>   free(line);  /* clean up the dynamic memory usage */
>   return 0;   /* function succeded */
> }
> </pre>
>
> -----Original Message-----
> From: Geoff Clare <gwc@xxxxxx>
> To: austin-group-l@xxxxxx
> Sent: Thu, Nov 5, 2009 5:24 am
> Subject: Re: [1003.1(2008)/Issue 7 0000177]: mismatch between  
> example description and code
>
>
>
>
>
>
>
>
>
>
> shwaresyst@xxxxxx <shwaresyst@xxxxxx> wrote, on 04 Nov 2009:
> >
> ----------------------------------------------------------------------
> > > (0000278) nick (manager) - 2009-11-04 18:56
> > > http://austingroupbugs.net/view.php?id=177#c278
> ----------------------------------------------------------------------
> > > A better example:
> > >
> > > <pre>
> > > #include <stdio.h>
> > > #include <limits.h>
> > > #include <unistd.h>
> > >
> > > void f()
> > > {
> > >   long line_max;
> > >   FILE *fp;
> > >       ...
> > >   if ((line_max = sysconf(_SC_LINE_MAX)) <= 0) {
> > >       line_max = LINE_MAX;
> > >   }
> > >   char line[line_max];
> > >   while (fgets(line, line_max, fp) != NULL) {
> > >       ...
> > >   }
> > >   return;
> > > }
> > >
> > > </pre>
> > Problem: line_max is declared as long. To use it as the array limit
> > declarator it would have to be '#define line_max =0;' but then
> sysconf()
> > couldn't be used to assign its runtime value. Suggested fix: change
> the
> > line[line_max] declaration to 'char *line;' declaration with a
> > malloc()/free() pair to assign line a value.
> C99 allows variable-length arrays in some circumstances.  I think
> this one is valid (at least, it is accepted by gcc -std=c99 and
> by the Sun Studio compiler when invoked as c99).
>
> However, since line_max could potentially be very large, it would
> be better to use malloc() so that if insufficient memory is
> available the program will see an error returned by malloc()
> instead of just crashing.
>
> The code does have a couple of problems:
>
> 1. The size of line[], and the argument to fgets(), should be  
> line_max+1
>
> 2. If sysconf() returns -1 to indicate that LINE_MAX is indeterminate
> then setting line_max to the LINE_MAX macro from <limits.h> is the
> wrong thing to do.  It would mean that the fgets() loop needs to
> handle partial lines, but the whole point of using a buffer sized
> to {LINE_MAX} is so that the program doesn't need to handle partial
> lines in the fgets() loop (except perhaps to detect them and report
> that the input is not a text file).
>
> One way to fix it would be to abandon the use of LINE_MAX altogether.
> Just have a fixed size buffer (of, say, 4096 bytes) and show how to
> handle partial lines in the fgets() loop.  Alternatively, have the
> code use getline() if sysconf() returns -1 and fgets() otherwise.
>
> --
> Geoff Clare <g.clare@xxxxxx>
> The Open Group, Thames Tower, Station Road, Reading, RG1 1LX, England