Re: can I know how to write a html parser in C

From: Walter Roberson (roberson_at_ibd.nrc-cnrc.gc.ca)
Date: 02/28/05 

Date: 28 Feb 2005 03:50:22 GMT

In article <1109560168.939988.157590@z14g2000cwz.googlegroups.com>,
WUV999U <usbharath.ganesh@gmail.com> wrote:
: op = fopen(argv[1],"r");

argv[1] might be NULL. You should be checking that you have the right
number of parameters before you use any of them.

:void htmlparse(FILE * op)
:{
: char line[81];

Are the lines truly limited to 80 characters of text? It is not
at all uncommon to encounter HTML in which the lines go on for
several hundred characters.

: char images[250];

That declares a single character array named 'images' with a maximum
null-terminated character string size of 249 characters. However,
since you are only fetching 80 characters per line, the maximum
image file name you are going to be able to extract is about 68
characters (once you remove the tag and quotes.)

If you want to allow for 250 images, then you should be declaring
either an array of char * pointers or else a "two dimensional"
array of characters.

: if (fgets(line,81,op) == NULL)

There's that magic number again, 81. Any time you have a number whose
meaning is not obvious and which is repeated, you should either
use a #define or store the value in a variable [which would have
implications on how you would write the code.]

: {
: printf("Error reading data");
: exit(0);
: }

Eventually you are going to run out of input and get NULL returned.
That isn't an error: it is a signal that your function should
finish up and return. As you have named the function 'htmlparse',
the reader would tend to assume that -all- the function does is
parse the input and extract certain information from it, but would
not act upon that information, so the reader would tend to assume
that you would return the list of images to the calling routine
and let it do whatever should be done with the list.

: puts(line);

Why do you need to output the line at that point? The input file
isn't going anywhere, so you are unlikely to need to duplicate the
input.

: if(line == "<img src")

That is never going to be true. That is going to compare the
*address* of the string "<img src" to the address of the character
array 'line'. Since "<img src" is a literal string, it is not going
to have the same address as your buffer.

You also cannot fix this just by using strcmp() instead of testing
the pointer: you need to be looking inside the line to find a place
on the line (not necessarily at the beginning) where the string
"<img src" occurs. Try strstr(). But watch out for comments and
for the possibility that you might be within a quoted string...

Note too that in the general case it is perfectly acceptable in HTML
for there to be a linebreak between the "<img" and "src". Are you
working with a very restricted subset of HTML? If so then it would
help a lot to describe what the subset is. Some HTML subsets are
very easy to parse, whereas HTML in general is fairly complex to
parse.

:well,, thats all i hav......... and m stuck here...

Ekkk!

No offense intended but you really haven't gotten very far
at all and have made a number of mistakes in what you posted.
Looking at this, we would tend to conclude that you are very
much a beginner at C (and possibly a beginner at programming
in general). Parsing general HTML is something that requires a
fair bit of experience to program correctly; if what you posted
is indeed representative of your C skills then you have no hope of
writing a generalized HTML img file name extractor in any reasonable
amount of time. Even a well-experienced programmer would take more
than "a day or two" to write a proper HTML parser from scratch.

[Of course, a well-experience programmer would know to *not*
write it from scratch if it could be avoided: there are a number
of already-written HTML parser libraries out there, and there
are programs such as "lynx" which could be canablized. Writing
from scratch would usually be reserved for instances in which
there were notable copyright or patent issues at stake.] 

-- 
   IEA408I: GETMAIN cannot provide buffer for WATLIB.

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