c - purpose of comparing pointer with (unsigned) -4000L -


in open source component, cjson,

#define is_error(ptr) ((unsigned long)ptr > (unsigned long)-4000l)

above statement used check validity of pointer shown below

json_object* reply = json_object_new_object(); if (!reply || is_error(reply)) {  . . . //error handling }

how comparing pointer (unsigned long)-4000l validates pointer?

the reason looks they're using pointer value contain "either pointer or error value".

look here: 

struct json_object* json_tokener_parse(const char *str) {   struct json_tokener* tok;   struct json_object* obj;    tok = json_tokener_new();   obj = json_tokener_parse_ex(tok, str, -1);   if(tok->err != json_tokener_success)     obj = error_ptr(-tok->err); // <<<<<---   json_tokener_free(tok);   return obj; }

the function returning special value pointer. err_ptr macro returns negative of error code, presumably because author assumes never valid pointer address.

here test demonstrates expected usage of macro, i.e. malformed json.

 new_obj = json_tokener_parse("{ foo }");   if(is_error(new_obj)) printf("got error expected\n");

so, reason using special value can hold "either pointer structure or error code". done union or struct, or other means, chose way.


Comments

Post a Comment

Popular posts from this blog

apache - Remove .php and add trailing slash in url using htaccess not loading css -

i have add following code htaccess file removing .php extension , add trailing slash urls. worked fine reduced page loading speed , css code not loading @ all. pages ugly. please help rewriteengine on rewritebase / ## hide .php extension snippet # externally redirect /dir/foo.php /dir/foo rewritecond %{the_request} ^[a-z]{3,}\s([^.]+)\.php [nc] rewriterule ^ %1/ [r,l] # add trailing slash rewritecond %{request_filename} !-f rewritecond %{request_uri} !/$ rewriterule . %{request_uri}/ [l,r=301] # internally forward /dir/foo /dir/foo.php rewritecond %{request_filename} !-d rewritecond %{request_filename}.php -f rewriterule ^(.*?)/?$ $1.php [l] we can rewrite urls using .htaccess files. before writing in .htaccess files need verify kind of server using. can check server adding a <?php phpinfo(); ?> in php page. ensure mod_rewrite module enabled in apache server. can identified checking phpinfo included page. common methods used url rewriting follows ...
Read more

javascript - jQuery show full size image on click -

pretty self explanatory not workking . i want display full image when user click on image... yes know there tons of plugins but i want keep simple. any idea ? css : .full { display: none; width: 500px; height: 500px; margin-left: 100px; margin-top: -300px; position: fixed; background-color: #f00; z-index: 6; } .thumbnail { width: 50px; height: 50px; } html: <img class="thumbnail" src="http://4.bp.blogspot.com/-uhbdp2esxng/txneptvi92i/aaaaaaaaaky/upfqis7zto0/s1600/rihanna-wallpaper.jpg"> jquery: $(document).ready(function() { $('.thumbnail').on('click', function() { var img = $('<img />',{src:this.src,'class':'full'}); $('.full').html(img).show(); console.log('done!'); }); }); try looking how jquery works. $(document).ready(function() { $('.thumbnail').on('click', function(...
Read more