RFC: strn(case)cmp supporting a negative length as its third paramter


strncmp doesn't supporting negative length:

   if (strncmp("prefix_num", "num", -3) === 0) {
        echo "they have same suffix\n";

running the above script in PHP 5.3.6 will result:

warning: Length must be greater than or equal to 0 in /tmp/1.php 

We need to write some codes like following one to make it works as expect:

   if (strncmp(substr("prefix_num", -3, 3), "num", 3) === 0) {
        echo "they have same suffix\n";


base on feature request: https://bugs.php.net/bug.php?id=36944 , I wrote a patch to make strn(case)cmp supporting negative length,

after patched, following script:

   if (strncmp("prefix_num", "num", -3) === 0) {
        echo "they have same suffix\n";

will work as expect.

and if the abs of the negative length is greater than any strlen of the first two parameters, then strn(case)cmp will work as strncmp in the reverse order with the abs value:

var_dump(strncmp("prefix_num", "num", -10));
var_dump(strncmp("mun_xiferp", "mun", 10));
/** output:

BC Breaks

  • Any strncmp call with a computed length which may end up being negative will now potentially return a match where it wouldn't before.




Do you want this feature in PHP 5.4
Real name yes no
arpad (arpad)  
cataphract (cataphract)  
derick (derick)  
felipe (felipe)  
gwynne (gwynne)  
iliaa (iliaa)  
klaussilveira (klaussilveira)  
laruence (laruence)  
levim (levim)  
pajoye (pajoye)  
pierrick (pierrick)  
rasmus (rasmus)  
rdohms (rdohms)  
salathe (salathe)  
stas (stas)  
tyrael (tyrael)  
Count: 11 5


  • 2011-07-28 Xinchen Hui: Initial RFC creation
  • 2011-07-28 Xinchen Hui: Updated patch to avoid making binary API changed
  • 2011-08-01 Xinchen Hui: Start voting
  • 2011-08-08 Xinchen Hui: Edited phpt
  • 2011-08-13 Xinchen Hui: Stop voting
  • 2011-08-14 Xinchen Hui: Open voting again, since there comes some new objections
  • 2011-12-13 Xinchen Hui: Won't apply now
rfc/strncmpnegativelen.txt · Last modified: 2013/05/28 08:23 by tyrael