I am not sure, is this just a wrong usage of ZEND API?

Not sure how that might be ...

Here's the signature for php_module_startup

int php_module_startup(sapi_module_struct *sf, zend_module_entry 
*additional_modules, uint num_additional_modules);

The parameter num_additional_modules suggests that more then one module_entry 
can be passed in the additional_modules argument, but it's only a pointer not an 
array of pointers.

Here's the signature for php_register_extensions, which is called from 
php_module_startup.

int php_register_extensions(zend_module_entry **ptr, int count TSRMLS_DC);

This more correctly takes an array of pointers.

I don't know why this wasn't fixed when first reported in 2004 - I couldn't find 
any more information than the thread I linked.

Could you add some (short) code that would help to reproduce the problem?

Sure - this will result in a segfault.

int register_moules(sapi_module_struct* sapi_module) {

  zend_module_entry modules[] = {
      my_module_1_entry,
      my_module_2_entry,
  };
  int module_count = sizeof(modules) / sizeof(zend_module_entry);

  return php_module_startup(sapi_module, modules, module_count);
}

Works just fine here.

Which architecture do you use? Maybe some rounding issues? 

However I would suggest you to manually set the modules count to the right numbers 
instead, to avoid any kind of architecture specific troubles.

Suggest you look again - it is not possible to get past this line without 
segfaulting if there is more than one extension.

https://github.com/php/php-src/blob/master/main/main.c#L2191

http://lxr.php.net/xref/PHP_5_4/main/main.c#1910 looks perfectly fine to me.

Which value do you actually pass as count? Also provide a backtrace.

I'll grab you a stack trace when I'm in to office tomorrow (need to remove my 
patch to get it to crash :)).... However you can pretty easily walk through the 
code.

int php_module_startup(sapi_module_struct *sf, zend_module_entry 
*additional_modules, uint num_additional_modules)

Takes a pointer to an array of zend_module_entry.

int php_register_extensions(zend_module_entry **ptr, int count TSRMLS_DC)


takes a pointer to an array of zend_module_entry pointers.

inside php_register_extensions it uses pointer arithmetic to enumerate the 
extensions. However, because php_module_startup has a pointer to an array of 
zend_module_entry rather than a pointer to an array of pointers of 
zend_module_entry it causes a segfault.

https://github.com/php/php-src/blob/master/main/main.c#L1961

https://github.com/php/php-src/blob/master/main/main.c#L1909

oh, stupid me, coffee++ :-)

You must be the 1st to call it with more than one ;-)

Well according to this I'm not :) 

http://marc.info/?l=php-internals&m=110121150631159&w=2

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https://github.com/php/php-src/commit/00a7b1ff7fed49df51d0392d47aa79f63bf1c66e