Managed shared memory segment value not aligned

[golderweb]

On our 32bit armv7l system the value object of shared memory segments created with managed_shared_memory::construct is not properly aligned.

The pointer to the value object is calculated in boost/interprocess/detail/segment_manager_helper.hhp (line 146 and following). This is done based on block_header::value_offset().

   void *value() const
   {
      return const_cast<char*>((reinterpret_cast<const char*>(this) + this->value_offset()));
   }

   size_type value_offset() const
   {
      return get_rounded_size(size_type(sizeof(block_header<size_type>)), size_type(m_value_alignment));
   }

This value_offset() tries to ensure proper alignment. But since the location of the block_header object is not guaranteed to be aligned for value type, the value object may end beeing unaligned.

The following minimal example triggers the bug.

#include <boost/interprocess/managed_shared_memory.hpp>
#include <string>
#include <iostream>

typedef struct head {
   uint32_t a;
   uint32_t b;
   struct{
      uint64_t tv_sec;
      uint32_t tv_nsec;
   } time;
} head_t;

typedef struct container64{
   head_t head;
   uint32_t a;
} container64_t;


int main(){

   std::string shmName("TestSHM");

   boost::interprocess::shared_memory_object::remove(shmName.c_str());
   boost::interprocess::managed_shared_memory managedShm(boost::interprocess::create_only, shmName.c_str(), 4096);

   // Create container
   auto container64 = managedShm.construct<container64_t>("Container64")();
   container64->a = 42;

   // Access container
   std::cerr << "Container64 data.a: " << container64->a << std::endl;
}

Compiler and flags: arm-linux-gnueabi-g++ (GCC) 7.3.0 arm-linux-gnueabi-g++ -march=armv7-a -marm -mfpu=neon -mfloat-abi=hard -mcpu=cortex-a8 -O0 -g3 -Wall -c -pthread

CtorArgN::construct() from boost/interprocess/detail/named_proxy.hpp results in following asembler code. The instruction 0004ca50: vst1.8 {d16-d17], [r3 :64] fails, since address in register r3 is not 64 bit aligned. This results in an alignment trap and programm beeing terminated by SIGBUS=0x7 signal.

;{  ::new((void*)mem, boost_container_new_t())T(boost::forward<Args>(get<IdxPack>(args_))...); }
0004ca38:   ldrb    r2, [r11, #-12]
0004ca3c:   ldr     r1, [r11, #-20] ; 0xffffffec
0004ca40:   mov     r0, #32
0004ca44:   bl      0x12664 <operator new(unsigned int, void*, boost_container_new_t)>
0004ca48:   mov     r3, r0
0004ca4c:   vmov.i32        q8, #0  ; 0x00000000
0004ca50:   vst1.8  {d16-d17}, [r3 :64]
0004ca54:   vorr    d18, d16, d16
0004ca58:   vstr    d18, [r3, #16]
0004ca5c:   vstr    d16, [r3, #24]
0004ca60:   nop     {0}
0004ca64:   sub     sp, r11, #4
0004ca68:   pop     {r11, pc}
´´´´

Tested with interprocess library from Boost 1.72.0 and 1.78.0 (current release)