2Unaligned Memory Accesses
   5:Author: Daniel Drake <>,
   6:Author: Johannes Berg <>
   8:With help from: Alan Cox, Avuton Olrich, Heikki Orsila, Jan Engelhardt,
   9  Kyle McMartin, Kyle Moffett, Randy Dunlap, Robert Hancock, Uli Kunitz,
  10  Vadim Lobanov
  13Linux runs on a wide variety of architectures which have varying behaviour
  14when it comes to memory access. This document presents some details about
  15unaligned accesses, why you need to write code that doesn't cause them,
  16and how to write such code!
  19The definition of an unaligned access
  22Unaligned memory accesses occur when you try to read N bytes of data starting
  23from an address that is not evenly divisible by N (i.e. addr % N != 0).
  24For example, reading 4 bytes of data from address 0x10004 is fine, but
  25reading 4 bytes of data from address 0x10005 would be an unaligned memory
  28The above may seem a little vague, as memory access can happen in different
  29ways. The context here is at the machine code level: certain instructions read
  30or write a number of bytes to or from memory (e.g. movb, movw, movl in x86
  31assembly). As will become clear, it is relatively easy to spot C statements
  32which will compile to multiple-byte memory access instructions, namely when
  33dealing with types such as u16, u32 and u64.
  36Natural alignment
  39The rule mentioned above forms what we refer to as natural alignment:
  40When accessing N bytes of memory, the base memory address must be evenly
  41divisible by N, i.e. addr % N == 0.
  43When writing code, assume the target architecture has natural alignment
  46In reality, only a few architectures require natural alignment on all sizes
  47of memory access. However, we must consider ALL supported architectures;
  48writing code that satisfies natural alignment requirements is the easiest way
  49to achieve full portability.
  52Why unaligned access is bad
  55The effects of performing an unaligned memory access vary from architecture
  56to architecture. It would be easy to write a whole document on the differences
  57here; a summary of the common scenarios is presented below:
  59 - Some architectures are able to perform unaligned memory accesses
  60   transparently, but there is usually a significant performance cost.
  61 - Some architectures raise processor exceptions when unaligned accesses
  62   happen. The exception handler is able to correct the unaligned access,
  63   at significant cost to performance.
  64 - Some architectures raise processor exceptions when unaligned accesses
  65   happen, but the exceptions do not contain enough information for the
  66   unaligned access to be corrected.
  67 - Some architectures are not capable of unaligned memory access, but will
  68   silently perform a different memory access to the one that was requested,
  69   resulting in a subtle code bug that is hard to detect!
  71It should be obvious from the above that if your code causes unaligned
  72memory accesses to happen, your code will not work correctly on certain
  73platforms and will cause performance problems on others.
  76Code that does not cause unaligned access
  79At first, the concepts above may seem a little hard to relate to actual
  80coding practice. After all, you don't have a great deal of control over
  81memory addresses of certain variables, etc.
  83Fortunately things are not too complex, as in most cases, the compiler
  84ensures that things will work for you. For example, take the following
  87        struct foo {
  88                u16 field1;
  89                u32 field2;
  90                u8 field3;
  91        };
  93Let us assume that an instance of the above structure resides in memory
  94starting at address 0x10000. With a basic level of understanding, it would
  95not be unreasonable to expect that accessing field2 would cause an unaligned
  96access. You'd be expecting field2 to be located at offset 2 bytes into the
  97structure, i.e. address 0x10002, but that address is not evenly divisible
  98by 4 (remember, we're reading a 4 byte value here).
 100Fortunately, the compiler understands the alignment constraints, so in the
 101above case it would insert 2 bytes of padding in between field1 and field2.
 102Therefore, for standard structure types you can always rely on the compiler
 103to pad structures so that accesses to fields are suitably aligned (assuming
 104you do not cast the field to a type of different length).
 106Similarly, you can also rely on the compiler to align variables and function
 107parameters to a naturally aligned scheme, based on the size of the type of
 108the variable.
 110At this point, it should be clear that accessing a single byte (u8 or char)
 111will never cause an unaligned access, because all memory addresses are evenly
 112divisible by one.
 114On a related topic, with the above considerations in mind you may observe
 115that you could reorder the fields in the structure in order to place fields
 116where padding would otherwise be inserted, and hence reduce the overall
 117resident memory size of structure instances. The optimal layout of the
 118above example is::
 120        struct foo {
 121                u32 field2;
 122                u16 field1;
 123                u8 field3;
 124        };
 126For a natural alignment scheme, the compiler would only have to add a single
 127byte of padding at the end of the structure. This padding is added in order
 128to satisfy alignment constraints for arrays of these structures.
 130Another point worth mentioning is the use of __attribute__((packed)) on a
 131structure type. This GCC-specific attribute tells the compiler never to
 132insert any padding within structures, useful when you want to use a C struct
 133to represent some data that comes in a fixed arrangement 'off the wire'.
 135You might be inclined to believe that usage of this attribute can easily
 136lead to unaligned accesses when accessing fields that do not satisfy
 137architectural alignment requirements. However, again, the compiler is aware
 138of the alignment constraints and will generate extra instructions to perform
 139the memory access in a way that does not cause unaligned access. Of course,
 140the extra instructions obviously cause a loss in performance compared to the
 141non-packed case, so the packed attribute should only be used when avoiding
 142structure padding is of importance.
 145Code that causes unaligned access
 148With the above in mind, let's move onto a real life example of a function
 149that can cause an unaligned memory access. The following function taken
 150from include/linux/etherdevice.h is an optimized routine to compare two
 151ethernet MAC addresses for equality::
 153  bool ether_addr_equal(const u8 *addr1, const u8 *addr2)
 154  {
 156        u32 fold = ((*(const u32 *)addr1) ^ (*(const u32 *)addr2)) |
 157                   ((*(const u16 *)(addr1 + 4)) ^ (*(const u16 *)(addr2 + 4)));
 159        return fold == 0;
 160  #else
 161        const u16 *a = (const u16 *)addr1;
 162        const u16 *b = (const u16 *)addr2;
 163        return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) == 0;
 164  #endif
 165  }
 167In the above function, when the hardware has efficient unaligned access
 168capability, there is no issue with this code.  But when the hardware isn't
 169able to access memory on arbitrary boundaries, the reference to a[0] causes
 1702 bytes (16 bits) to be read from memory starting at address addr1.
 172Think about what would happen if addr1 was an odd address such as 0x10003.
 173(Hint: it'd be an unaligned access.)
 175Despite the potential unaligned access problems with the above function, it
 176is included in the kernel anyway but is understood to only work normally on
 17716-bit-aligned addresses. It is up to the caller to ensure this alignment or
 178not use this function at all. This alignment-unsafe function is still useful
 179as it is a decent optimization for the cases when you can ensure alignment,
 180which is true almost all of the time in ethernet networking context.
 183Here is another example of some code that could cause unaligned accesses::
 185        void myfunc(u8 *data, u32 value)
 186        {
 187                [...]
 188                *((u32 *) data) = cpu_to_le32(value);
 189                [...]
 190        }
 192This code will cause unaligned accesses every time the data parameter points
 193to an address that is not evenly divisible by 4.
 195In summary, the 2 main scenarios where you may run into unaligned access
 196problems involve:
 198 1. Casting variables to types of different lengths
 199 2. Pointer arithmetic followed by access to at least 2 bytes of data
 202Avoiding unaligned accesses
 205The easiest way to avoid unaligned access is to use the get_unaligned() and
 206put_unaligned() macros provided by the <asm/unaligned.h> header file.
 208Going back to an earlier example of code that potentially causes unaligned
 211        void myfunc(u8 *data, u32 value)
 212        {
 213                [...]
 214                *((u32 *) data) = cpu_to_le32(value);
 215                [...]
 216        }
 218To avoid the unaligned memory access, you would rewrite it as follows::
 220        void myfunc(u8 *data, u32 value)
 221        {
 222                [...]
 223                value = cpu_to_le32(value);
 224                put_unaligned(value, (u32 *) data);
 225                [...]
 226        }
 228The get_unaligned() macro works similarly. Assuming 'data' is a pointer to
 229memory and you wish to avoid unaligned access, its usage is as follows::
 231        u32 value = get_unaligned((u32 *) data);
 233These macros work for memory accesses of any length (not just 32 bits as
 234in the examples above). Be aware that when compared to standard access of
 235aligned memory, using these macros to access unaligned memory can be costly in
 236terms of performance.
 238If use of such macros is not convenient, another option is to use memcpy(),
 239where the source or destination (or both) are of type u8* or unsigned char*.
 240Due to the byte-wise nature of this operation, unaligned accesses are avoided.
 243Alignment vs. Networking
 246On architectures that require aligned loads, networking requires that the IP
 247header is aligned on a four-byte boundary to optimise the IP stack. For
 248regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
 249architectures this constant has the value 2 because the normal ethernet
 250header is 14 bytes long, so in order to get proper alignment one needs to
 251DMA to an address which can be expressed as 4*n + 2. One notable exception
 252here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
 253addresses can be very expensive and dwarf the cost of unaligned loads.
 255For some ethernet hardware that cannot DMA to unaligned addresses like
 2564*n+2 or non-ethernet hardware, this can be a problem, and it is then
 257required to copy the incoming frame into an aligned buffer. Because this is
 258unnecessary on architectures that can do unaligned accesses, the code can be
 259made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so::
 262                skb = original skb
 263        #else
 264                skb = copy skb
 265        #endif