Byte Order, Size, and Alignment¶

By default, C types are represented in the machine’s native format and byteorder, and properly aligned by skipping pad bytes if necessary (according to therules used by the C compiler).This behavior is chosen sothat the bytes of a packed struct correspond exactly to the memory layoutof the corresponding C struct.Whether to use native byte orderingand padding or standard formats depends on the application.

Alternatively, the first character of the format string can be used to indicatethe byte order, size and alignment of the packed data, according to thefollowing table:

If the first character is not one of these,'@' is assumed.

>>>importstruct>>>struct.pack('>h',1023)b'\x03\xff'>>>struct.pack('<h',1023)b'\xff\x03'

Native byte order is big-endian or little-endian, depending on thehost system. For example, Intel x86, AMD64 (x86-64), and Apple M1 arelittle-endian; IBM z and many legacy architectures are big-endian.Usesys.byteorder to check the endianness of your system.

Native size and alignment are determined using the C compiler’ssizeof expression. This is always combined with native byte order.

Standard size depends only on the format character; see the table intheFormat Characters section.

Note the difference between'@' and'=': both use native byte order, butthe size and alignment of the latter is standardized.

The form'!' represents the network byte order which is always big-endianas defined inIETF RFC 1700.

There is no way to indicate non-native byte order (force byte-swapping); use theappropriate choice of'<' or'>'.

Notes:

1. Padding is only automatically added between successive structure members.No padding is added at the beginning or the end of the encoded struct.

2. No padding is added when using non-native size and alignment, e.g.with ‘<’, ‘>’, ‘=’, and ‘!’.

3. To align the end of a structure to the alignment requirement of aparticular type, end the format with the code for that type with a repeatcount of zero. SeeExamples.

Format Characters¶

Format characters have the following meaning; the conversion between C andPython values should be obvious given their types. The ‘Standard size’ columnrefers to the size of the packed value in bytes when using standard size; thatis, when the format string starts with one of'<','>','!' or'='. When using native size, the size of the packed value isplatform-dependent.

Notes:

1. The'?' conversion code corresponds to the_Bool typedefined by C standards since C99. In standard mode, it isrepresented by one byte.

2. When attempting to pack a non-integer using any of the integer conversioncodes, if the non-integer has a__index__() method then that method iscalled to convert the argument to an integer before packing.

   Changed in version 3.2:Added use of the__index__() method for non-integers.

3. The'n' and'N' conversion codes are only available for the nativesize (selected as the default or with the'@' byte order character).For the standard size, you can use whichever of the other integer formatsfits your application.

4. For the'f','d' and'e' conversion codes, the packedrepresentation uses the IEEE 754 binary32, binary64 or binary16 format (for'f','d' or'e' respectively), regardless of the floating-pointformat used by the platform.

5. The'P' format character is only available for the native byte ordering(selected as the default or with the'@' byte order character). The byteorder character'=' chooses to use little- or big-endian ordering basedon the host system. The struct module does not interpret this as nativeordering, so the'P' format is not available.

6. The IEEE 754 binary16 “half precision” type was introduced in the 2008revision of theIEEE 754 standard. It has a signbit, a 5-bit exponent and 11-bit precision (with 10 bits explicitly stored),and can represent numbers between approximately6.1e-05 and6.5e+04at full precision. This type is not widely supported by C compilers: on atypical machine, an unsigned short can be used for storage, but not for mathoperations. See the Wikipedia page on thehalf-precision floating-pointformat for more information.

7. When packing,'x' inserts one NUL byte.

8. The'p' format character encodes a “Pascal string”, meaning a shortvariable-length string stored in afixed number of bytes, given by the count.The first byte stored is the length of the string, or 255, whichever issmaller. The bytes of the string follow. If the string passed in topack() is too long (longer than the count minus 1), only the leadingcount-1 bytes of the string are stored. If the string is shorter thancount-1, it is padded with null bytes so that exactly count bytes in allare used. Note that forunpack(), the'p' format character consumescount bytes, but that the string returned can never contain more than 255bytes.

9. For the's' format character, the count is interpreted as the length of thebytes, not a repeat count like for the other format characters; for example,'10s' means a single 10-byte string mapping to or from a singlePython byte string, while'10c' means 10separate one byte character elements (e.g.,cccccccccc) mappingto or from ten different Python byte objects. (SeeExamplesfor a concrete demonstration of the difference.)If a count is not given, it defaults to 1. For packing, the string istruncated or padded with null bytes as appropriate to make it fit. Forunpacking, the resulting bytes object always has exactly the specified numberof bytes. As a special case,'0s' means a single, empty string (while'0c' means 0 characters).

A format character may be preceded by an integral repeat count. For example,the format string'4h' means exactly the same as'hhhh'.

Whitespace characters between formats are ignored; a count and its format mustnot contain whitespace though.

When packing a valuex using one of the integer formats ('b','B','h','H','i','I','l','L','q','Q'), ifx is outside the valid range for that formatthenstruct.error is raised.

For the'?' format character, the return value is eitherTrue orFalse. When packing, the truth value of the argument object is used.Either 0 or 1 in the native or standard bool representation will be packed, andany non-zero value will beTrue when unpacking.

Examples¶

Pack and unpack integers of three different sizes, using big endianordering:

>>>fromstructimport*>>>pack(">bhl",1,2,3)b'\x01\x00\x02\x00\x00\x00\x03'>>>unpack('>bhl',b'\x01\x00\x02\x00\x00\x00\x03')(1, 2, 3)>>>calcsize('>bhl')7

Attempt to pack an integer which is too large for the defined field:

>>>pack(">h",99999)Traceback (most recent call last):  File"<stdin>", line1, in<module>struct.error:'h' format requires -32768 <= number <= 32767

Demonstrate the difference between's' and'c' formatcharacters:

>>>pack("@ccc",b'1',b'2',b'3')b'123'>>>pack("@3s",b'123')b'123'

Unpacked fields can be named by assigning them to variables or by wrappingthe result in a named tuple:

>>>record=b'raymond\x32\x12\x08\x01\x08'>>>name,serialnum,school,gradelevel=unpack('<10sHHb',record)>>>fromcollectionsimportnamedtuple>>>Student=namedtuple('Student','name serialnum school gradelevel')>>>Student._make(unpack('<10sHHb',record))Student(name=b'raymond   ', serialnum=4658, school=264, gradelevel=8)

The ordering of format characters may have an impact on size in nativemode since padding is implicit. In standard mode, the user isresponsible for inserting any desired padding.Note inthe firstpack call below that three NUL bytes were added after thepacked'#' to align the following integer on a four-byte boundary.In this example, the output was produced on a little endian machine:

>>>pack('@ci',b'#',0x12131415)b'#\x00\x00\x00\x15\x14\x13\x12'>>>pack('@ic',0x12131415,b'#')b'\x15\x14\x13\x12#'>>>calcsize('@ci')8>>>calcsize('@ic')5

The following format'llh0l' results in two pad bytes being addedat the end, assuming the platform’s longs are aligned on 4-byte boundaries:

>>>pack('@llh0l',1,2,3)b'\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'

Native Formats¶

When constructing format strings which mimic native layouts, thecompiler and machine architecture determine byte ordering and padding.In such cases, the@ format character should be used to specifynative byte ordering and data sizes. Internal pad bytes are normally insertedautomatically. It is possible that a zero-repeat format code will beneeded at the end of a format string to round up to the correctbyte boundary for proper alignment of consecutive chunks of data.

Consider these two simple examples (on a 64-bit, little-endianmachine):

>>>calcsize('@lhl')24>>>calcsize('@llh')18

Data is not padded to an 8-byte boundary at the end of the secondformat string without the use of extra padding. A zero-repeat formatcode solves that problem:

>>>calcsize('@llh0l')24

The'x' format code can be used to specify the repeat, but fornative formats it is better to use a zero-repeat format like'0l'.

By default, native byte ordering and alignment is used, but it isbetter to be explicit and use the'@' prefix character.

Standard Formats¶

When exchanging data beyond your process such as networking or storage,be precise. Specify the exact byte order, size, and alignment. Donot assume they match the native order of a particular machine.For example, network byte order is big-endian, while many popular CPUsare little-endian. By defining this explicitly, the user need notcare about the specifics of the platform their code is running on.The first character should typically be< or>(or!). Padding is the responsibility of the programmer. Thezero-repeat format character won’t work. Instead, the user mustexplicitly add'x' pad bytes where needed. Revisiting theexamples from the previous section, we have:

>>>calcsize('<qh6xq')24>>>pack('<qh6xq',1,2,3)==pack('@lhl',1,2,3)True>>>calcsize('@llh')18>>>pack('@llh',1,2,3)==pack('<qqh',1,2,3)True>>>calcsize('<qqh6x')24>>>calcsize('@llh0l')24>>>pack('@llh0l',1,2,3)==pack('<qqh6x',1,2,3)True

The above results (executed on a 64-bit machine) aren’t guaranteed tomatch when executed on different machines. For example, the examplesbelow were executed on a 32-bit machine:

>>>calcsize('<qqh6x')24>>>calcsize('@llh0l')12>>>pack('@llh0l',1,2,3)==pack('<qqh6x',1,2,3)False