# StrPack.jl — Structured Stream I/O¶

This package performs serialization of structures to and deserialization from binary data streams and string objects.

## Example¶

Let’s create a C library as follows:

struct teststruct {
int      int1;
float    float1;
};

void getvalues(struct teststruct* ts)
{
ts->int1 = 7;
ts->float1 = 3.7;
}


The getvalues function just fills the two fields with specified values. Compile this as a shared library, which on Linux is achieved with gcc -fPIC teststruct.c -shared -o libteststruct.so.

Let’s also create the Julia analog of this structure:

using StrPack

@struct type TestStruct
int1::Int32
float1::Float32
end
#define a convenience constructor for 0.2 versions of Julia (in 0.3 this is automatic)
if VERSION < v"0.3.0-pre"
TestStruct(i, f) = TestStruct(convert(Int32, i), convert(Float32, f))
end


Note that C’s int corresponds to Int32. Let’s initialize an object of this type:

s = TestStruct(-1, 1.2)


We can pack s into a form suitable to pass as the input to our C function getvalues, which we do in the following way:

iostr = IOBuffer()
pack(iostr, s)


It’s worth seeing what has happened here:

julia> iostr
IOBuffer([0xff, 0xff, 0xff, 0xff, 0x9a, 0x99, 0x99, 0x3f],9)


The first 4 bytes correspond to the Int32 representation of -1, and the last 4 to the Float32 representation of 1.2. In other words, this is just a packed memory buffer encoding s. (There are subtleties such as data alignment, endian status, etc. strpack knows about this stuff, and users who need to control its behavior manually can do so.)

Now we load our library and make a ccall:

const libtest = dlopen("libteststruct")
ccall(dlsym(libtest, :getvalues), Void, (Ptr{Void},), iostr.data)


The C function getvalues stores its output in the buffer we provided as input. We unpack this buffer back into a Julia type:

seek(iostr, 0)   # "rewind" to the beginning of the buffer
s2 = unpack(iostr, TestStruct)


Voila! You have the result back.

## Macros¶

@struct(type, strategy, endianness)

Create and register a structural Julia type with StrPack. The type argument uses an extended form of the standard Julia type syntax to define the size of arrays and strings. Each element must declare its type, and each type must be reducible to a bits type or array or composite of bits types.:

@struct type StructuralType
a::Float64 # a bits type
b::Array{Int32,2}(4, 4) # an array of bits types
c::ASCIIString(8) # a string with a fixed number of bytes
end


## Methods¶

pack(io, composite[, asize, strategy, endianness])

Create a packed buffer representation of composite in stream io, using array and string sizes fixed by asize and data alignment coded by strategy with endianness endianness. If the optional arguments are not provided, then T, the type of composite, is expected to have been created with the @struct macro.

unpack(io, T[, asize, strategy, endianness])

Extract an instance of the Julia composite type T from the packed representation in the stream io. If the optional arguments are not provided, then T is expected to have been created with the @struct macro.

show_struct_layout(T[, asize, strategy][, width, bytesize])

Print a graphical representation of the memory layout of the packed type T. If asize and strategy are not provided, then T is expected to have been created with the @struct macro. The display will show width bytes in each row, with each byte taking up bytesize characters.

## Endianness¶

StrPack supports both big-endian (also known as network-ordered) and little-endian streams. The symbols :BigEndian, :LittleEndian, and :NativeEndian can be passed as endianness arguments in StrPack macros and methods.

## Packing strategies¶

To support arbitrary ABIs, StrPack defines a number of “packing strategies”–that is, instructions for inserting padding bytes–as well as allowing the user to define their own.

The predefined strategies are:

align_default
Each bits type is aligned to the next higher power of two. Arrays and structures are aligned to the largest alignment required by any of their members.
align_packed
No padding is inserted. Equivalent to __attribute__ (( __packed__ )).
align_x86_pc_linux_gnu
The x86 Linux ABI, which uses 4 byte alignment for Int64, Uint64, and Float64.
align_native
The native C ABI specified by the host platform.
align_packmax(n)
The default alignment is used up to a limit of n bytes. Equvalent to #pragma pack(n).
align_structpack(n)
The default alignment is used for bitstypes, but aggregate types are aligned to n bytes. Equivalent to __attribute__ (( align(n) ))
align_table(ttable)
The alignments will be taken from ttable, a Dict mapping types to alignments in bytes. Otherwise, the default alignment will be used.

The align_packmax, align_structpack, and align_table strategies can be composed. For example:

align_structpack(align_table({Special => 2}), 4)


will align structures to 4 bytes, except for Special, which will be aligned to 2 bytes.

Completely custom alignment strategies can be defined by constructing a DataAlign type.

DataAlign([ttable, ]default, aggregate)

Create a padding strategy. ttable is an optional Dict mapping types to alignments in bytes. default is a function from Type to Integer which should return the required alignment for bitstypes not in ttable. aggregate is a function from Vector{Type} to Integer which should return the requirement for composite (also known as structure or aggreggate) types.