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##################################
Expressions and Control Structures
##################################
.. index :: if, else, while, for, break, continue, return, switch, goto
Control Structures
===================
Most of the control structures from C/JavaScript are available in Solidity
except for `switch` `goto`
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there is: `if` `else` `while` `for` `break` `continue` `return` `? :`
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the usual semantics known from C / JavaScript.
Parentheses can *not* be omitted for conditionals, but curly brances can be omitted
around single-statement bodies.
Note that there is no type conversion from non-boolean to boolean types as
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there is in C and JavaScript, so `if (1) { ... }` *not* valid Solidity.
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.. index :: ! function;call, function;internal, function;external
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.. _function-calls:
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Function Calls
==============
Internal Function Calls
-----------------------
Functions of the current contract can be called directly ("internally"), also recursively, as seen in
this nonsensical example::
contract c {
function g(uint a) returns (uint ret) { return f(); }
function f() returns (uint ret) { return g(7) + f(); }
}
These function calls are translated into simple jumps inside the EVM. This has
the effect that the current memory is not cleared, i.e. passing memory references
to internally-called functions is very efficient. Only functions of the same
contract can be called internally.
External Function Calls
-----------------------
The expression `this.g(8);`
will be called "externally", via a message call and not directly via jumps.
Functions of other contracts have to be called externally. For an external call,
all function arguments have to be copied to memory.
When calling functions
of other contracts, the amount of Wei sent with the call and the gas can be specified::
contract InfoFeed {
function info() returns (uint ret) { return 42; }
}
contract Consumer {
InfoFeed feed;
function setFeed(address addr) { feed = InfoFeed(addr); }
function callFeed() { feed.info.value(10).gas(800)(); }
}
Note that the expression `InfoFeed(addr)`
that "we know that the type of the contract at the given address is `InfoFeed`
this does not execute a constructor. We could also have used `function setFeed(InfoFeed _feed) { feed = _feed; }` `feed.info.value(10).gas(800)`
only (locally) sets the value and amount of gas sent with the function call and only the
parentheses at the end perform the actual call.
Named Calls and Anonymous Function Parameters
---------------------------------------------
Function call arguments can also be given by name, in any order, and the names
of unused parameters (especially return parameters) can be omitted.
::
contract c {
function f(uint key, uint value) { ... }
function g() {
// named arguments
f({value: 2, key: 3});
}
// omitted parameters
function func(uint k, uint) returns(uint) {
return k;
}
}
Order of Evaluation of Expressions
==================================
The evaluation order of expressions is not specified (more formally, the order
in which the children of one node in the expression tree are evaluated is not
specified, but they are of course evaluated before the node itself). It is only
guaranteed that statements are executed in order and short-circuiting for
boolean expressions is done.
.. index :: ! assignment
Assignment
==========
.. index :: ! assignment;destructuring
Destructuring Assignments and Returning Multiple Values
-------------------------------------------------------
Solidity internally allows tuple types, i.e. a list of objects of potentially different types whose size is a constant at compile-time. Those tuples can be used to return multiple values at the same time and also assign them to multiple variables (or LValues in general) at the same time::
contract C {
uint[] data;
function f() returns (uint, bool, uint) {
return (7, true, 2);
}
function g() {
// Declares and assigns the variables. Specifying the type explicitly is not possible.
var (x, b, y) = f();
// Assigns to a pre-existing variable.
(x, y) = (2, 7);
// Common trick to swap values -- does not work for non-value storage types.
(x, y) = (y, x);
// Components can be left out (also for variable declarations).
// If the tuple ends in an empty component,
// the rest of the values are discarded.
(data.length,) = f(); // Sets the length to 7
// The same can be done on the left side.
(,data[3]) = f(); // Sets data[3] to 2
// Components can only be left out at the left-hand-side of assignments, with
// one exception:
(x,) = (1,);
// (1,) is the only way to specify a 1-component tuple, because (1) is
// equivalent to 1.
}
}
Complications for Arrays and Structs
------------------------------------
The semantics of assignment are a bit more complicated for non-value types like arrays and structs.
Assigning *to* a state variable always creates an independent copy. On the other hand, assigning to a local variable creates an independent copy only for elementary types, i.e. static types that fit into 32 bytes. If structs or arrays (including `bytes` `string` *do* change the state.
.. index :: ! exception, ! throw
Exceptions
==========
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There are some cases where exceptions are thrown automatically (see below). You can use the `throw` `send` `call` `delegatecall` `callcode` `false`
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Catching exceptions is not yet possible.
In the following example, we show how `throw` `send`
contract Sharer {
function sendHalf(address addr) returns (uint balance) {
if (!addr.send(msg.value/2))
throw; // also reverts the transfer to Sharer
return this.balance;
}
}
Currently, there are three situations, where exceptions happen automatically in Solidity:
1. If you access an array beyond its length (i.e. `x[i]` `i >= x.length`
2. If a function called via a message call does not finish properly (i.e. it runs out of gas or throws an exception itself).
3. If a non-existent function on a library is called or Ether is sent to a library.
Internally, Solidity performs an "invalid jump" when an exception is thrown and thus causes the EVM to revert all changes made to the state. The reason for this is that there is no safe way to continue execution, because an expected effect did not occur. Because we want to retain the atomicity of transactions, the safest thing to do is to revert all changes and make the whole transaction (or at least call) without effect.
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.. index :: ! assembly, ! asm, ! evmasm
Inline Assembly
===============
For more fine-grained control especially in order to enhance the language by writing libraries,
it is possible to interleave Solidity statements with inline assembly in a language close
to the one of the virtual machine. Due to the fact that the EVM is a stack machine, it is
often hard to address the correct stack slot and provide arguments to opcodes at the correct
point on the stack. Solidity's inline assembly tries to facilitate that and other issues
arising when writing manual assembly by the following features:
* functional-style opcodes: `mul(1, add(2, 3))` `push1 3 push1 2 add push1 1 mul`
* assembly-local variables: `let x := add(2, 3) let y := mload(0x40) x := add(x, y)`
* access to external variables: `function f(uint x) { assembly { x := sub(x, 1) } }`
* labels: `let x := 10 repeat: x := sub(x, 1) jumpi(repeat, eq(x, 0))`
We now want to describe the inline assembly language in detail.
.. warning ::
Inline assembly is still a relatively new feature and might change if it does not prove useful,
so please try to keep up to date.
Example
-------
The following example provides library code to access the code of another contract and
load it into a `bytes`
idea is that assembly libraries will be used to enhance the language in such ways.
.. code ::
library GetCode {
function at(address _addr) returns (bytes o_code) {
assembly {
// retrieve the size of the code, this needs assembly
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let size := extcodesize(_addr)
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// allocate output byte array - this could also be done without assembly
// by using o_code = new bytes(size)
o_code := mload(0x40)
// new "memory end" including padding
mstore(0x40, add(o_code, and(add(add(size, 0x20), 0x1f), bnot(0x1f))))
// store length in memory
mstore(o_code, size)
// actually retrieve the code, this needs assembly
extcodecopy(_addr, add(o_code, 0x20), 0, size)
}
}
}
Inline assemmbly could also be beneficial in cases where the optimizer fails to produce
efficient code. Please be aware that assembly is much more difficult to write because
the compiler does not perform checks, so you should use it for complex things only if
you really know what you are doing.
.. code ::
library VectorSum {
// This function is less efficient because the optimizer currently fails to
// remove the bounds checks in array access.
function sumSolidity(uint[] _data) returns (uint o_sum) {
for (uint i = 0; i < _data.length; ++i)
o_sum += _data[i];
}
// We know that we only access the array in bounds, so we can avoid the check.
// 0x20 needs to be added to an array because the first slot contains the
// array length.
function sumAsm(uint[] _data) returns (uint o_sum) {
for (uint i = 0; i < _data.length; ++i)
assembly { o_sum := mload(add(add(_data, 0x20), i)) }
}
}
Syntax
------
Inline assembly parses comments, literals and identifiers exactly as Solidity, so you can use the
usual `//` `/* */` `assembly { ... }`
these curly braces, the following can be used (see the later sections for more details)
- literals, i.e. `0x123` `42` `"abc"`
- opcodes (in "instruction style"), e.g. `mload sload dup1 sstore`
- opcode in functional style, e.g. `add(1, mlod(0))`
- labels, e.g. `name:`
- variable declarations, e.g. `let x := 7` `let x := add(y, 3)`
- identifiers (externals, labels or assembly-local variables), e.g. `jump(name)` `3 x add`
- assignments (in "instruction style"), e.g. `3 =: x`
- assignments in functional style, e.g. `x := add(y, 3)`
- blocks where local variables are scoped inside, e.g. `{ let x := 3 { let y := add(x, 1) } }`
Opcodes
-------
This document does not want to be a full description of the Ethereum virtual machine, but the
following list can be used as a reference of its opcodes.
If an opcode takes arguments (always from the top of the stack), they are given in parentheses.
Note that the order of arguments can be seed to be reversed in non-functional style (explained below).
Opcodes marked with `-` `*`
special and all others push exactly one item onte the stack.
In the following, `mem[a...b)` `a`
(excluding) position `b` `storage[p]` `p`
The opcodes `pushi` `jumpdest`
+-----------------------+------+---------------------------------------------------------------+
| stop + `-`
+-----------------------+------+---------------------------------------------------------------+
| add(x, y) | | x + y |
+-----------------------+------+---------------------------------------------------------------+
| sub(x, y) | | x - y |
+-----------------------+------+---------------------------------------------------------------+
| mul(x, y) | | x * y |
+-----------------------+------+---------------------------------------------------------------+
| div(x, y) | | x / y |
+-----------------------+------+---------------------------------------------------------------+
| sdiv(x, y) | | x / y, for signed numbers in two's complement |
+-----------------------+------+---------------------------------------------------------------+
| mod(x, y) | | x % y |
+-----------------------+------+---------------------------------------------------------------+
| smod(x, y) | | x % y, for signed numbers in two's complement |
+-----------------------+------+---------------------------------------------------------------+
| exp(x, y) | | x to the power of y |
+-----------------------+------+---------------------------------------------------------------+
| bnot(x) | | ~x, every bit of x is negated |
+-----------------------+------+---------------------------------------------------------------+
| lt(x, y) | | 1 if x < y, 0 otherwise |
+-----------------------+------+---------------------------------------------------------------+
| gt(x, y) | | 1 if x > y, 0 otherwise |
+-----------------------+------+---------------------------------------------------------------+
| slt(x, y) | |1 if x < y, 0 otherwise, for signed numbers in two's complement|
+-----------------------+------+---------------------------------------------------------------+
| sgt(x, y) | |1 if x > y, 0 otherwise, for signed numbers in two's complement|
+-----------------------+------+---------------------------------------------------------------+
| eq(x, y) | | 1 if x == y, 0 otherwise |
+-----------------------+------+---------------------------------------------------------------+
| not(x) | | 1 if x == 0, 0 otherwise |
+-----------------------+------+---------------------------------------------------------------+
| and(x, y) | | bitwise and of x and y |
+-----------------------+------+---------------------------------------------------------------+
| or(x, y) | | bitwise or of x and y |
+-----------------------+------+---------------------------------------------------------------+
| xor(x, y) | | bitwise xor of x and y |
+-----------------------+------+---------------------------------------------------------------+
| byte(n, x) | | nth byte of x, where the most significant byte is the 0th byte|
+-----------------------+------+---------------------------------------------------------------+
| addmod(x, y, m) | | (x + y) % m with arbitrary precision arithmetics |
+-----------------------+------+---------------------------------------------------------------+
| mulmod(x, y, m) | | (x * y) % m with arbitrary precision arithmetics |
+-----------------------+------+---------------------------------------------------------------+
| signextend(i, x) | | sign extend from (i*8+7)th bit counting from least significant|
+-----------------------+------+---------------------------------------------------------------+
| sha3(p, n) | | keccak(mem[p...(p+n))) |
+-----------------------+------+---------------------------------------------------------------+
| jump(label) | `-`
+-----------------------+------+---------------------------------------------------------------+
| jumpi(label, cond) | `-`
+-----------------------+------+---------------------------------------------------------------+
| pc | | current position in code |
+-----------------------+------+---------------------------------------------------------------+
| pop | `*`
+-----------------------+------+---------------------------------------------------------------+
| dup1 ... dup16 | | copy ith stack slot to the top (counting from top) |
+-----------------------+------+---------------------------------------------------------------+
| swap1 ... swap1 | `*`
+-----------------------+------+---------------------------------------------------------------+
| mload(p) | | mem[p..(p+32)) |
+-----------------------+------+---------------------------------------------------------------+
| mstore(p, v) | `-`
+-----------------------+------+---------------------------------------------------------------+
| mstore8(p, v) | `-`
+-----------------------+------+---------------------------------------------------------------+
| sload(p) | | storage[p] |
+-----------------------+------+---------------------------------------------------------------+
| sstore(p, v) | `-`
+-----------------------+------+---------------------------------------------------------------+
| msize | | size of memory, i.e. largest accessed memory index |
+-----------------------+------+---------------------------------------------------------------+
| gas | | gas still available to execution |
+-----------------------+------+---------------------------------------------------------------+
| address | | address of the current contract / execution context |
+-----------------------+------+---------------------------------------------------------------+
| balance(a) | | wei balance at address a |
+-----------------------+------+---------------------------------------------------------------+
| caller | | call sender (excluding delegatecall) |
+-----------------------+------+---------------------------------------------------------------+
| callvalue | | wei sent together with the current call |
+-----------------------+------+---------------------------------------------------------------+
| calldataload(p) | | call data starting from position p (32 bytes) |
+-----------------------+------+---------------------------------------------------------------+
| calldatasize | | size of call data in bytes |
+-----------------------+------+---------------------------------------------------------------+
| calldatacopy(t, f, s) | `-`
+-----------------------+------+---------------------------------------------------------------+
| codesize | | size of the code of the current contract / execution context |
+-----------------------+------+---------------------------------------------------------------+
| codecopy(t, f, s) | `-`
+-----------------------+------+---------------------------------------------------------------+
| extcodesize(a) | | size of the code at address a |
+-----------------------+------+---------------------------------------------------------------+
|extcodecopy(a, t, f, s)| `-`
+-----------------------+------+---------------------------------------------------------------+
| create(v, p, s) | | create new contract with code mem[p..(p+s)) and send v wei |
| | | and return the new address |
+-----------------------+------+---------------------------------------------------------------+
| call(g, a, v, in, | | call contract at address a with input mem[in..(in+insize)] |
| insize, out, outsize) | | providing g gas and v wei and output area |
| | | mem[out..(out+outsize)] returting 1 on error (out of gas) |
+-----------------------+------+---------------------------------------------------------------+
| callcode(g, a, v, in, | | identical to call but only use the code from a and stay |
| insize, out, outsize) | | in the context of the current contract otherwise |
+-----------------------+------+---------------------------------------------------------------+
| delegatecall(g, a, in,| | identical to callcode but also keep `caller` `callvalue`
| insize, out, outsize) | | |
+-----------------------+------+---------------------------------------------------------------+
| return(p, s) | `*`
+-----------------------+------+---------------------------------------------------------------+
| selfdestruct(a) | `*`
+-----------------------+------+---------------------------------------------------------------+
| log0(p, s) | `-`
+-----------------------+------+---------------------------------------------------------------+
| log1(p, s, t1) | `-`
+-----------------------+------+---------------------------------------------------------------+
| log2(p, s, t1, t2) | `-`
+-----------------------+------+---------------------------------------------------------------+
| log3(p, s, t1, t2, t3)| `-`
+-----------------------+------+---------------------------------------------------------------+
| log4(p, s, t1, t2, t3,| `-`
| t4) | | |
+-----------------------+------+---------------------------------------------------------------+
| origin | | transaction sender |
+-----------------------+------+---------------------------------------------------------------+
| gasprice | | gas price of the transaction |
+-----------------------+------+---------------------------------------------------------------+
| blockhash(b) | |hash of block nr b - only for last 256 blocks excluding current|
+-----------------------+------+---------------------------------------------------------------+
| coinbase | | current mining beneficiary |
+-----------------------+------+---------------------------------------------------------------+
| timestamp | | timestamp of the current block in seconds since the epoch |
+-----------------------+------+---------------------------------------------------------------+
| number | | current block number |
+-----------------------+------+---------------------------------------------------------------+
| difficulty | | difficulty of the current block |
+-----------------------+------+---------------------------------------------------------------+
| gaslimit | | block gas limit of the current block |
+-----------------------+------+---------------------------------------------------------------+
Literals
--------
You can use integer constants by typing them in decimal or hexadecimal notation and an
appropriate `PUSHi`
to add 2 and 3 resulting in 5 and then computes the bitwise and with the string "abc".
Strings are stored left-aligned and cannot be longer than 32 bytes.
.. code ::
assembly { 2 3 add "abc" and }
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Functional Style
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-----------------
You can type opcode after opcode in the same way they will end up in bytecode. For example
adding `3` `0x80`
.. code ::
3 0x80 mload add 0x80 mstore
As it is often hard to see what the actual arguments for certain opcodes are,
Solidity inline assembly also provides a "functional style" notation where the same code
would be written as follows
.. code ::
mstore(0x80, add(mload(0x80), 3))
Functional style and instructional style can be mixed, but any opcode inside a
functional style expression has to return exactly one stack slot (most of the opcodes do).
Note that the order of arguments is reversed in functional-style as opposed to the instruction-style
way. If you use functional-style, the first argument will end up on the stack top.
Access to External Variables
----------------------------
Solidity variables and other identifiers can be accessed by simply using their name.
For storage and memory variables, this will push the address and not the value onto the
stack. Also note that non-struct and non-array storage variable addresses occupy two slots
on the stack: One for the address and one for the byte offset inside the storage slot.
In assignments (see below), we can even use local Solidity variables to assign to.
.. code ::
contract c {
uint b;
function f(uint x) returns (uint r) {
assembly {
b pop // remove the offset, we know it is zero
sload
x
mul
=: r // assign to return variable r
}
}
}
Labels
------
Another problem in EVM assembly is that `jump` `jumpi`
which can change easily. Solidity inline assembly provides labels to make the use of
jumps easier. The following code computes an element in the Fibonacci series.
.. code ::
{
let n := calldataload(4)
let a := 1
let b := a
loop:
jumpi(loopend, eq(n, 0))
a add swap1
n := sub(n, 1)
jump(loop)
loopend:
mstore(0, a)
return(0, 0x20)
}
Please note that automatically accessing stack variables can only work if the
assembler knows the current stack height. This fails to work if the jump source
and target have different stack heights. It is still fine to use such jumps,
you should just not access any stack variables (even assembly variables) in that case.
Furthermore, the stack height analyser goes through the code opcode by opcode
(and not according to control flow), so in the following case, the assembler
will have a wrong impression about the stack height at label `two`
.. code ::
{
jump(two)
one:
// Here the stack height is 1 (because we pushed 7),
// but the assembler thinks it is 0 because it reads
// from top to bottom.
// Accessing stack variables here will lead to errors.
jump(three)
two:
7 // push something onto the stack
jump(one)
three:
}
Declaring Assembly-Local Variables
----------------------------------
You can use the `let`
inline assembly and actually only in the current `{...}`
is that the `let`
for the variable and automatically removed again when the end of the block
is reached. You need to provide an initial value for the variable which can
be just `0`
.. code ::
contract c {
function f(uint x) returns (uint b) {
assembly {
let v := add(x, 1)
mstore(0x80, v)
{
let y := add(sload(v), 1)
b := y
} // y is "deallocated" here
b := add(b, v)
} // v is "deallocated" here
}
}
Assignments
-----------
Assignments are possible to assembly-local variables and to function-local
variables. Take care that when you assign to variables that point to
memory or storage, you will only change the pointer and not the data.
There are two kinds of assignments: Functional-style and instruction-style.
For functionaly-style assignments (`variable := value`
functional-style expression that results in exactly one stack value
and for instruction-style (`=: variable`
For both ways, the colon points to the name of the variable.
.. code ::
assembly {
let v := 0 // functional-style assignment as part of variable declaration
let g := add(v, 2)
sload(10)
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=: v // instruction style assignment, puts the result of sload(10) into v
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}
Things to Avoid
---------------
Inline assembly might have a quite high-level look, but it actually is extremely
low-level. The only thing the assembler does for you is re-arranging
functional-style opcodes, managing jump labels, counting stack height for
variable access and removing stack slots for assembly-local variables when the end
of their block is reached. Especially for those two last cases, it is important
to know that the assembler only counts stack height from top to bottom, not
necessarily following control flow. Furthermore, operations like swap will only
swap the contents of the stack but not the location of variables.
Conventions in Solidity
-----------------------
In contrast to EVM assembly, Solidity knows types which are narrower than 256 bits,
e.g. `uint24`
treat them as 256 bit numbers and the higher-order bits are only cleaned at the
point where it is necessary, i.e. just shortly before they are written to memory
or before comparisons are performed. This means that if you access such a variable
from within inline assembly, you might have to manually clean the higher order bits
first.
Solidity manages memory in a very simple way: There is a "free memory pointer"
at position `0x40`
from that point on and update the pointer accordingly.
Elements in memory arrays in Solidity always occupy multiples of 32 bytes (yes, this is
even true for `byte[]` `bytes` `string`
arrays are pointers to memory arrays. The length of a dynamic array is stored at the
first slot of the array and then only the array elements follow.
.. warning ::
Statically-sized memory arrays do not have a length field, but it will be added soon
to allow better convertibility between statically- and dynamically-sized arrays, so
please do not rely on that.
