Defines the semantics of computer memory storage for the purpose of the C abstract machine.
The data storage (memory) available to a C program is one or more contiguous sequences of bytes. Each byte in memory has a unique address.
A byte is the smallest addressable unit of memory. It is defined as a contiguous sequence of bits, large enough to hold any member of the basic execution character set (the 96 characters that are required to be single-byte). C supports bytes of sizes 8 bits and greater.
The types char
, unsigned char
, and signed char
use one byte for both storage and value representation. The number of bits in a byte is accessible as CHAR_BIT
.
For use of bytes to representation values of other fundamental types (including big-endian and little-endian memory layouts), see object representation.
A memory location is.
struct S { char a; // memory location #1 int b : 5; // memory location #2 int c : 11, // memory location #2 (continued) : 0, d : 8; // memory location #3 struct { int ee : 8; // memory location #4 } e; } obj; // The object 'obj' consists of 4 separate memory locations
Threads and data racesA thread of execution is a flow of control within a program that begins with the invocation of a top-level function by Any thread can potentially access any object in the program (objects with automatic and thread-local storage duration may still be accessed by another thread through a pointer). Different threads of execution are always allowed to access (read and modify) different memory locations concurrently, with no interference and no synchronization requirements. (note that it is not safe to concurrently update two non-atomic bit-fields in the same structure if all members declared between them are also (non-zero-length) bit-fields, no matter what the sizes of those intervening bit-fields happen to be). When an evaluation of an expression writes to a memory location and another evaluation reads or modifies the same memory location, the expressions are said to conflict. A program that has two conflicting evaluations has a data race unless either.
If a data race occurs, the behavior of the program is undefined. (in particular, mtx_lock of the same mutex by another thread, which makes it possible to use mutex locks to guard against data races) Memory orderWhen a thread reads a value from a memory location, it may see the initial value, the value written in the same thread, or the value written in another thread. See | (since C11) |
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