Memory model
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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.
Byte
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 (MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 38 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE that are required to be single-byte). C supports bytes of sizes 8 bits and greater.
The MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 36 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE char, unsigned char, and signed char use one byte for both storage and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 35 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. The number of bits in a byte is accessible as MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 34 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
For use of bytes to representation values of other fundamental types (including big-endian and little-endian memory layouts), see MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 32 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
Memory location
A memory location is
- an object of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 31 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (arithmetic type, pointer type, enumeration type)
- or the largest contiguous sequence of MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 30 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE of non-zero length
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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 29 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE or other means. Any thread can potentially access any object in the program (objects with automatic and thread-local MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 27 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE 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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 26 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE 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
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 22 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (in particular, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 21 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE is synchronized-with, and therefore, happens-before MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 20 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE 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 MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 08 HOURS 54 MINUTES 18 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for details on the order in which writes made from threads become visible to other threads. |
(since 哋它亢11) |
References
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 3.6 byte (p: 4)
- 3.14 memory location (p: 5)
- 5.1.2.4 Multi-threaded executions and data races (p: 17-21)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 3.6 byte (p: 4)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 1.6 DEFINITIONS OF TERMS