📄 portability.html
字号:
<html><!-- #BeginTemplate "/Templates/tmpl.dwt" --><head><!-- #BeginEditable "doctitle" --> <title>PTypes: Portability and Performance Issues</title><!-- #EndEditable --> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link rel="stylesheet" href="styles.css"></head><body bgcolor="#FFFFFF" leftmargin="40" marginwidth="40"><p><a href="../index.html"><img src="title-1.7.gif" width="213" height="34" alt="C++ Portable Types Library (PTypes) Version 1.7" border="0"></a> <hr noshade><!-- #BeginEditable "body" --> <p class="hpath"><a href="index.html">Top</a>: Portability and performance issues</p><p><br><b> 64-bit integers</b>: most compilers provide <span class="lang">long long</span> type modifier for declaring 64-bit integers, but at least two of them, Microsoft C/C++ and Borland C/C++, use quite unaesthetic nonstandard keyword <span class="lang">__int64</span>. PTypes offers its own wrapper type <span class="lang">large</span> (in <pport.h>), which is a portable signed 64-bit integer. <span class="lang">Unsigned large</span> is also acceptable. For portable 64-bit string conversions use <span class="lang">itostring()</span> and <span class="lang">stringtoi()</span> declared in <ptypes.h>.</p><p>Another incompatibility exists for printf-style formatted output routines: Windows uses "I64" type modifier for 64-bit integers, whereas standard-compliant libraries recognize "ll" (ell-ell). PTypes solves this problem in its <span class="lang">outstm::putf()</span> method: you can safely use "ll" modifier on all platforms.</p><p><b>End-of-line sequences</b>: PTypes input stream routines automatically recognize three types of EOL sequences: LF (Unix), CR (Mac) and CR-LF (DOS/Win) and set <span class="lang">instm::eol</span> property accordingly.</p><p>As for output streams, unlike many Windows-based C run-time libraries, PTypes does NOT make any translations and always sends your data as is. In order to generate an EOL sequence appropriate to the target platform (i.e. the system your application is running on), use <span class="lang">outstm::puteol()</span> and <span class="lang">outstm::putline(string)</span>. Some protocols and data formats, however, require some predetermined EOL sequence regardless of the target platform, e.g. HTTP always uses CR-LF, PostScript uses CR, etc. In such cases you can use standard C escape sequences to send appropriate codes to the output stream, e.g. "\r\n" or "\r".</p><p><b>File names</b>: you may always use Unix-style path names (with slash '/') when working with files through PTypes streaming classes. Since drive specifiers in path names are specific only to Windows, try to use relative paths in your program whenever possible. For additional notes on pipe naming, please see introduction for the <a href="streams.namedpipe.html">namedpipe</a> class.</p><p><b>Arithmetic operations and multithreading</b>: a simple increment and decrement operation (i++ and i--) in a multithreaded application may NOT be thread-safe on most RISC CPU's and on i386-based multiprocessor systems. Even if you are not interested in the resulting value of an increment/decrement operation, still use PTypes <span class="lang">pincrement()</span> and <span class="lang">pdecrement()</span> (in <pasync.h>) on variables shared between threads.</p><p><b>Performance of dynamic strings</b>: most of the string operations rely on dynamic memory reallocation. PTypes tries to avoid frequent use of physical reallocation by aligning the dynamic string buffers on 16, 32, 64 or 4096-byte boundaries, depending on the logical size of a string. Physical reallocation highly depends on the performance of the standard <span class="lang">realloc()</span> function, which may vary from OS to OS. Linux, for example, performs reallocations up to 30 times faster than Windows on a similar computer. PTypes dynamic strings also depend on the performance of the atomic increment and decrement functions (see below).</p><p><b>Performance of atomic increment/decrement/exchange operations</b>: on Intel-based platforms PTypes implements atomic operations through short assembly language code. On other platforms, however, these operations use slower mutex locking (see <span class="lang">src/patomic.cxx</span>). PTypes is in desperate need of developers who can write SMP-safe atomic increment, decrement and exchange for Sparc, PPC, Alpha and MIPS in assembly language. This would speed up dynamic strings and other parts of the library on these platforms.</p><p><b>Reentrancy of synchronization objects</b>: the POSIX standard does not specify whether synchronization objects should be reentrant or not. For instance, if you use recursive locks on a mutex object the behaviour of your application may vary on different systems. Most POSIX systems, such like Linux, provide a faster but non-reentrant implementation of synchronization primitives.</p><p><b>Read/write lock portability issues</b>: the <span class="lang">rwlock</span> class is built on top of the POSIX rwlock interface, wherever available, or uses the internal implementation (see <span class="lang">src/rwlock.cxx</span>). Various system-level implementations use different prioritization schemes: they may be 'fair' with respect to both reader and writer threads, or a higher priority may be given to one of them. Unfortunately the behaviour of rwlock is not standardized yet. PTypes' internal implementation uses the 'fair' scheme which we believe is the best and the only DoS-proof (denial-of-service) among other algorithms. On the other hand, replacing the system-level rwlock with our own implementation on all systems would lead to a preformance penalty compared to using 'native' objects. We set this issue aside for some time and we are open to discuss it with interested/experienced developers.</p><p class="seealso">See also: <a href="streams.outstm.html">outstm</a>, <a href="streams.namedpipe.html">namedpipe</a>, <a href="string.html">string</a>, <a href="async.utils.html">Multithreading Utilities</a></p><!-- #EndEditable --><hr size="1"><a href="../index.html" class="ns">PTypes home</a></body><!-- #EndTemplate --></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -