2002-02-17 21:56:45 +00:00
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<title>libsm Overview</title>
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<h1> libsm Overview </h1>
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2014-05-22 03:45:17 +00:00
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<br> $Id: index.html,v 1.14 2001-02-13 21:21:25 gshapiro Exp $
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2002-02-17 21:56:45 +00:00
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<h2> Introduction </h2>
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Libsm is a library of generally useful C abstractions.
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Libsm stands alone; it depends on no other sendmail libraries,
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and the only sendmail header files it depends on are its own,
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which reside in <tt>../include/sm</tt>.
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<h2> Contents </h2>
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Here is the current set of packages:
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<blockquote>
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<a href="gen.html"> gen: general definitions </a><br>
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<a href="debug.html"> debug: debugging and tracing </a><br>
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<a href="assert.html"> assert: assertion handling and aborts </a><br>
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<a href="heap.html"> heap: memory allocation </a><br>
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<a href="exc.html"> exc: exception handling </a><br>
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<a href="rpool.html"> rpool: resource pools </a><br>
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<a href="cdefs.html"> cdefs: C language portability macros </a><br>
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<a href="io.html"> io: buffered i/o </a><br>
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</blockquote>
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<h2> Naming Conventions </h2>
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Some of the symbols defined by libsm
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come from widely used defacto or dejure standards.
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Examples include <tt>size_t</tt> (from the C 89 standard),
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<tt>bool</tt> (from the C 99 standard),
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<tt>strerror</tt> (from Posix),
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and <tt>__P</tt> (from BSD and Linux).
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In these cases, we use the standard name rather than
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inventing a new name.
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We import the name from the appropriate header file when possible,
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or define it ourselves when necessary.
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When you are using one of these abstractions, you must include
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the appropriate libsm header file.
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For example, when you are using <tt>strerror</tt>, you must
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include <tt><sm/string.h></tt> instead of <tt><string.h></tt>.
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<p>
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When we aren't implementing a standard interface,
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we use a naming convention that attempts to maximize portability
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across platforms, and minimize conflicts with other libraries.
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Except for a few seemingly benign exceptions,
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all names begin with <tt>SM_</tt>, <tt>Sm</tt> or <tt>sm_</tt>.
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<p>
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The ISO C, Posix and Unix standards forbid applications
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from using names beginning with <tt>__</tt> or <tt>_[A-Z]</tt>,
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and place restrictions on what sorts of names can begin
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with <tt>_[a-z]</tt>. Such names are reserved for the compiler and
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the standard libraries.
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For this reason, we avoid defining any names that begin
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with <tt>_</tt>.
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For example, all libsm header file idempotency macros have the form
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<tt>SM_FOO_H</tt> (no leading <tt>_</tt>).
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<p>
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Type names begin with <tt>SM_</tt> and end with <tt>_T</tt>.
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Note that the Posix standard reserves all identifiers ending
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with <tt>_t</tt>.
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<p>
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All functions that are capable of raising an exception
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have names ending in <tt>_x</tt>, and developers are
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encouraged to use this convention when writing new code.
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This naming convention may seem unnecessary at first,
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but it becomes extremely useful during maintenance,
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when you are attempting to reason about the correctness
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of a block of code,
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and when you are trying to track down exception-related bugs
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in existing code.
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<h2> Coding Conventions </h2>
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The official style for function prototypes in libsm header files is
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<blockquote><pre>
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extern int
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foo __P((
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int _firstarg,
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int _secondarg));
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</pre></blockquote>
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The <tt>extern</tt> is useless, but required for stylistic reasons.
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The parameter names are optional; if present they are lowercase
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and begin with _ to avoid namespace conflicts.
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Each parameter is written on its own line to avoid very long lines.
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<p>
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For each structure <tt>struct sm_foo</tt> defined by libsm,
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there is a typedef:
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<blockquote><pre>
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typedef struct sm_foo SM_FOO_T;
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</pre></blockquote>
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and there is a global variable which is defined as follows:
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<blockquote><pre>
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const char SmFooMagic[] = "sm_foo";
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</pre></blockquote>
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The first member of each structure defined by libsm is
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<blockquote><pre>
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const char *sm_magic;
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</pre></blockquote>
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For all instances of <tt>struct sm_foo</tt>,
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<tt>sm_magic</tt> contains <tt>SmFooMagic</tt>,
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which points to a unique character string naming the type.
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It is used for debugging and run time type checking.
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<p>
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Each function with a parameter declared <tt>SM_FOO_T *foo</tt>
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contains the following assertion:
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<blockquote><pre>
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SM_REQUIRE_ISA(foo, SmFooMagic);
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</pre></blockquote>
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which is equivalent to
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<blockquote><pre>
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SM_REQUIRE(foo != NULL && foo->sm_magic == SmFooMagic);
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</pre></blockquote>
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When an object of type <tt>SM_FOO_T</tt> is deallocated,
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the member <tt>sm_magic</tt> is set to <tt>NULL</tt>.
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That will cause the above assertion to fail if a dangling pointer is used.
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<h2> Additional Design Goals </h2>
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Here are some of my design goals:
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<ul>
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<p>
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<li>The sm library is self contained; it does not depend on any other
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sendmail libraries or header files.
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<p>
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<li>The sm library must be compatible with shared libraries,
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even on platforms with weird implementation restrictions.
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I assume that a shared library can export global variables;
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the debug package relies on this assumption.
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I do not assume that if an application redefines a function defined
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in a shared library, the shared library will use the version of the
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function defined in the application in preference to the version
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that it defines.
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For this reason, I provide interfaces for registering handler functions
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in cases where an application might need to override standard behaviour.
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<p>
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<li>The sm library must be compatible with threads.
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The debug package presents a small problem: I don't want
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sm_debug_active to acquire and release a lock.
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So I assume that
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there exists an integral type <tt>SM_ATOMIC_INT_T</tt>
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(see <a href="gen.html"><tt><sm/gen.h></tt></a>)
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that can be accessed and updated atomically.
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I assume that locking must be used to guard updates and accesses to
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any other type, and I have designed the interfaces accordingly.
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</ul>
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</body>
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