127 lines
4.5 KiB
HTML
127 lines
4.5 KiB
HTML
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<html>
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<head>
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<title>stdarg</title>
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<link rel="stylesheet" type="text/css" media="all" href="../man.css">
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</head>
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<body bgcolor=#ffffff>
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<h2 align=center>stdarg</h2>
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<h4 align=center>OS/161 Reference Manual</h4>
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<h3>Name</h3>
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<p>
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stdarg - handle functions with variable arguments
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</p>
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<h3>Library</h3>
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<p>
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Standard C Library (libc, -lc)
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</p>
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<h3>Synopsis</h3>
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<p>
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<tt>#include <stdarg.h></tt><br>
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<br>
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<tt>va_start(va_list </tt><em>ap</em><tt>,
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</tt><em>start-argument</em><tt>);</tt><br>
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<br>
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<tt>va_end(va_list </tt><em>ap</em><tt>);</tt><br>
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<br>
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<em>type</em><br>
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<tt>va_arg(va_list </tt><em>ap</em><tt>, </tt><em>type</em><tt>);</tt><br>
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<br>
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<tt>va_copy(va_list </tt><em>dest</em><tt>,
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va_list </tt><em>src</em><tt>);</tt><br>
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</p>
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<h3>Description</h3>
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<p>
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Functions where the number of arguments is not fixed at compile time
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can be written using the stdarg facility. This provides a type,
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<tt>va_list</tt>, and the macros listed above. These allow iterating
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through the arguments.
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</p>
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<p>
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<tt>va_start</tt> initializes a <tt>va_list</tt> <em>ap</em> to point
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to the current function's arguments. The <em>start-argument</em>
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argument should be the name of the last fixed parameter in the calling
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sequence.
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(There must be at least one fixed parameter.)
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</p>
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<p>
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<tt>va_end</tt> cleans up a <tt>va_list</tt> once it is no longer
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needed. While failure to use <tt>va_end</tt> may have no effect on
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some architectures (in fact, in some cases <tt>va_end</tt> does
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nothing at all) on other architectures it may be fatal.
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</p>
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<p>
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<tt>va_arg</tt> retrieves the next argument, which is presumed to be
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of type <em>type</em>. The function must have some way to determine
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what types to expect, and how many arguments, as this information
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cannot be extracted from the argument list itself. To rewind, use
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<tt>va_end</tt> and then <tt>va_start</tt> again.
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</p>
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<p>
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Remember that default C argument promotions occur when passing the
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variable arguments. There is no run-time checking of any kind, and
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little to no compile-time checking: if you use <tt>va_arg</tt> to
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retrieve a type different from that which was passed, you will
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silently get garbage for that and (usually) all subsequent arguments.
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</p>
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<p>
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<tt>va_copy</tt> assigns a copy of <em>src</em> to
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<em>dest</em>. Subsequent operations on either will not affect the
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other. Both copies need to be cleaned up with <tt>va_end</tt>.
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</p>
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<h3>Restrictions</h3>
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<p>
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Because the <tt>va_list</tt> is not necessarily a simple type, but may
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involve pointers to state maintained elsewhere, it is not necessarily
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a simple value. Thus, assigning <tt>va_list</tt> objects to each other
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with `=', memcpy, or the like, or passing them to functions, may not
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give multiple independent objects. When in doubt, use
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<tt>va_copy</tt>, or invoke <tt>va_start</tt> multiple times.
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</p>
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<h3>Return Values</h3>
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<p>
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<tt>va_start</tt>, <tt>va_end</tt>, and <tt>va_copy</tt> do not return
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anything. <tt>va_arg</tt> returns the value of the requested argument.
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</p>
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</body>
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</html>
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