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diff --git a/share/doc/gcc/Incompatibilities.html b/share/doc/gcc/Incompatibilities.html new file mode 100644 index 0000000..fea6a2f --- /dev/null +++ b/share/doc/gcc/Incompatibilities.html @@ -0,0 +1,314 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. 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If several +identical-looking string constants are used, GCC stores only one +copy of the string. + +<a name="index-mktemp_002c-and-constant-strings"></a> +<p>One consequence is that you cannot call <code>mktemp</code> with a string +constant argument. The function <code>mktemp</code> always alters the +string its argument points to. +</p> +<a name="index-sscanf_002c-and-constant-strings"></a> +<a name="index-fscanf_002c-and-constant-strings"></a> +<a name="index-scanf_002c-and-constant-strings"></a> +<p>Another consequence is that <code>sscanf</code> does not work on some very +old systems when passed a string constant as its format control string +or input. This is because <code>sscanf</code> incorrectly tries to write +into the string constant. Likewise <code>fscanf</code> and <code>scanf</code>. +</p> +<p>The solution to these problems is to change the program to use +<code>char</code>-array variables with initialization strings for these +purposes instead of string constants. +</p> +</li><li> <code>-2147483648</code> is positive. + +<p>This is because 2147483648 cannot fit in the type <code>int</code>, so +(following the ISO C rules) its data type is <code>unsigned long int</code>. +Negating this value yields 2147483648 again. +</p> +</li><li> GCC does not substitute macro arguments when they appear inside of +string constants. For example, the following macro in GCC + +<div class="smallexample"> +<pre class="smallexample">#define foo(a) "a" +</pre></div> + +<p>will produce output <code>"a"</code> regardless of what the argument <var>a</var> is. +</p> +</li><li> <a name="index-setjmp-incompatibilities"></a> +<a name="index-longjmp-incompatibilities"></a> +When you use <code>setjmp</code> and <code>longjmp</code>, the only automatic +variables guaranteed to remain valid are those declared +<code>volatile</code>. This is a consequence of automatic register +allocation. Consider this function: + +<div class="smallexample"> +<pre class="smallexample">jmp_buf j; + +foo () +{ + int a, b; + + a = fun1 (); + if (setjmp (j)) + return a; + + a = fun2 (); + /* <span class="roman"><code>longjmp (j)</code> may occur in <code>fun3</code>.</span> */ + return a + fun3 (); +} +</pre></div> + +<p>Here <code>a</code> may or may not be restored to its first value when the +<code>longjmp</code> occurs. If <code>a</code> is allocated in a register, then +its first value is restored; otherwise, it keeps the last value stored +in it. +</p> +<a name="index-W-3"></a> +<p>If you use the <samp>-W</samp> option with the <samp>-O</samp> option, you will +get a warning when GCC thinks such a problem might be possible. +</p> +</li><li> Programs that use preprocessing directives in the middle of macro +arguments do not work with GCC. For example, a program like this +will not work: + +<div class="smallexample"> +<pre class="smallexample">foobar ( +#define luser + hack) +</pre></div> + +<p>ISO C does not permit such a construct. +</p> +</li><li> K&R compilers allow comments to cross over an inclusion boundary +(i.e. started in an include file and ended in the including file). + +</li><li> <a name="index-external-declaration-scope"></a> +<a name="index-scope-of-external-declarations"></a> +<a name="index-declaration-scope"></a> +Declarations of external variables and functions within a block apply +only to the block containing the declaration. In other words, they +have the same scope as any other declaration in the same place. + +<p>In some other C compilers, an <code>extern</code> declaration affects all the +rest of the file even if it happens within a block. +</p> +</li><li> In traditional C, you can combine <code>long</code>, etc., with a typedef name, +as shown here: + +<div class="smallexample"> +<pre class="smallexample">typedef int foo; +typedef long foo bar; +</pre></div> + +<p>In ISO C, this is not allowed: <code>long</code> and other type modifiers +require an explicit <code>int</code>. +</p> +</li><li> <a name="index-typedef-names-as-function-parameters"></a> +PCC allows typedef names to be used as function parameters. + +</li><li> Traditional C allows the following erroneous pair of declarations to +appear together in a given scope: + +<div class="smallexample"> +<pre class="smallexample">typedef int foo; +typedef foo foo; +</pre></div> + +</li><li> GCC treats all characters of identifiers as significant. According to +K&R-1 (2.2), “No more than the first eight characters are significant, +although more may be used.”. Also according to K&R-1 (2.2), “An +identifier is a sequence of letters and digits; the first character must +be a letter. The underscore _ counts as a letter.”, but GCC also +allows dollar signs in identifiers. + +</li><li> <a name="index-whitespace"></a> +PCC allows whitespace in the middle of compound assignment operators +such as ‘<samp>+=</samp>’. GCC, following the ISO standard, does not +allow this. + +</li><li> <a name="index-apostrophes"></a> +<a name="index-_0027"></a> +GCC complains about unterminated character constants inside of +preprocessing conditionals that fail. Some programs have English +comments enclosed in conditionals that are guaranteed to fail; if these +comments contain apostrophes, GCC will probably report an error. For +example, this code would produce an error: + +<div class="smallexample"> +<pre class="smallexample">#if 0 +You can't expect this to work. +#endif +</pre></div> + +<p>The best solution to such a problem is to put the text into an actual +C comment delimited by ‘<samp>/*…*/</samp>’. +</p> +</li><li> Many user programs contain the declaration ‘<samp>long time ();</samp>’. In the +past, the system header files on many systems did not actually declare +<code>time</code>, so it did not matter what type your program declared it to +return. But in systems with ISO C headers, <code>time</code> is declared to +return <code>time_t</code>, and if that is not the same as <code>long</code>, then +‘<samp>long time ();</samp>’ is erroneous. + +<p>The solution is to change your program to use appropriate system headers +(<code><time.h></code> on systems with ISO C headers) and not to declare +<code>time</code> if the system header files declare it, or failing that to +use <code>time_t</code> as the return type of <code>time</code>. +</p> +</li><li> <a name="index-float-as-function-value-type"></a> +When compiling functions that return <code>float</code>, PCC converts it to +a double. GCC actually returns a <code>float</code>. If you are concerned +with PCC compatibility, you should declare your functions to return +<code>double</code>; you might as well say what you mean. + +</li><li> <a name="index-structures"></a> +<a name="index-unions"></a> +When compiling functions that return structures or unions, GCC +output code normally uses a method different from that used on most +versions of Unix. As a result, code compiled with GCC cannot call +a structure-returning function compiled with PCC, and vice versa. + +<p>The method used by GCC is as follows: a structure or union which is +1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union +with any other size is stored into an address supplied by the caller +(usually in a special, fixed register, but on some machines it is passed +on the stack). The target hook <code>TARGET_STRUCT_VALUE_RTX</code> +tells GCC where to pass this address. +</p> +<p>By contrast, PCC on most target machines returns structures and unions +of any size by copying the data into an area of static storage, and then +returning the address of that storage as if it were a pointer value. +The caller must copy the data from that memory area to the place where +the value is wanted. GCC does not use this method because it is +slower and nonreentrant. +</p> +<p>On some newer machines, PCC uses a reentrant convention for all +structure and union returning. GCC on most of these machines uses a +compatible convention when returning structures and unions in memory, +but still returns small structures and unions in registers. +</p> +<a name="index-fpcc_002dstruct_002dreturn-1"></a> +<p>You can tell GCC to use a compatible convention for all structure and +union returning with the option <samp>-fpcc-struct-return</samp>. +</p> +</li><li> <a name="index-preprocessing-tokens"></a> +<a name="index-preprocessing-numbers"></a> +GCC complains about program fragments such as ‘<samp>0x74ae-0x4000</samp>’ +which appear to be two hexadecimal constants separated by the minus +operator. Actually, this string is a single <em>preprocessing token</em>. +Each such token must correspond to one token in C. Since this does not, +GCC prints an error message. Although it may appear obvious that what +is meant is an operator and two values, the ISO C standard specifically +requires that this be treated as erroneous. + +<p>A <em>preprocessing token</em> is a <em>preprocessing number</em> if it +begins with a digit and is followed by letters, underscores, digits, +periods and ‘<samp>e+</samp>’, ‘<samp>e-</samp>’, ‘<samp>E+</samp>’, ‘<samp>E-</samp>’, ‘<samp>p+</samp>’, +‘<samp>p-</samp>’, ‘<samp>P+</samp>’, or ‘<samp>P-</samp>’ character sequences. (In strict C90 +mode, the sequences ‘<samp>p+</samp>’, ‘<samp>p-</samp>’, ‘<samp>P+</samp>’ and ‘<samp>P-</samp>’ cannot +appear in preprocessing numbers.) +</p> +<p>To make the above program fragment valid, place whitespace in front of +the minus sign. This whitespace will end the preprocessing number. +</p></li></ul> + +<hr> +<div class="header"> +<p> +Next: <a href="Fixed-Headers.html#Fixed-Headers" accesskey="n" rel="next">Fixed Headers</a>, Previous: <a href="Interoperation.html#Interoperation" accesskey="p" rel="previous">Interoperation</a>, Up: <a href="Trouble.html#Trouble" accesskey="u" rel="up">Trouble</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |