| 1 | \chapter{Background} |
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| 2 | |
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| 3 | This chapter states facts about the prior work, upon which my contributions build. |
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| 4 | Each receives a justification of the extent to which its statement is phrased to provoke controversy or surprise. |
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| 5 | |
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| 6 | \section{C} |
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| 7 | |
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| 8 | \subsection{Common knowledge} |
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| 9 | |
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| 10 | The reader is assumed to have used C or \CC for the coursework of at least four university-level courses, or have equivalent experience. |
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| 11 | The current discussion introduces facts, unaware of which, such a functioning novice may be operating. |
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| 12 | |
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| 13 | % TODO: decide if I'm also claiming this collection of facts, and test-oriented presentation is a contribution; if so, deal with (not) arguing for its originality |
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| 14 | |
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| 15 | \subsection{Convention: C is more touchable than its standard} |
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| 16 | |
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| 17 | When it comes to explaining how C works, I like illustrating definite program semantics. |
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| 18 | I prefer doing so, over a quoting manual's suggested programmer's intuition, or showing how some compiler writers chose to model their problem. |
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| 19 | To illustrate definite program semantics, I devise a program, whose behaviour exercises the point at issue, and I show its behaviour. |
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| 20 | |
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| 21 | This behaviour is typically one of |
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| 22 | \begin{itemize} |
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| 23 | \item my statement that the compiler accepts or rejects the program |
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| 24 | \item the program's printed output, which I show |
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| 25 | \item my implied assurance that its assertions do not fail when run |
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| 26 | \end{itemize} |
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| 27 | |
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| 28 | The compiler whose program semantics is shown is |
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| 29 | \begin{lstlisting} |
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| 30 | $ gcc --version |
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| 31 | gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0 |
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| 32 | \end{lstlisting} |
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| 33 | running on Architecture @x86_64@, with the same environment targeted. |
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| 34 | |
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| 35 | Unless explicit discussion ensues about differences among compilers or with (versions of) the standard, it is further implied that there exists a second version of GCC and some version of Clang, running on and for the same platform, that give substantially similar behaviour. |
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| 36 | In this case, I do not argue that my sample of major Linux compilers is doing the right thing with respect to the C standard. |
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| 37 | |
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| 38 | |
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| 39 | \subsection{C reports many ill-typed expressions as warnings} |
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| 40 | |
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| 41 | TODO: typeset |
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| 42 | \lstinputlisting[language=C, firstline=13, lastline=56]{bkgd-c-tyerr.c} |
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| 43 | |
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| 44 | |
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| 45 | \section{C Arrays} |
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| 46 | |
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| 47 | \subsection{C has an array type (!)} |
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| 48 | |
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| 49 | TODO: typeset |
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| 50 | \lstinputlisting[language=C, firstline=35, lastline=116]{bkgd-carray-arrty.c} |
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| 51 | |
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| 52 | My contribution is enabled by recognizing |
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| 53 | \begin{itemize} |
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| 54 | \item There is value in using a type that knows how big the whole thing is. |
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| 55 | \item The type pointer to (first) element does not. |
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| 56 | \item C \emph{has} a type that knows the whole picture: array, e.g. @T[10]@. |
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| 57 | \item This type has all the usual derived forms, which also know the whole picture. A usefully noteworthy example is pointer to array, e.g. @T(*)[10]@. |
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| 58 | \end{itemize} |
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| 59 | |
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| 60 | Each of these sections, which introduces another layer of of the C arrays' story, |
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| 61 | concludes with an \emph{Unfortunate Syntactic Reference}. |
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| 62 | It shows how to spell the types under discussion, |
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| 63 | along with interactions with orthogonal (but easily confused) language features. |
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| 64 | Alterrnate spellings are listed withing a row. |
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| 65 | The simplest occurrences of types distinguished in the preceding discussion are marked with $\triangleright$. |
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| 66 | The Type column gives the spelling used in a cast or error message (though note Section TODO points out that some types cannot be casted to). |
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| 67 | The Declaration column gives the spelling used in an object declaration, such as variable or aggregate member; parameter declarations (section TODO) follow entirely different rules. |
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| 68 | |
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| 69 | After all, reading a C array type is easy: just read it from the inside out, and know when to look left and when to look right! |
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| 70 | |
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| 71 | |
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| 72 | \CFA-specific spellings (not yet introduced) are also included here for referenceability; these can be skipped on linear reading. |
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| 73 | The \CFA-C column gives the, more fortunate, ``new'' syntax of section TODO, for spelling \emph{exactly the same type}. |
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| 74 | This fortunate syntax does not have different spellings for types vs declarations; |
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| 75 | a declaration is always the type followed by the declared identifier name; |
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| 76 | for the example of letting @x@ be a \emph{pointer to array}, the declaration is spelled: |
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| 77 | \begin{lstlisting} |
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| 78 | [ * [10] T ] x; |
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| 79 | \end{lstlisting} |
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| 80 | The \CFA-Full column gives the spelling of a different type, introduced in TODO, which has all of my contributed improvements for safety and ergonomics. |
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| 81 | |
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| 82 | \noindent |
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| 83 | \textbf{Unfortunate Syntactic Reference} |
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| 84 | |
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| 85 | \noindent |
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| 86 | \begin{tabular}{llllll} |
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| 87 | & Description & Type & Declaration & \CFA-C & \CFA-Full \\ \hline |
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| 88 | $\triangleright$ & val. |
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| 89 | & @T@ |
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| 90 | & @T x;@ |
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| 91 | & @[ T ]@ |
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| 92 | & |
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| 93 | \\ \hline |
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| 94 | & \pbox{20cm}{ \vspace{2pt} val.\\ \footnotesize{no writing the val.\ in \lstinline{x}} }\vspace{2pt} |
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| 95 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T} \\ \lstinline{T const} } |
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| 96 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T x;} \\ \lstinline{T const x;} } |
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| 97 | & @[ const T ]@ |
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| 98 | & |
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| 99 | \\ \hline |
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| 100 | $\triangleright$ & ptr.\ to val. |
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| 101 | & @T *@ |
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| 102 | & @T * x;@ |
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| 103 | & @[ * T ]@ |
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| 104 | & |
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| 105 | \\ \hline |
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| 106 | & \pbox{20cm}{ \vspace{2pt} ptr.\ to val.\\ \footnotesize{no writing the ptr.\ in \lstinline{x}} }\vspace{2pt} |
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| 107 | & @T * const@ |
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| 108 | & @T * const x;@ |
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| 109 | & @[ const * T ]@ |
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| 110 | & |
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| 111 | \\ \hline |
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| 112 | & \pbox{20cm}{ \vspace{2pt} ptr.\ to val.\\ \footnotesize{no writing the val.\ in \lstinline{*x}} }\vspace{2pt} |
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| 113 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T *} \\ \lstinline{T const *} } |
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| 114 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T * x;} \\ \lstinline{T const * x;} } |
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| 115 | & @[ * const T ]@ |
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| 116 | & |
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| 117 | \\ \hline |
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| 118 | $\triangleright$ & ar.\ of val. |
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| 119 | & @T[10]@ |
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| 120 | & @T x[10];@ |
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| 121 | & @[ [10] T ]@ |
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| 122 | & @[ array(T, 10) ]@ |
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| 123 | \\ \hline |
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| 124 | & \pbox{20cm}{ \vspace{2pt} ar.\ of val.\\ \footnotesize{no writing the val.\ in \lstinline{x[5]}} }\vspace{2pt} |
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| 125 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T[10]} \\ \lstinline{T const[10]} } |
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| 126 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T x[10];} \\ \lstinline{T const x[10];} } |
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| 127 | & @[ [10] const T ]@ |
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| 128 | & @[ const array(T, 10) ]@ |
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| 129 | \\ \hline |
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| 130 | & ar.\ of ptr.\ to val. |
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| 131 | & @T*[10]@ |
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| 132 | & @T *x[10];@ |
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| 133 | & @[ [10] * T ]@ |
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| 134 | & @[ array(* T, 10) ]@ |
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| 135 | \\ \hline |
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| 136 | & \pbox{20cm}{ \vspace{2pt} ar.\ of ptr.\ to val.\\ \footnotesize{no writing the ptr.\ in \lstinline{x[5]}} }\vspace{2pt} |
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| 137 | & @T * const [10]@ |
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| 138 | & @T * const x[10];@ |
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| 139 | & @[ [10] const * T ]@ |
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| 140 | & @[ array(const * T, 10) ]@ |
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| 141 | \\ \hline |
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| 142 | & \pbox{20cm}{ \vspace{2pt} ar.\ of ptr.\ to val.\\ \footnotesize{no writing the val.\ in \lstinline{*(x[5])}} }\vspace{2pt} |
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| 143 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T * [10]} \\ \lstinline{T const * [10]} } |
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| 144 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T * x[10];} \\ \lstinline{T const * x[10];} } |
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| 145 | & @[ [10] * const T ]@ |
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| 146 | & @[ array(* const T, 10) ]@ |
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| 147 | \\ \hline |
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| 148 | $\triangleright$ & ptr.\ to ar.\ of val. |
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| 149 | & @T(*)[10]@ |
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| 150 | & @T (*x)[10];@ |
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| 151 | & @[ * [10] T ]@ |
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| 152 | & @[ * array(T, 10) ]@ |
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| 153 | \\ \hline |
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| 154 | & \pbox{20cm}{ \vspace{2pt} ptr.\ to ar.\ of val.\\ \footnotesize{no writing the ptr.\ in \lstinline{x}} }\vspace{2pt} |
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| 155 | & @T(* const)[10]@ |
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| 156 | & @T (* const x)[10];@ |
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| 157 | & @[ const * [10] T ]@ |
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| 158 | & @[ const * array(T, 10) ]@ |
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| 159 | \\ \hline |
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| 160 | & \pbox{20cm}{ \vspace{2pt} ptr.\ to ar.\ of val.\\ \footnotesize{no writing the val.\ in \lstinline{(*x)[5]}} }\vspace{2pt} |
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| 161 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T(*)[10]} \\ \lstinline{T const (*) [10]} } |
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| 162 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T (*x)[10];} \\ \lstinline{T const (*x)[10];} } |
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| 163 | & @[ * [10] const T ]@ |
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| 164 | & @[ * const array(T, 10) ]@ |
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| 165 | \\ \hline |
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| 166 | & ptr.\ to ar.\ of ptr.\ to val. |
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| 167 | & @T*(*)[10]@ |
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| 168 | & @T *(*x)[10];@ |
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| 169 | & @[ * [10] * T ]@ |
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| 170 | & @[ * array(* T, 10) ]@ |
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| 171 | \\ \hline |
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| 172 | \end{tabular} |
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| 173 | |
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| 174 | |
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| 175 | \subsection{Arrays decay and pointers diffract} |
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| 176 | |
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| 177 | TODO: typeset |
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| 178 | \lstinputlisting[language=C, firstline=4, lastline=26]{bkgd-carray-decay.c} |
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| 179 | |
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| 180 | |
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| 181 | So, C provides an implicit conversion from @float[10]@ to @float*@, as described in ARM-6.3.2.1.3: |
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| 182 | |
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| 183 | \begin{quote} |
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| 184 | Except when it is the operand of the @sizeof@ operator, or the unary @&@ operator, or is a |
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| 185 | string literal used to initialize an array |
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| 186 | an expression that has type ``array of type'' is |
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| 187 | converted to an expression with type ``pointer to type'' that points to the initial element of |
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| 188 | the array object |
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| 189 | \end{quote} |
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| 190 | |
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| 191 | This phenomenon is the famous ``pointer decay,'' which is a decay of an array-typed expression into a pointer-typed one. |
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| 192 | |
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| 193 | It is worthy to note that the list of exception cases does not feature the occurrence of @a@ in @a[i]@. |
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| 194 | Thus, subscripting happens on pointers, not arrays. |
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| 195 | |
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| 196 | Subscripting proceeds first with pointer decay, if needed. Next, ARM-6.5.2.1.2 explains that @a[i]@ is treated as if it were @(*((a)+(i)))@. |
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| 197 | ARM-6.5.6.8 explains that the addition, of a pointer with an integer type, is defined only when the pointer refers to an element that is in an array, with a meaning of ``@i@ elements away from,'' which is valid if @a@ is big enough and @i@ is small enough. |
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| 198 | Finally, ARM-6.5.3.2.4 explains that the @*@ operator's result is the referenced element. |
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| 199 | |
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| 200 | Taken together, these rules also happen to illustrate that @a[i]@ and @i[a]@ mean the same thing. |
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| 201 | |
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| 202 | Subscripting a pointer when the target is standard-inappropriate is still practically well-defined. |
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| 203 | While the standard affords a C compiler freedom about the meaning of an out-of-bound access, |
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| 204 | or of subscripting a pointer that does not refer to an array element at all, |
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| 205 | the fact that C is famously both generally high-performance, and specifically not bound-checked, |
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| 206 | leads to an expectation that the runtime handling is uniform across legal and illegal accesses. |
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| 207 | Moreover, consider the common pattern of subscripting on a malloc result: |
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| 208 | \begin{lstlisting} |
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| 209 | float * fs = malloc( 10 * sizeof(float) ); |
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| 210 | fs[5] = 3.14; |
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| 211 | \end{lstlisting} |
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| 212 | The @malloc@ behaviour is specified as returning a pointer to ``space for an object whose size is'' as requested (ARM-7.22.3.4.2). |
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| 213 | But program says \emph{nothing} more about this pointer value, that might cause its referent to \emph{be} an array, before doing the subscript. |
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| 214 | |
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| 215 | Under this assumption, a pointer being subscripted (or added to, then dereferenced) |
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| 216 | by any value (positive, zero, or negative), gives a view of the program's entire address space, |
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| 217 | centred around the @p@ address, divided into adjacent @sizeof(*p)@ chunks, |
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| 218 | each potentially (re)interpreted as @typeof(*p)@. |
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| 219 | |
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| 220 | I call this phenomenon ``array diffraction,'' which is a diffraction of a single-element pointer |
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| 221 | into the assumption that its target is in the middle of an array whose size is unlimited in both directions. |
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| 222 | |
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| 223 | No pointer is exempt from array diffraction. |
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| 224 | |
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| 225 | No array shows its elements without pointer decay. |
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| 226 | |
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| 227 | A further pointer--array confusion, closely related to decay, occurs in parameter declarations. |
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| 228 | ARM-6.7.6.3.7 explains that when an array type is written for a parameter, |
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| 229 | the parameter's type becomes a type that I summarize as being the array-decayed type. |
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| 230 | The respective handlings of the following two parameter spellings shows that the array-spelled one is really, like the other, a pointer. |
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| 231 | \lstinputlisting[language=C, firstline=40, lastline=44]{bkgd-carray-decay.c} |
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| 232 | As the @sizeof(x)@ meaning changed, compared with when run on a similarly-spelled local variariable declaration, |
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| 233 | GCC also gives this code the warning: ```sizeof' on array function parameter `x' will return size of `float *'.'' |
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| 234 | |
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| 235 | The caller of such a function is left with the reality that a pointer parameter is a pointer, no matter how it's spelled: |
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| 236 | \lstinputlisting[language=C, firstline=60, lastline=63]{bkgd-carray-decay.c} |
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| 237 | This fragment gives no warnings. |
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| 238 | |
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| 239 | The shortened parameter syntax @T x[]@ is a further way to spell ``pointer.'' |
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| 240 | Note the opposite meaning of this spelling now, compared with its use in local variable declarations. |
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| 241 | This point of confusion is illustrated in: |
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| 242 | \lstinputlisting[language=C, firstline=80, lastline=87]{bkgd-carray-decay.c} |
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| 243 | The basic two meanings, with a syntactic difference helping to distinguish, |
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| 244 | are illustrated in the declarations of @ca@ vs.\ @cp@, |
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| 245 | whose subsequent @edit@ calls behave differently. |
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| 246 | The syntax-caused confusion is in the comparison of the first and last lines, |
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| 247 | both of which use a literal to initialze an object decalared with spelling @T x[]@. |
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| 248 | But these initialized declarations get opposite meanings, |
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| 249 | depending on whether the object is a local variable or a parameter. |
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| 250 | |
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| 251 | |
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| 252 | In sumary, when a funciton is written with an array-typed parameter, |
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| 253 | \begin{itemize} |
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| 254 | \item an appearance of passing an array by value is always an incorrect understanding |
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| 255 | \item a dimension value, if any is present, is ignorred |
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| 256 | \item pointer decay is forced at the call site and the callee sees the parameter having the decayed type |
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| 257 | \end{itemize} |
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| 258 | |
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| 259 | Pointer decay does not affect pointer-to-array types, because these are already pointers, not arrays. |
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| 260 | As a result, a function with a pointer-to-array parameter sees the parameter exactly as the caller does: |
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| 261 | \lstinputlisting[language=C, firstline=100, lastline=110]{bkgd-carray-decay.c} |
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| 262 | |
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| 263 | |
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| 264 | \noindent |
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| 265 | \textbf{Unfortunate Syntactic Reference} |
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| 266 | |
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| 267 | \noindent |
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| 268 | (Parameter declaration; ``no writing'' refers to the callee's ability) |
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| 269 | |
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| 270 | \noindent |
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| 271 | \begin{tabular}{llllll} |
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| 272 | & Description & Type & Param. Decl & \CFA-C \\ \hline |
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| 273 | $\triangleright$ & ptr.\ to val. |
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| 274 | & @T *@ |
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| 275 | & \pbox{20cm}{ \vspace{2pt} \lstinline{T * x,} \\ \lstinline{T x[10],} \\ \lstinline{T x[],} }\vspace{2pt} |
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| 276 | & \pbox{20cm}{ \vspace{2pt} \lstinline{[ * T ]} \\ \lstinline{[ [10] T ]} \\ \lstinline{[ [] T ]} } |
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| 277 | \\ \hline |
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| 278 | & \pbox{20cm}{ \vspace{2pt} ptr.\ to val.\\ \footnotesize{no writing the ptr.\ in \lstinline{x}} }\vspace{2pt} |
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| 279 | & @T * const@ |
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| 280 | & \pbox{20cm}{ \vspace{2pt} \lstinline{T * const x,} \\ \lstinline{T x[const 10],} \\ \lstinline{T x[const],} }\vspace{2pt} |
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| 281 | & \pbox{20cm}{ \vspace{2pt} \lstinline{[ const * T ]} \\ \lstinline{[ [const 10] T ]} \\ \lstinline{[ [const] T ]} } |
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| 282 | \\ \hline |
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| 283 | & \pbox{20cm}{ \vspace{2pt} ptr.\ to val.\\ \footnotesize{no writing the val.\ in \lstinline{*x}} }\vspace{2pt} |
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| 284 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T *} \\ \lstinline{T const *} } |
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| 285 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const T * x,} \\ \lstinline{T const * x,} \\ \lstinline{const T x[10],} \\ \lstinline{T const x[10],} \\ \lstinline{const T x[],} \\ \lstinline{T const x[],} }\vspace{2pt} |
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| 286 | & \pbox{20cm}{ \vspace{2pt} \lstinline{[* const T]} \\ \lstinline{[ [10] const T ]} \\ \lstinline{[ [] const T ]} } |
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| 287 | \\ \hline |
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| 288 | $\triangleright$ & ptr.\ to ar.\ of val. |
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| 289 | & @T(*)[10]@ |
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| 290 | & \pbox{20cm}{ \vspace{2pt} \lstinline{T (*x)[10],} \\ \lstinline{T x[3][10],} \\ \lstinline{T x[][10],} }\vspace{2pt} |
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| 291 | & \pbox{20cm}{ \vspace{2pt} \lstinline{[* [10] T]} \\ \lstinline{[ [3] [10] T ]} \\ \lstinline{[ [] [10] T ]} } |
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| 292 | \\ \hline |
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| 293 | & ptr.\ to ptr.\ to val. |
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| 294 | & @T **@ |
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| 295 | & \pbox{20cm}{ \vspace{2pt} \lstinline{T ** x,} \\ \lstinline{T *x[10],} \\ \lstinline{T *x[],} }\vspace{2pt} |
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| 296 | & \pbox{20cm}{ \vspace{2pt} \lstinline{[ * * T ]} \\ \lstinline{[ [10] * T ]} \\ \lstinline{[ [] * T ]} } |
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| 297 | \\ \hline |
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| 298 | & \pbox{20cm}{ \vspace{2pt} ptr.\ to ptr.\ to val.\\ \footnotesize{no writing the val.\ in \lstinline{**argv}} }\vspace{2pt} |
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| 299 | & @const char **@ |
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| 300 | & \pbox{20cm}{ \vspace{2pt} \lstinline{const char *argv[],} \\ \footnotesize{(others elided)} }\vspace{2pt} |
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| 301 | & \pbox{20cm}{ \vspace{2pt} \lstinline{[ [] * const char ]} \\ \footnotesize{(others elided)} } |
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| 302 | \\ \hline |
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| 303 | \end{tabular} |
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| 304 | |
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| 305 | |
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| 306 | |
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| 307 | \subsection{Lengths may vary, checking does not} |
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| 308 | |
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| 309 | When the desired number of elements is unknown at compile time, |
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| 310 | a variable-length array is a solution: |
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| 311 | \begin{lstlisting} |
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| 312 | int main( int argc, const char *argv[] ) { |
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| 313 | |
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| 314 | assert( argc == 2 ); |
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| 315 | size_t n = atol( argv[1] ); |
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| 316 | assert( 0 < n && n < 1000 ); |
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| 317 | |
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| 318 | float a[n]; |
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| 319 | float b[10]; |
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| 320 | |
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| 321 | // ... discussion continues here |
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| 322 | } |
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| 323 | \end{lstlisting} |
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| 324 | This arrangement allocates @n@ elements on the @main@ stack frame for @a@, |
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| 325 | just as it puts 10 elements on the @main@ stack frame for @b@. |
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| 326 | The variable-sized allocation of @a@ is provided by @alloca@. |
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| 327 | |
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| 328 | In a situation where the array sizes are not known to be small enough |
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| 329 | for stack allocation to be sensible, |
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| 330 | corresponding heap allocations are achievable as: |
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| 331 | \begin{lstlisting} |
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| 332 | float *ax1 = malloc( sizeof( float[n] ) ); |
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| 333 | float *ax2 = malloc( n * sizeof( float ) ); |
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| 334 | float *bx1 = malloc( sizeof( float[1000000] ) ); |
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| 335 | float *bx2 = malloc( 1000000 * sizeof( float ) ); |
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| 336 | \end{lstlisting} |
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| 337 | |
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| 338 | |
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| 339 | VLA |
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| 340 | |
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| 341 | Parameter dependency |
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| 342 | |
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| 343 | Checking is best-effort / unsound |
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| 344 | |
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| 345 | Limited special handling to get the dimension value checked (static) |
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| 346 | |
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| 347 | |
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| 348 | |
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| 349 | \subsection{C has full-service, dynamically sized, multidimensional arrays (and \CC does not)} |
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| 350 | |
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| 351 | In C and \CC, ``multidimensional array'' means ``array of arrays.'' Other meanings are discussed in TODO. |
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| 352 | |
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| 353 | Just as an array's element type can be @float@, so can it be @float[10]@. |
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| 354 | |
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| 355 | While any of @float*@, @float[10]@ and @float(*)[10]@ are easy to tell apart from @float@, |
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| 356 | telling them apart from each other may need occasional reference back to TODO intro section. |
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| 357 | The sentence derived by wrapping each type in @-[3]@ follows. |
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| 358 | |
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| 359 | While any of @float*[3]@, @float[3][10]@ and @float(*)[3][10]@ are easy to tell apart from @float[3]@, |
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| 360 | telling them apart from each other is what it takes to know what ``array of arrays'' really means. |
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| 361 | |
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| 362 | |
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| 363 | Pointer decay affects the outermost array only |
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| 364 | |
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| 365 | |
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| 366 | TODO: unfortunate syntactic reference with these cases: |
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| 367 | |
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| 368 | \begin{itemize} |
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| 369 | \item ar. of ar. of val (be sure about ordering of dimensions when the declaration is dropped) |
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| 370 | \item ptr. to ar. of ar. of val |
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| 371 | \end{itemize} |
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| 372 | |
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| 373 | |
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| 374 | |
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| 375 | |
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| 376 | |
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| 377 | \subsection{Arrays are (but) almost values} |
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| 378 | |
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| 379 | Has size; can point to |
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| 380 | |
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| 381 | Can't cast to |
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| 382 | |
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| 383 | Can't pass as value |
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| 384 | |
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| 385 | Can initialize |
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| 386 | |
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| 387 | Can wrap in aggregate |
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| 388 | |
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| 389 | Can't assign |
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| 390 | |
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| 391 | |
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| 392 | \subsection{Returning an array is (but) almost possible} |
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| 393 | |
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| 394 | |
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| 395 | |
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| 396 | |
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| 397 | \subsection{The pointer-to-array type has been noticed before} |
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| 398 | |
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| 399 | |
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| 400 | \section{\CFA} |
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| 401 | |
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| 402 | Traditionally, fixing C meant leaving the C-ism alone, while providing a better alternative beside it. |
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| 403 | (For later: That's what I offer with array.hfa, but in the future-work vision for arrays, the fix includes helping programmers stop accidentally using a broken C-ism.) |
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| 404 | |
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| 405 | \subsection{\CFA features interacting with arrays} |
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| 406 | |
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| 407 | Prior work on \CFA included making C arrays, as used in C code from the wild, |
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| 408 | work, if this code is fed into @cfacc@. |
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| 409 | The quality of this this treatment was fine, with no more or fewer bugs than is typical. |
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| 410 | |
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| 411 | More mixed results arose with feeding these ``C'' arrays into preexisting \CFA features. |
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| 412 | |
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| 413 | A notable success was with the \CFA @alloc@ function, |
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| 414 | which type information associated with a polymorphic return type |
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| 415 | replaces @malloc@'s use of programmer-supplied size information. |
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| 416 | \begin{lstlisting} |
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| 417 | // C, library |
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| 418 | void * malloc( size_t ); |
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| 419 | // C, user |
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| 420 | struct tm * el1 = malloc( sizeof(struct tm) ); |
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| 421 | struct tm * ar1 = malloc( 10 * sizeof(struct tm) ); |
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| 422 | |
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| 423 | // CFA, library |
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| 424 | forall( T * ) T * alloc(); |
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| 425 | // CFA, user |
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| 426 | tm * el2 = alloc(); |
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| 427 | tm (*ar2)[10] = alloc(); |
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| 428 | \end{lstlisting} |
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| 429 | The alloc polymorphic return compiles into a hidden parameter, which receives a compiler-generated argument. |
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| 430 | This compiler's argument generation uses type information from the left-hand side of the initialization to obtain the intended type. |
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| 431 | Using a compiler-produced value eliminates an opportunity for user error. |
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| 432 | |
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| 433 | TODO: fix in following: even the alloc call gives bad code gen: verify it was always this way; walk back the wording about things just working here; assignment (rebind) seems to offer workaround, as in bkgd-cfa-arrayinteract.cfa |
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| 434 | |
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| 435 | Bringing in another \CFA feature, reference types, both resolves a sore spot of the last example, and gives a first example of an array-interaction bug. |
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| 436 | In the last example, the choice of ``pointer to array'' @ar2@ breaks a parallel with @ar1@. |
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| 437 | They are not subscripted in the same way. |
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| 438 | \begin{lstlisting} |
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| 439 | ar1[5]; |
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| 440 | (*ar2)[5]; |
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| 441 | \end{lstlisting} |
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| 442 | Using ``reference to array'' works at resolving this issue. TODO: discuss connection with Doug-Lea \CC proposal. |
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| 443 | \begin{lstlisting} |
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| 444 | tm (&ar3)[10] = *alloc(); |
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| 445 | ar3[5]; |
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| 446 | \end{lstlisting} |
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| 447 | The implicit size communication to @alloc@ still works in the same ways as for @ar2@. |
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| 448 | |
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| 449 | Using proper array types (@ar2@ and @ar3@) addresses a concern about using raw element pointers (@ar1@), albeit a theoretical one. |
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| 450 | TODO xref C standard does not claim that @ar1@ may be subscripted, |
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| 451 | because no stage of interpreting the construction of @ar1@ has it be that ``there is an \emph{array object} here.'' |
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| 452 | But both @*ar2@ and the referent of @ar3@ are the results of \emph{typed} @alloc@ calls, |
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| 453 | where the type requested is an array, making the result, much more obviously, an array object. |
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| 454 | |
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| 455 | The ``reference to array'' type has its sore spots too. TODO see also @dimexpr-match-c/REFPARAM_CALL (under TRY_BUG_1)@ |
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| 456 | |
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| 457 | |
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| 458 | |
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| 459 | TODO: I fixed a bug associated with using an array as a T. I think. Did I really? What was the bug? |
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