eth-summaries/semester3/spca/parts/01_c/01_basics/02_declarations.tex

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\subsubsection{Declarations}
We have already seen a few examples for how \texttt{C} handles declarations.
In concept they are similar (and scoping works the same) to most other \texttt{C}-like programming languages, including \texttt{Java}.

\inputcodewithfilename{c}{code-examples/00_c/00_basics/}{02_declarations.c}

\newpage
A peculiarity of \texttt{C} is that the bit-count is not defined by the language, but rather the hardware it is compiled for.
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\begin{fullTable}{llll}{\texttt{C} data type & typical 32-bit & ia32  & x86-64}{Comparison of byte-sizes for each datatype on different architectures}
    \texttt{char}               & 1              & 1     & 1       \\
    \texttt{short}              & 2              & 2     & 2       \\
    \texttt{int}                & 4              & 4     & 4       \\
    \texttt{long}               & 4              & 4     & 8       \\
    \texttt{long long}          & 8              & 8     & 8       \\
    \texttt{float}              & 4              & 4     & 4       \\
    \texttt{double}             & 4              & 8     & 8       \\
    \texttt{long double}        & 8              & 10/12 & 16      \\
\end{fullTable}

\drmvspace
\warn{Type format} Be however aware that this table uses the \texttt{LP64} format for the x86-64 sizes
and this is the format all UNIX-Systems use (i.e. Linux, BSD, Darwin (the Mac Kernel)).
64 bit Windows however uses \texttt{LLP64}, i.e. \texttt{int} and \texttt{long} have the same size (32) and \texttt{long long} and pointers are 64 bit.


\content{Integers} By default, integers in \lC\ are \texttt{signed}, to declare an unsigned integer, use \texttt{unsigned int}.
Since it is hard and annoying to remember the number of bytes that are in each data type, \texttt{C99} has introduced the extended integer types,
which can be imported from \texttt{stdint.h} and are of form \texttt{int<bit count>\_t} and \texttt{uint<bit count>\_t},
where we substitute the \texttt{<bit count>} with the number of bits (have to correspond to a valid type of course).


\content{Booleans} Another notable difference of \texttt{C} compared to other languages is that \texttt{C} doesn't natively have a \texttt{boolean} type,
by convention a \texttt{short} is used to represent it, where any non-zero value means \texttt{true} and \texttt{0} means \texttt{false}.
Since boolean types are quite handy, the \texttt{!} syntax for negation turns any non-zero value of any integer type into zero and vice-versa.
\texttt{C99} has added support for a bool type via \texttt{stdbool.h}, which however is still an integer.


\content{Implicit casts} Notably, \texttt{C} doesn't have a very rigid type system and lower bit-count types are implicitly cast to higher bit-count data types, i.e.
if you add a \texttt{short} and an \texttt{int}, the \texttt{short} is cast to \texttt{short} (bits 16-31 are set to $0$) and the two are added.
Explicit casting between almost all types is also supported.
Some will force a change of bit representation, but most won't (notably, when casting to and from \texttt{float}-like types, minus to \texttt{void})


\content{Expressions} Every \lC\ statement is also an expression, see above code block for example.


\content{Void} The \texttt{void} type has \bi{no} value and is used for untyped pointers and declaring functions with no return value


\content{Structs} Are like classes in OOP, but they contain no logic.
We can assign copy a struct by assignment and they behave just like everything else in \texttt{C} when used as an argument for functions
in that they are passed by value and not by reference.
You can of course pass it also by reference (like any other data type) by setting the argument to type \texttt{struct mystruct * name} and then calling the function using
\texttt{func(\&test)} assuming \texttt{test} is the name of your struct


\content{Typedef} To define a custom type using \texttt{typedef <type it represents> <name of the new type>}.

You may also use \texttt{typedef} on structs using \texttt{typedef struct <struct tag> <name of the new alias>},
you can thus instead of e.g. \verb|struct list_el my_list;| write \verb|list my_list;|, if you have used \verb|typedef struct list_el list;| before.
It is even possible to do this:
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\begin{code}{c}
    typedef struct list_el {
        unsigned long val;
        struct list_el *next;
    } list_el;

    struct list_el my_list;
    list_el my_other_list;
\end{code}
\rmvspace

\content{Namespaces}
\lC\ has a few different namespaces, i.e. you can have the one of the same name in each namespace (i.e. you can have \texttt{struct a}, \texttt{int a}, etc).
The following namespaces were covered:
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\begin{itemize}[noitemsep]
    \item Label names (used for \texttt{goto})
    \item Tags (for \texttt{struct}, \texttt{union} and \texttt{enum})
    \item Member names one namespace for each \texttt{struct}, \texttt{union} and \texttt{enum}
    \item Everything else mostly (types, variable names, etc, including typedef)
\end{itemize}