?? chapter 4 variables and constants -- valvano.htm
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block. In C local variables must be defined at the beginning of a block. The
following example is proper C++ code, but results in a syntax error in
C.</FONT></P>
<DIR>
<P><CODE>void sub(void){ int x; /* a valid local variable declaration
*/<BR> x=1;<BR> int
y; /* This declaration is improper
*/<BR> y=2;<BR>}</CODE></P></DIR>
<ADDRESS>Listing 4-13: Example showing an illegal local variable
declaration</ADDRESS>
<P> </P>
<P><B><I><FONT face=Helvetica,Arial><A
name=DECLARATIONS></A>Declarations</FONT></I></B></P>
<P><FONT face="Times New Roman,Times">Unlike BASIC and FORTRAN, which will
automatically declare variables when they are first used, every variable in C
must be declared first. This may seem unnecessary, but when we consider how much
time is spent debugging BASIC and FORTRAN programs simply because misspelled
variable names are not caught for us, it becomes obvious that the time spent
declaring variables beforehand is time well spent. Declarations also force us to
consider the precision (8-bit, 16-bit etc.) and format (unsigned vs. signed) of
each variable.</FONT></P>
<P><FONT face="Times New Roman,Times">As we saw in <A
href="http://www.ece.utexas.edu/~valvano/embed/chap1/chap1.htm">Chapter 1</A>,
describing a variable involves two actions. The first action is declaring its
type and the second action is defining it in memory (reserving a place for it).
Although both of these may be involved, we refer to the C construct that
accomplishes them as a <I>declaration</I>. As we saw above, if the declaration
is preceded by <B>extern</B> it only declares the type of the variables, without
reserving space for them. In such cases, the definition must exist in another
source file. Failure to do so, will result in an unresolved reference error at
link time.</FONT></P>
<P><FONT face="Times New Roman,Times">Table 4-1 contains examples of legitimate
variable declarations. Notice that the declarations are introduced by one or
type keywords that states the data type of the variables listed. The keyword
<B>char</B> declares 8-bit values, <B>int</B> declares 16-bit values,
<B>short</B> declares 16-bit values and <B>long</B> declares 32-bit values.
Unless the modifier <B>unsigned</B> is present, the variables declared by these
statements are assumed by the compiler to contain signed values. You could add
the keyword <B>signed</B> before the data type to clarify its type.</FONT></P>
<P><FONT face="Times New Roman,Times">When more than one variable is being
declared, they are written as a list with the individual names separated by
commas. Each declaration is terminated with a semicolon as are all simple C
statements.</FONT></P>
<P><FONT face="Times New Roman,Times">
<TABLE bgColor=white border=0 cellPadding=0 cellSpacing=0 width=511>
<TBODY>
<TR>
<TD><FONT face="Times New Roman,Times" size=2>Declaration</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>Comment</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>Range</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">unsigned char
uc;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>8-bit unsigned
number</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>0 to +255</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">char
c1,c2,c3;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>three 8-bit signed
numbers</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>-128 to +127</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">unsigned int
ui;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>16-bit unsigned
number</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>0 to +65535</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">int i1,i2;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>two 16-bit signed
numbers</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>-32768 to
+32767</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">unsigned short
us;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>16-bit unsigned
number</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>0 to +65535</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">short s1,s2;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>two 16-bit signed
numbers</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>-32768 to
+32767</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">long
l1,l2,l3,l4;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>four signed 32 bit
integers</FONT></TD>
<TD><FONT face="Times New Roman,Times" size=2>-2147483648L to
2147483647L</FONT></TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">float f1,f2;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>two 32-bit floating
numbers</FONT></TD>
<TD>not recommended</TD></TR>
<TR>
<TD><CODE><FONT face="Times New Roman,Times">double
d1,d2;</FONT></CODE></TD>
<TD><FONT face="Times New Roman,Times" size=2>two 64-bit floating
numbers</FONT></TD>
<TD>not recommended</TD></TR></TBODY></TABLE></FONT></P>
<ADDRESS>Table 4-1: Variable Declarations</ADDRESS>
<ADDRESS> </ADDRESS>
<P><FONT face="Times New Roman,Times">ICC11 version 4 does not support long
integers, and ICC12 does not support unsigned long integers. ICC11 and ICC12
compilers allow the <B>register</B> modifier for automatic variables, but the
compilers still define the register locals on the stack. The keywords <B>char
int short long</B> specifies the precision of the variable. The following tables
shows the available modifiers for variables.</FONT></P>
<P>
<TABLE border=0 cellSpacing=0 width=521>
<TBODY>
<TR>
<TD bgColor=white vAlign=center width="16%"><FONT
face="Times New Roman,Times">Modifier</FONT></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">Comment</FONT></TD></TR>
<TR>
<TD bgColor=white vAlign=center width="16%"><CODE>auto</CODE></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">automatic, allocated on the
stack</FONT></TD></TR>
<TR>
<TD bgColor=white vAlign=center width="16%"><CODE>extern</CODE></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">defined in some other program</FONT>
file</TD></TR>
<TR>
<TD bgColor=white vAlign=center width="16%"><CODE>static</CODE></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">permanently allocated </FONT></TD></TR>
<TR>
<TD bgColor=white width="16%"><CODE>register</CODE></TD>
<TD bgColor=white width="84%">attempt to implement an automatic using a
register instead of on the stack</TD></TR></TBODY></TABLE></P>
<ADDRESS>Table 4-2: Variable storage classes</ADDRESS>
<ADDRESS> </ADDRESS>
<P>
<TABLE border=0 cellSpacing=0 width=521>
<TBODY>
<TR>
<TD bgColor=white vAlign=center width="16%"><FONT
face="Times New Roman,Times">Modifier</FONT></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">Comment</FONT></TD></TR>
<TR>
<TD bgColor=white vAlign=center width="16%"><CODE>volatile</CODE></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">can change value by means other than the
current program</FONT></TD></TR>
<TR>
<TD bgColor=white vAlign=center width="16%"><CODE>const</CODE></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">fixed value, defined in the source code and
can not be changed during execution</FONT></TD></TR>
<TR>
<TD bgColor=white vAlign=center width="16%"><CODE>unsigned</CODE></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">range starts with 0 includes only positive
values</FONT></TD></TR>
<TR>
<TD bgColor=white vAlign=center width="16%"><CODE>signed</CODE></TD>
<TD bgColor=white vAlign=center width="84%"><FONT
face="Times New Roman,Times">range includes both negative and positive
values</FONT></TD></TR></TBODY></TABLE></P>
<ADDRESS>Table 4-3 Variable modifiers</ADDRESS>
<P> </P>
<P><FONT face="Times New Roman,Times">As we shall see, a similar syntax is used
to declare pointers, arrays, and functions (<A
href="http://www.ece.utexas.edu/~valvano/embed/chap7/chap7.htm">Chapters 7</A>,
<A href="http://www.ece.utexas.edu/~valvano/embed/chap8/chap8.htm">8</A>, and <A
href="http://www.ece.utexas.edu/~valvano/embed/chap10/chap10.htm">10</A>).</FONT></P>
<P><B><I><FONT face=Helvetica,Arial><A name=CHARACTER></A>Character
Variables</FONT></I></B></P>
<P><FONT face="Times New Roman,Times">Character variables are stored as 8-bit
quantities. When they are fetched from memory, they are always promoted
automatically to 16-bit integers. Unsigned 8-bit values are promoted by adding 8
zeros into the most significant bits. Signed values are promoted by coping the
sign bit (bit7) into the 8 most significant bits.</FONT></P>
<P><FONT face="Times New Roman,Times">There is a confusion when signed and
unsigned variables are mixed into the same expression. It is good programming
practice to avoid such confusions. As with integers, when a signed character
enters into an operation with an unsigned quantity, the character is interpreted
as though it was unsigned. The result of such operations is also unsigned. When
a signed character joins with another signed quantity, the result is also
signed.</FONT></P>
<DIR>
<P><CODE>char x; /* signed 8 bit global */<BR>unsigned short
y; /* unsigned signed 16 bit global */<BR>void
sub(void){<BR> y=y+x;<BR>/* x treated as unsigned even
though defined as signed */<BR>}</CODE></P></DIR>
<ADDRESS>Listing 4-13: An example showing the mixture of signed and unsigned
variables</ADDRESS>
<P><FONT face="Times New Roman,Times">There is also a need to change the size of
characters when they are stored, since they are represented in the CPU as 16-bit
values. In this case, however, it matters not whether they are signed or
unsigned. Obviously there is only one reasonable way to put a 16-bit quantity
into an 8-bit location. When the high-order byte is chopped off, an error might
occur. It is the programmer's responsibility to ensure that significant bits are
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