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UNCC ECGR 4101 - C Review and Dissection III

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C Programming Language Review and Dissection IIITodayPointersMore about PointersWhat else are pointers used for?Pointers and the M16C ISASpecifying AreasPointers and the M16C ISA (II)StringsFormatted String Creationsprintf Examples – strings and integerssprintf Examples – floating-pointsprintf Examples – More IntegersString Operations in string.hMore String OperationsSlide 165-1Embedded SystemsC Programming Language Review and Dissection IIILecture 5Embedded Systems 5-2TodayPointersStringsFormatted Text OutputReading Assignment:–Patt & Patel “Pointers and Arrays”•Chapter 16 in 2nd edition•Chapter 17 in 1st edition and online notesEmbedded Systems 5-3PointersA pointer variable holds the address of the data, rather than the data itselfTo make a pointer point to variable a, we can specify the address of a–address operator &The data is accessed by dereferencing (following) the pointer–indirection operator * works for reads and writesAssigning a new value to a pointer variable changes where the variable points, not the datavoid main ( ) { int i, j; int *p1, *p2; i = 4; j = 3; p1 = &i; p2 = &j; *p1 = *p1+*p2; p2 = p1;}123456ijp1p21&243Adx600602604606ijp1p2343600ijp1p2443600602ijp1p2573600602ijp1p2673600600Embedded Systems 5-4More about PointersIncrementing and decrementing pointers to array elements–Increment operator ++ makes pointer advance to next element (next larger address)–Decrement operator -- makes pointer move to previous element (next smaller address)–These use the size of the variable’s base type (e.g. int, char, float) to determine what to add•p1++ corresponds to p1 = p1 + sizeof(int);•sizeof is C macro which returns size of type in bytesPre and post–Putting the ++/-- before the pointer causes inc/dec before pointer is used•int *p=100, *p2;–p2 = ++p; assigns 102 to integer pointer p2, and p is 102 afterwards–Putting the ++/-- after the pointer causes inc/dec after pointer is used•char *q=200, *q2;–q2 = q--; assigns 200 to character pointer q2, and q is 199 afterwardsint a[18];int * p;p = &a[5];*p = 5; /* a[5]=5 */p++;*p = 7; /* a[6]=7 */p--;*p = 3; /* a[5]=3 */Embedded Systems 5-5What else are pointers used for?Data structures which reference each other–lists–trees–etc.Exchanging information between procedures –Passing arguments (e.g. a structure) quickly – just pass a pointer –Returning a structureAccessing elements within arrays (e.g. string)Embedded Systems 5-6Pointers and the M16C ISAAddress space of M16C is 1 megabyte–Need 20 bits to address thisThis space is divided into two areas–Near: 64 kilobytes from 00000h to 0FFFFh can be addressed with a 16-bit pointer (top 4 bits of 20-bit address are 0)•Pointer is shorter (2 bytes)•Pointer operations are faster•Note: internal RAM and SFRs are in this space–Far: Entire 1 megabyte area from 00000h to FFFFFh can be addressed with a 20-bit pointer•Pointer is longer (4 bytes used (1.5 bytes wasted!))•Pointer operations are slower, since ALU operates on 16 bits at a timeDetails in section 2.3 of M16C C Programming ManualEmbedded Systems 5-7Specifying AreasBy default, RAM data is near, ROM data is farEmbedded Systems 5-8Pointers and the M16C ISA (II)Default locations–Near area: RAM data•data, bss–Far area: ROM data•rom, program•const dataPointer sizes chosen by compiler based on area holding type of data–Near pointer (16 bits) used for near data–Far pointer (32 bits) used for far dataNC30 doesn’t recognize near/far keywords?–int * near near_data; does not compile–int * far far_data; does not compile–int near near_data; does not compile–int far far_data; does not compileEmbedded Systems 5-9StringsSee Section 16.3.4 of Patt & Patel.There is no “string” type in C. Instead an array of characters is used - char a[44]The string is terminated by a NULL character (value of 0, represented in C by \0).–Need an extra array element to store this nullExample–char str[10] = “testing”;t e s t i n g \0str[0]str[1]strstr[2]Embedded Systems 5-10Formatted String CreationCommon family of functions defined in stdio.h–printf: print to standard output–sprintf: print to a string–fprintf: print to a fileSyntax: sprintf(char *str, char * frmt, arg1, arg2, arg3 .. );–str: destination –fmt: format specifying what to print and how to interpret arguments•%d: signed decimal integer•%f: floating point•%x: unsigned hexadecimal integer•%c: one character•%s: null-terminated string–arg1, etc: arguments to be converted according to format stringEmbedded Systems 5-11sprintf Examples – strings and integerschar s1[30], s2[30];int a=5, b=10, c=-30;char ch=‘$’;sprintf(s1, “Testing”);sprintf(s2, “a=%d, b=%d”, a, b);sprintf(s1, “b=%x, c=%d”, b, c);sprintf(s1, “b=0x%x”, b);sprintf(s2, “s1=%s”, s1);sprintf(s1, “%c %c”, ch, s2);Testings1a=5, b=10s2b=a, c=-30s1b=0xas1s1=b=0xas2$ ss1Embedded Systems 5-12sprintf Examples – floating-pointVariation on %f format specifier–%-w.pf•- = left-justify. Optional•w = minimum field width (# of symbols)•p = precision (digits after decimal point)Examples3.140000s1float f1=3.14, f2=9.991, f3=-19110.331;char s1[30], s2[30];sprintf(s1, “%f”, f1);sprintf(s1, “%f”, f3);sprintf(s1, “%4.1f”, f2);-19110.3s110.0s1Embedded Systems 5-13sprintf Examples – More IntegersVariation on %d format specifier for integers (d/i/o/x/u)–%-w.pd•- = left justify. Optional•w = minimum field width (# of symbols)•p = precision (digits). Zero pad as neededExamples 442s1int a=442, b=1, c=-11;char s1[30], s2[30];sprintf(s1, “%5d”, a);sprintf(s1, “%-4d”, b);sprintf(s1, “%4d”, b);sprintf(s1, “%-5.4d”, c);1 s1 1s1-011 s1Embedded Systems 5-14String Operations in string.hCopy ct to s including terminating null character. Returns a pointer to s. –char* strcpy(char* s, const char* ct); s1 = “cheese”;s2 = “limburger”;strcpy(s1, s2); /* s1 = limburger */Concatenate the characters of ct to s. Terminate s with the null character and return a pointer to it.–char* strcat(char* s, const char* ct);s1 = “cheese”;s2 = “ puffs”;strcat(s1, s2); /* s1 = cheese puffs */Embedded Systems 5-15More String OperationsConcatenate at most n characters of ct to s. Terminate s with the null character and return a pointer to it.–char* strncat(char* s, const char* ct, int n);s1 = “cheese”;s2 = “ puffs”;strncat(s1, s2, 4); /* cheese puf */Compares two strings. The


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UNCC ECGR 4101 - C Review and Dissection III

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