C Programming for Embedded Systems
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1. FILE os_frobble c DESCRPITION os_frobble c contains some OS functions include os_frobble h g_callback is a static global pointer to a void function with one int parameter g_callback currently points to nothing as it is initialized to null static callback_i_v_t g_callback 0 This function saves p_callback in g_callback so that when some event occurs os_frobble_event_handler will call p_callback void os_frobble_callback_request callback_i_v_t p_callback g_callback p_callback This function gets called when some external event happens that the OS detects It will call the user callback function stored in g_callback to let it know that the event happened void os_frobble_event_handler int x assume x is somehow assigned a value Check that g_callback points to something before calling the callback function This will avoid attempting to access a null pointer which is generally a bad thing if g_callback g_callback x 11 https en wikipedia org wiki Callback_ computer_programming 12 It s really not a new type but it is okay to think of it as being one c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 5 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for2. Subtract 4 from sp to allocate 4 bytes on stack for local variable x Line 2 SETTA 0x01 laddr r0 0x400004C0 r0 amp SETTA load r1 r0 r1 SETTA ori r1 r1 9 r1 SETTA 0x09 store r0 r1 SETTA SETTA 0x09 Suppose an external hw event clears bit 3 of SETTA after the line above is executed Line 23 x SETTA amp 0x08 gt gt 3 load r1 r0 r1 SETTA andi r1 r1 8 r1 SETTA amp 0x08 lsr r1 r1 3 r1 SETTA amp 0x08 gt gt 3 store r0 r1 SETTA SETTA amp 0x08 gt gt 3 14 Of course if we turned off optimization then this problem would not occur but in general optimization is a good thing because it will make the generated machine language code more efficient c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 7 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems Another more problematic situation would occur if we wrote something like this where SETTA is not volatile Loop while bit 3 of SETTA is 0 while SETTA amp 0x08 The optimized assembly language code for this loop is note we only r1 with SETTA amp 0x08 once laddr r0 0x400004C0 r0 amp SETTA load r1 r0 r1 SETTA not r1 r1 r1 SETTA andi r1 8 r1 SETTA amp 0x08 beginloop cmp r1 0 Drop out of loop be
3. Error const pointer const_ptr1 is read only and cannot be changed to point to something else const_ptr1 amp y Okay The data type of the thing to which const_ptr1 points x is int x is now 5 const_ptr1 5 Okay const_ptr2 is a const pointer to an int const pointers do not have to be initialized when defined and if not are initialized to NULL Note that const_ptr2 cannot be changed see the next statement int const const_ptr2 Error const pointer const_ptr2 is read only and cannot be changed to point to something else const_ptr2 amp x Okay const_ptr_const1 is a const pointer to a const int const pointers do not have to be initialized when defined and if not are initialized to NULL Note that const_ptr_const1 cannot be changed see the next statement int const const const_ptr_const1 c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 12 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems Errors Since const_ptr_const1 is a read only const pointer it cannot be changed to point to something else const_ptr_const1 amp x const_ptr_const1 amp C_INT1 Okay const_ptr_const2 is a const pointer to a const int Even though x is not an const int as far as const_ptr_const2 is concerned the int to which it points x is now const and cannot be changed int const const const_pt
4. ptr2 amp C_INT1 Questionable Behavior is undefined On my Linux system the program crashes with a segmentation fault ptr2 5 Okay ptr_const1 is an initialized non const pointer to a const int C_INT1 int const ptr_const1 amp C_INT1 Error The const int to which ptr_const1 points is read only ptr_const1 5 Okay Since ptr_const1 is a non const pointer it can be changed to point to a different const int ptr_const1 amp C_INT2 Okay ptr_const2 is an uninitialized non const pointer to a const int int const ptr_const2 Okay ptr_const2 is now an initialized non const pointer to a const int C_INT2 ptr_const2 amp C_INT2 Error The const int to which ptr_const2 points is read only ptr_const2 5 Okay Since ptr_const2 is a non const pointer it can be changed to point to a different const int ptr_const2 amp C_INT2 Okay ptr_const3 is an uninitialized non const pointer to an int x This seems like it might be illegal since the data type of the thing to which ptr_const3 points is supposed to be int const but what happens is that as far as ptr_const3 is concerned x is now treated as const read only int const ptr_const3 amp x Error Since x via ptr_const3 is read only the value of x cannot be changed by dereferencing ptr_const3 ptr_const3 5 Okay const_ptr1 is an initialized const pointer to an int x int const const_ptr1 amp x
5. 0000 0xE0 z lt lt 2 1110 0000 lt lt 2 gt 1000 0000 0x80 On unsigned integer types consisting of b bits shifting a left by n will multiply a by 2 n but you must be careful to avoid the overflow which will occur when a lt lt n is too large to be represented in b bits Overflow can be avoid by ensuring that a 2 n 2 b 1 or a 2 b 1 2 n Example a is a uint16_t variable and we wish to multiply a 14 567 by n 16 2 4 using the left shift trick Will this result in overflow To avoid overflow it must be true that a 2 b 1 2 n or 14 567 2 16 1 2 4 or 14 567 4095 9375 which is false therefore overflow will occur 6 http www cprogramming com tutorial bitwise_operators html 7 http en wikipedia org wiki Bitwise_operations_in_C 8 typedef unsigned char uint8_t See global h in newbbproj zip c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 2 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems On b bit signed integer types shifting a nonnegative integer left by n will also multiply by 2 n but you must be careful not to trash the sign bit The largest nonnegative integer that may be safely shifted left n times is 2 b 1 1 2 n It is never wise to try this trick on a negative integer the result is undefined 1 3 6 Bitwise S
6. 1 5 Function Pointers and Callback Functions A callback 11 function or simply callback is a function f passed an argument to some other function g that is to be called by g when some event occurs that the f needs to be notified of Callbacks can be blocking synchronous or nonblocking asynchronous A blocking callback g does not return until f is called back A nonblocking callback would one where g returns before calling f although g will call f at some future time We will only discuss nonblocking callbacks A very common use of a callback and this is how we will use them is for a application user function f to request the operating system or a library function g to call f when some event occurs that is detected by the operating system or library function Here is a typical example FILE os_frobble h DESCRIPTION os_frobble h contains a typedef for a callback function pointer type ifndef OS_FROBBLE_H define OS_FROBBLE_H callback_i_v_t is a new type12 which is a function pointer to a function with an int parameter that returns nothing typedef void callback_i_v_t int endif
7. CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems Table of Contents 1 C Programming for Embedded Systems 1 1 Structure Types 1 2 Enumerated Types 1 3 Bit Manipulation Operators 1 4 Function Pointers 1 5 Function Pointers and Callback Functions 1 6 Volatile Reserved Word 1 7 __attribute__ 1 8 C and Const Correctness 1 C Programming for Embedded Systems In this section we will review some features of the C programming language that are useful in embedded systems program and will introduce some new features that you may not have been exposed to before 1 1 Structure Types 1 You should have learned all about structure types in CSE220 Take the time now to review Struct c 2 on the course website 1 2 Enumerated Types3 Suppose we wish to create our own data type to represent colors The traditional way of doing this in C is to use define preprocessor macros and typedef define COLOR_RED 0 define COLOR_GREEN 1 define COLOR_BLUE 2 typedef unsigned int color color c COLOR_RED color d c c 99 Error What color is c With the addition of the const reserved word to ANSI C the use of define is discouraged 4 so a better way to do this would be int const5 COLOR_RED 0 int const COLOR_GREEN 1 int const COLOR_BLUE 2 typedef unsigned int color color c COLOR_RED color d c c 99 Error What color is c There s nothin
8. Embedded Systems FILE gromulate c DESCRIPTION gromulate c contains user application code include lt os_frobble h gt This function initializes the gromulate module and in the process it requests the os_frobble module to call gromulate_callback when some event occurs void gromulate_init os_frobble_callback_request gromulate_callback This callback function will be called by the os_frobble module when the external event happens void gromulate_callback int p Do something to handle the event Essentially the gromulate user module is delegating the responsibility of detecting the external event to the os_frobble operating system module which notifies the gromulate module via the callback function when the event has occurred We will see examples of this and use this technique when we talk about interrupts later in the course 1 6 Volatile Reserved Word 13 The volatile reserved word was added to the C language standard specifically to support real time and embedded systems programming Why Suppose we are accessing an 8 bit hardware register named SETTA that controls a general purpose I O pin We want to write a 1 to bits 0 and 3 of this register without a
9. You cannot turn green into yellow since green 0 is const return 0 The primary use of const is to practice defensive programming i e to reduce your chances of introducing bugs into your code If something is not going to be changed or should not be changed make it const When to use const Whenever you can Finally in the old days numeric constants were traditionally created using the define preprocessor directive but today the recommendation is to use const for a variety of reasons one of the best of which is that the name associated with a define disappears after the preprocessor finishes its job so consequently when debugging you will essentially be dealing with magic numbers With const and exporting symbolic debugging information to your executable you will see the names of your consts rather than their values That alone is good enough reason to use const 18 Which means it is implementation specific you should not count on your code behaving in the same way when you change from the Microsoft Visual Studio compiler to the GCC C compiler c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 13 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems References ARM ARM ARM Architecture Reference Manual Rev I ARM 2005 ARM CALL Procedure Call Standard for the ARM Architecture Rev E 2 09 ARM 2012 ARM ISQR ARM
10. _attribute__ specification is __attribute__ attribute list where the attribute list is a possibly empty comma separated sequence of attributes We will only discuss the attributes that may be applicable to our CSE325 projects alias Causes the declaration to be emitted to the linker as a new name for another symbol e g 15 https msdn microsoft com en us library dabb5z75 aspx 16 https gcc gnu org onlinedocs gcc Attribute Syntax html Attribute Syntax c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 8 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems void __f __attribute__ alias __f void f int main f Calls __f return 0 On the surface this may not seem to useful However consider the use of alias and weak in bare_startup c void bare_default_isr __asm bkpt Weak definitions of interrupt service routines point to bare_default_isr if not implemented void nmi_isr __attribute__ weak alias bare_default_isr void hardfault_isr __attribute__ weak alias bare_default_isr void svc_isr __attribute__ weak alias bare_default_isr void portscd_isr __attribute__ weak alias bare_default_isr Define and initialize the interrupt vector table Place the vector table in its own section named vectortable in the o file Th
11. ak_symbol c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 10 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems user hpp void lib_funct user cpp include user hpp void lib_funct g_delay Increment the value of a global variable include user hpp int main lib_funct Calls lib_funct in user cpp which overwrites default lib_funct in lib cpp Now if lib_funct is not overridden in user cpp delete user hpp and user cpp lib hpp void lib_funct lib cpp include lib hpp __attribute__ weak __ void lib_funct g_delay Decrement the value of a global variable int main lib_funct Calls lib_funct in lib cpp because it is not overwritten 1 8 C and Const Correctness The reserved word const is a type qualifier and when applied to a variable tells the compiler variable is read only i e cannot be changed int w w is an int variable and is uninitialized It s value is undefined int x 10 x is an int variable and is initialized to 10 int const y Error const variables must be initialized when defined int const z 10 z is a read only int variable that cannot be changed It s data type is int const w 20 Okay w is now 20 x 20 Okay x is now 20 y 20 Error const variable y is read only z 20 Error const
12. and Thumb 2 Instruction Set Quick Reference Card Rev M ARM 2008 ARM M0 DUG Cortex M0 Device User s Guide ARM 2012 ARM M0 TRM Cortex M0 Technical Reference Manual Rev r0p1 ARM 2012 ARM V6M ARMv6 M Architecture Reference Manual Rev C 2010 BARR Barr M Programming Embedded Systems in C and C O Reilly 1999 BERGER Berger A Embedded Systems Design CMP Books 2002 CW BUILD CodeWarrior Kinetis Build Tools Reference Manual Rev 10 6 Freescale 2014 FRDM KL46 PIN FRDM KL46Z Pinout FRDM KL46Z QSG FRDM KL46Z Quick Start Guide Rev 1 Freescale 2013 FRDM KL46 SCH FRDM KL46Z Schematic Rev C Freescale 2013 FRDM KL46Z UM FRDM KL46Z User s Manual Rev 1 0 Freescale HEATH Heath S Embedded Systems Design 2nd ed Newnes 2003 KL46 SDS Kinetis KL46 Sub Family Data Sheet Rev 5 Freescale 2014 KL46 SRM Kinetis KL46 Sub Family Reference Manual Rev 3 Freescale 2013 NOER Noergaard T Embedded Systems Architecture 1st ed Newnes 2005 OPENSDA OpenSDA User s Guide Rev 0 93 Freescale 2012 VAHID Vahid F and Givargis T Embedded System Design John Wiley amp Sons 2002 WHITE White E Making Embedded Systems O Reilly 2011 c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 14
13. e linker script will use this section name to make sure the vector table is placed in the flash at 0x0000_0000 void const bare_vector_table __attribute__ section vectortable void amp __init_sp Initial value of the SP register bare_startup Reset vector will jump here and begin executing code nmi_isr Nonmaskable interrupt service routine hardfault_isr Hardfault interrupt service routine portscd_isr Single interrupt vector for Port C and Port D interrupt The function declaration employing the weak and alias attributes e g nmi_isr declares a function that provides a default implementation which can be overwritten by the user That is if the user defines nmi_isr that function definition will be called in place of the nmi_isr function that is declared weak The alias attribute specifies that nmi_isr is an alias for bare_default_isr so when nmi_isr is called bare_default_isr will be the function that is actually called Putting weak and alias together then specifies that if nmi_isr is not overwritten bare_default_isr will be called if nmi_isr is overwritten then the user can provide their own code that will be executed when nmi_isr is called interrupt Specifies that the function is an interrupt service routine ISR so the compiler will generate special prologue and epilogue code appropriate for ISR s I ll explain this in more detail when we discuss interrupts For examp
14. ffecting the other bits and later on we are going to read the value of bit 3 Assume the address of SETTA is 0x4000_04C0 001 define SETTA uint8_t 0x400004C0 SETTA is equiv to the dereferenced value of addr 0x4000_04C0 002 int x x is an uninitialized variable 003 SETTA 0x09 Set bits 0 and 3 025 x SETTA amp 0x08 gt gt 3 Set x to bit 3 of SETTA This would be the C code after the preprocessor expands the SETTA macro 001 int x 002 unsigned char 0x400004C0 0x09 023 x unsigned char 0x400004C0 amp 0x08 gt gt 3 Where the second line is equivalent to unsigned char 0x400004C0 unsigned char 0x400004C0 0x01 Now consider the assembly language code that a C compiler might generate for these statements this is pseudo assembly language ah I just love assembly language such power Line 1 int x sub sp sp 4 Subtract 4 from sp to allocate 4 bytes on stack for local variable x Line 2 SETTA 0x01 laddr r0 0x400004C0 r0 amp SETTA load r1 r0 r1 SETTA ori r1 r1 9 r1 SETTA 0x09 store r0 r1 SETTA SETTA 0x09 13 http publications gbdirect co uk c_book chapter8 const_and_volatile html c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 6 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programm
15. function that has int and int parameters and return a pointer to an int the second parameter p_a is an int int gromulate fptr_i_ip_ip_t p_some_funct int p_a int x 1 Calls the function that some_funct points to Passes a an int and the address of x an int some_funct returns a pointer to an integer so we return that pointer return p_some_funct p_a amp x void glorp Defines funct_ptr to be a function pointer variable to a function which has parameters of types int and double and returns an int And what is the data type of funct_ptr It is int int double int funct_ptr int double Make funct_ptr point to foo funct_ptr foo Call foo indirectly via funct_ptr Passes an int and a double Assigns the returned int to a int a funct_ptr 2 3 1 Call gromulate passing a pointer to bar as the argument This indirectly calls bar Assigns the integer pointer returned from bar to p int p gromulate bar 9 10 http www cprogramming com tutorial function pointers html c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 4 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems Call gromulate passing a pointer to baz as the argument This indirectly calls baz Assigns the integer pointer returned from baz to q int q gromulate baz 5
16. g necessarily wrong with this but enumerated types provide a better way with a few minor advantages one of which depends on your IDE Consider the declaration of an enumerated type for colors 1 Good Tutorial http www programiz com c programming c structures 2 See Struct c posted on the course website 3 Good Tutorial http www programiz com c programming c enumeration See Enum c posted on the course website 4 Why Because const enables the C compiler to perform more accurate static type checking 5 const int COLOR_RED 0 is equivalent to int const COLOR_RED 0 c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 1 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems Advantages to enums 1 Proper use of enumerated types should improve the readability of the code when you create an enumerated type in C you are not really creating a new type but rather an enumerated type is just a mapping from magic numbers to identifiers 2 enum values can be auto generated by default the values are 0 1 2 so this saves a bit of typing 3 Some compilers can emit a warning if a switch statement does not include cases for all enumerated values 4 In some IDE s the editor will syntax color enumerated values Also the debugger will display the identifier for the enumerated values rather than the integer values Thi
17. hift Right Operator gt gt The operation a gt gt n will shift the bits in variable a to the right n times filling vacated bit positions with 0 uint8_t x 0x3C z z x gt gt 2 0011 1100 gt gt 2 gt 0000 1111 0x0F which is int 60 4 z gt gt 3 0000 1111 gt gt 3 gt 0000 0001 0x01 which is int 15 4 On unsigned integer types shifting right by n will divide integer division the operand by 2 n On signed integer types shifting a nonnegative integer right by n will also divide by 2 n It is never wise to try this trick on a negative integer the result is undefined 9 1 3 7 Setting Bits To set a bit means to put a 1 into a specific bit position Boolean algebra a 0 a a 1 1 Example Set bit 3 of uint8_t variable x mask must have 1 in bit 3 and 0 s in other bits use bitwise OR x 1 lt lt 3 or x 0x08 Example Set bits 3 and 6 of x mask must have 1 s in bits 3 and 6 and 0 s in other bits Example Set bit n of x mask must have 1 in bit n and 0 s in other bits Example Set bits n and m of x mask must have 1 s in bits n and m and 0 s in other bits 1 3 8 Clearing Bits To clear a bit means to put a 0 into a bit position Boolean algebra a 0 0 a 1 a Example Clear bit 3 of uint8_t variable x mask must have 0 in bit 3 and 1 s in other bits use bitwise AND x amp 1 lt lt 3 or x amp 0xF7 Example Clear bit
18. ing for Embedded Systems Suppose an external hw event clears bit 3 of SETTA after the line above is executed Line 23 x SETTA amp 0x08 gt gt 3 andi r1 r1 8 r1 SETTA amp 0x08 lsr r1 r1 3 r1 SETTA amp 0x08 gt gt 3 store r0 r1 SETTA SETTA amp 0x08 gt gt 3 Now trace through this code So the problem is that the compiler in optimizing the code 14 had no idea that SETTA could change value between lines 2 and 23 and for normal microcontroller registers that would not happen but SETTA is not just any other microcontroller register It is connected to a peripheral which can change the value of any bit in SETTA at any time So between the ori and andi instructions some event could occur that would change the value of bit 3 from 0 to 1 or 1 to 0 Therefore the value assigned to x would be the old value of bit 3 of SETTA This could of course lead to a bug which we would like to prevent Thus the addition of the volatile reserved word to the language What volatile does is inform the compiler that the object to which volatile is applied is subject to change in ways that the compiler cannot know This forces the compiler to always access the memory location to load the value of the object every time it is used So to define register SETTA we would write define SETTA volatile uint8 0x400004C0 The generated assembly language code would be Line 1 int x sub sp sp 4
19. le __attribute__ interrupt timer0_isr naked This is an ARM specific attribute which permits the compiler to construct the requisite function definition while allowing the body of the function to be assembly code The function will not have the usual prologue epilogue code that saves registers on the stack at the beginning of the function restores registers at the end and returns from the function For example void bar __asm bl bar c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 9 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems __attribute__ naked void foo __asm bl foo Consider the C compiler generated assembly language bar push r7 lr add r7 sp 0 bl bar mov sp r7 pop r7 pc foo bl foo section Normally for a function named foo the GNU C compiler will place the instructions for foo in the text section of the o file Our CW project settings are configured such that each function is placed in its own unique text section e g foo will be output to text foo There are times when it is desirable to specify the name of the section in which the object code is placed For example in the code for bare_startup c scroll to line 98 and you will see void const bare_vector_table __attribute__ section vectortable which specifies to the co
20. mpiler that the contents of bare_vector_table are to be written to a section named vectortable in bare_startup o This section is used in the MKL46Z256_flash ld linker script to ensure that bare_vector_table is written to the m_interrupts text segment unused Specifies that the programmer is aware that a variable or function parameter is not used within the body of a function e g int some_funct int x __attribute__ unused double y return x 99 Note this attribute would not be required if we had not enabled the Extra warnings Wextra check box in Step 11 of 3 5 of Project 1 You may wonder why one would go to the trouble of defining a function parameter that is not used in the function I find this attribute to be useful during debugging and testing at times when I want to see if I can get my code to run correctly when I do not pass an argument for y the attribute allows me to try this out without having to locate and modify all calls to some_funct weak 17 When applied to a function weak causes the function to be emitted to the linker as a weak symbol rather than as a strong symbol This is primarily useful in defining library functions containing default code that may be overwritten by the user For example lib hpp void lib_funct lib cpp include lib hpp __attribute__ weak void lib_funct g_delay Decrement the value of a global variable 17 https en wikipedia org wiki We
21. q endloop if SETTA amp 0x08 is zero During the 12th pass of this loop an external event changes bit 3 of SETTA to 1 bra beginloop Continue looping endloop With the volatile reserved word applied to SETTA the assembly language code would be note we load r1 with SETTA amp 0x08 each time the loop condition is evaluated beginloop laddr r0 0x400004C0 r0 amp SETTA load r1 r0 r1 SETTA not r1 r1 r1 SETTA andi r1 8 r1 SETTA amp 0x08 cmp r1 0 Drop out of loop beq endloop if SETTA amp 0x08 is zero During the 12th pass of this loop an external event changes bit 3 of SETTA to 1 bra beginloop Continue looping endloop Voil Bugs be gone When we talk about interrupt service routines I will explain another situation where using volatile is critical 1 7 __attribute__ Ref CW BUILD 5 4 Most C compilers have nonstandard ways of specifying additional information to the compiler which it uses when compiling the code For example Microsoft C and C compilers introduced the __declspec 15 extension to the C language it not part of the language standard but many other C and C compilers support it The GNU C compiler uses the __attribute__ 16 extension to do this CodeWarrior uses the GNU C compiler on the backend to compile your code so CW supports some but not all of the GCC __attribute__ extensions The general syntax of an _
22. r_const2 amp x Error The const int to which const_ptr_const2 points x is read only and cannot be changed const_ptr_const2 10 Okay const_ptr_const3 is a const pointer to a const int C_INT1 int const const const_ptr_const3 amp C_INT1 Error The const int to which const_ptr_const3 points C_INT1 is read only and cannot be changed const_ptr_const3 10 Error Since const_ptr_const3 is a const pointer it cannot be changed to point to something else const_ptr_const3 amp C_INT2 return 0 Note that making a pointer to a const point to a non const is legal but the non const is treated as if it were const i e it is read only It is also legal to make a pointer to a non const point to a const but the behavior is undefined 18 For example gcc will complain with a warning message about non const pointer ptr2 being made to point to a const C_INT1 but if you ignore the warning and proceed to attempt to change the value of C_INT1 by dereferencing ptr2 then be prepared for something bad to happen in this case my program crashed with a segmentation fault The elements of an array may be const typedef int rgb_t 3 A color expressed as Red Green and Blue components int main rgb_t const red Error red is const and must be initialized at definition rgb_t const green 0 255 0 Okay green is initialized and its data type is actually int const green 0 255 Error
23. s 3 and 6 of x mask must have 0 in bits 3 and 6 and 1 s in other bits Example Clear bit n of x mask must have 0 in bit n and 1 s in other bits Example Clear bits n and m of x mask must have 0 s in bits n and m and 1 s in other bits 9 Most ISA s contain at least two shift right instructions one to perform a logical shift right and one to perform an arithmetic shift right Logical shift right fills vacated bit positions with 0 s and most certainly would not work on a negative integer Arithmetic shift right fills vacated bit positions with the sign bit and would therefore work on a negative integer However which of these is used is compiler dependent so unfortunately you should not depend on it working if you wish to write portable C code c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 3 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems 1 3 9 Testing Bits for Set Example Test bit n of x to see if it is set mask must have 1 in bit n and 0 s in other bits use bitwise AND if x amp 1 lt lt n 0 or if x amp 1 lt lt n 1 3 10 Testing Bits for Clear Example Test bit n of x to see if it is clear mask must have 1 in bit n and 0 s in other bits use bitwise AND 1 4 Function Pointers10 The name of a C function is an identifier Every function has an associated memory address which is
24. s last advantage is probably the most useful one 1 3 Bit Manipulation Operators6 7 There are six bit manipulation operators in C 1 3 1 Bitwise AND Operator amp Remember amp amp is logical AND uint8_t8 x 0x3C y 0xA5 z 0011 1100 0x3C x 0010 0100 0x24 z z x amp y amp 1010 0101 0xA5 y amp 0011 1100 0x3C x z amp x gt 0010 0100 0x24 z x amp y gt 0010 0100 0x24 z amp x 1 3 2 Bitwise OR Operator Remember is logical OR uint8_t x 0x3C y 0xA5 z 0011 1100 0x3C x 1011 1101 0xBD z z x y 1010 0101 0xA5 y 0011 1100 0x3C x z x gt 1011 1101 0xBD z x y gt 1011 1101 0xBD z x 1 3 3 Bitwise Complement Operator Forms the one s complement of the operand uint8_t x 0x3C z x 0011 1100 gt 1100 0011 0xC3 1 3 4 Bitwise Exclusive OR XOR Operator is called the bitwise inclusive OR operator Exclusive OR is 0 if the operands are the same and 1 if the operands are different uint8_t x 0x3C y 0xA5 z 0011 1100 0x3C x 1001 1001 0x99 z z x y 1010 0101 0xA5 y 0011 1100 0x3C x z x 1001 1001 0x99 z x y gt 1010 0101 0xA5 z x 1 3 5 Bitwise Shift Left Operator lt lt The operation a lt lt n will shift the bits in variable a to the left n times filling new bit positions with 0 uint8_t x 0x3C z z x lt lt 3 0011 1100 lt lt 3 gt 1110
25. the address of the first instruction of the function A pointer variable is a variable that contains the memory address of an identifier Putting all of this together means that we can create a pointer variable which contains the address of a function i e a function pointer The basic syntax for declaring a function pointer variable is nasty funct return type funct ptr var name parameter list For example foo is a function has an int and double parameter and returns an int int foo int x double y bar is a function that has an int and int parameter and returns a pointer to an int int bar int x int y baz is a function that has an int and int parameter and returns a pointer to an int int baz int x int y Define a new type named fptr_i_ip_ip t which is a function pointer to a function that has parameters of types int and int and the function returns an int The i part of fptr_i_ip_ip_t specifies that the first parameter is an int the first ip part specifies that the second parameter is an int pointer the third ip specifies that the return type is an int pointer What is the data type of a function pointer variable defined to be of type fptr_i_ip_ip_t It is int int int typedef int fptr_i_ip_ip_t int int I prefix my parameter names in c with a leading p_ gromulate has two parameter the first named p_some_funct is a function pointer to a
26. variable z is read only A function s formal parameters are variables and can also be defined as const void foo int x int const y x 5 Okay x is now 5 y 15 Error const variable y is read only void bar int a 10 a is an int variable with value 10 int b 20 b is an int variable with value 20 foo a b The value of a 10 is copied into x and the value of b 20 is copied into y Since C has only one parameter passing technique pass by value when foo is called the values of a and b are copied into the memory locations allocated for x and y Within foo the value of x can be changed but the value of y cannot c Kevin R Burger Computer Science amp Engineering Arizona State University Fall 2015 Page 11 CSE325 Embedded Microprocessor Systems Course Notes Part 2 C Programming for Embedded Systems Note that changing x does not affect the value of a i e on return from foo the value of a is still 10 The keyword const may also be applied to pointers int main x and y are non const ints int x 1 y 2 C_INT1 and C_INT2 are const ints i e they are read only int const C_INT1 3 int const C_INT2 4 Okay The data type of ptr1 is pointer to int and it points to an int x int ptr1 amp x Okay Changes x to 5 indirection via ptr1 ptr1 5 Warning The data type of ptr2 is pointer to int but it points to a const int int
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