Embedded C

Sizeof int

4 bytes

Why ++n executes faster than n+1

++n is a single assembly instruction. n+1 uses other instructions to load n into the adder in addition to just using the increment instruction

Sizeof char

1 byte

Sizeof pointer

8 bytes on 64 bit machine, 4 bytes on 32 bit machine

What is the difference between hard real-time and soft real-time OS?

A Hard real-time system strictly adheres to the deadline associated with the task. If the system fails to meet the deadline, even once, the system is considered to have failed. In case of a soft real-time system, missing a deadline is acceptable. In this type of system, a critical real-time task gets priority over other tasks and retains that priority until it completes.

What is dangling pointer?

A pointer initially holding a valid address, but later the held address is released.

Can static variables be declared in a header file?

A static variable cannot be declared without defining it. A static variable can be defined in the header file. But doing so, the result will be having a private copy of that variable in each source file which includes the header file. So it will be wise not to declare a static variable in header file, unless you are dealing with a different scenario.

What are Pipes?

Communication between 2 procs. One proc writes to the pipe, one reads from the pipe.

What are inline functions?

Compilers replace the function call with all the code from the function. This removes a function call but increases code size

What is needed for RTOS

Context switching latency should be short, Interrupt latency should be short, Interrupt dispatch latency should be short, Reliable and time bound inter process mechanisms, Should support kernel preemption

Is Count Down_to_Zero Loop better than Count_Up_Loops?

Count to zero is better because CPU's have optimized functions for comparison to 0

How to reduce interrupt latency?

Interrupt latency can be minimized by writing short ISR routine and by not delaying interrupts for more time.

What is Shared Memory?

Fastest form of interprocess communication, Common block of virtual memory shared by multiple processes,Permission is read-only or read-write for a process, Mutual exclusion constraints are not part of the shared-memory facility but must be provided by the processes using the shared memory

What is spin lock?

If a resource is locked, a thread that wants to access that resource may repetitively check whether the resource is available. During that time, the thread may loop and check the resource without doing any useful work. Such a lock is termed as spin lock.

What is priority inversion?

If two tasks share a resource, the one with higher priority will run first. However, if the lower-priority task is using the shared resource when the higher-priority task becomes ready, then the higher-priority task must wait for the lower-priority task to finish. In this scenario, even though the task has higher priority it needs to wait for the completion of the lower-priority task with the shared resource. This is called priority inversion.

Can a pointer be volatile?

If we see the declaration volatile int *p, it means that the pointer itself is not volatile and points to an integer that is volatile. This is to inform the compiler that pointer p is pointing to an integer and the value of that integer may change unexpectedly even if there is no code indicating so in the program.

Why cannot arrays be passed by values to functions?

In C, the array name itself represents the address of the first element. So, even if we pass the array name as argument, it will be passed as reference and not its address.

What is the difference between static linking and dynamic linking ?

In static linking, all the library modules used in the program are placed in the final executable file making it larger in size. This is done by the linker. If the modules used in the program are modified after linking, then re-compilation is needed. The advantage of static linking is that the modules are present in an executable file. We don't want to worry about compatibility issues. In case of dynamic linking, only the names of the module used are present in the executable file and the actual linking is done at run time when the program and the library modules both are present in the memory. That is why, the executables are smaller in size. Modification of the library modules used does not force re-compilation. But dynamic linking may face compatibility issues with the library modules used.

What happens when recursive functions are declared inline?

Inlining an recursive function reduces the overhead of saving context on stack. But, inline is merely a suggestion to the compiler and it does not guarantee that a function will be inlined. Obviously, the compiler won't be able to inline a recursive function infinitely. It may not inline it at all or it may inline it, just a few levels deep.

What are IPCs

Inter Process Communications

What is ISR?

Interrupt Service Routine. Handles hardware interrupts

What makes a function reentrant?

It can be interrupted. It should not use global or static variables. It should not modify its own code. It should not call other non-reentrant functions.

What is a binary semaphore

It can have only two values (0 and 1). The semaphore value is set to 1 by the process in charge, when the resource is available.

What is a counting semaphore

It can have value greater than one. It is used to control access to a pool of resources.

What are Named Pipes or FIFO?

It is an extension to the traditional pipe concept on Unix. A traditional pipe is "unnamed" and lasts only as long as the process.Usually a named pipe appears as a file and generally processes attach to it for inter-process communication. A FIFO file is a special kind of file on the local storage which allows two or more processes to communicate with each other by reading/writing to/from this file. call mkfifo to use

What is a message queue?

Message queues store "messages"—packets of data that applications create for other applications to consume—in the order they are transmitted until the consuming application can process them. This enables messages to wait safely until the receiving application is ready, so if there is a problem with the network or receiving application, the messages in the message queue are not lost.

What is difference between binary semaphore and mutex?

Mutual exclusion and synchronization can be used by binary semaphore while mutex is used only for mutual exclusion; A mutex can be released by the same thread which acquired it. Semaphore values can be changed by other thread also; From an ISR, a mutex can not be used; The advantage of semaphores is that, they can be used to synchronize two unrelated processes trying to access the same resource; Semaphores can act as mutex, but the opposite is not possible.

What is return type of ISR?

No return type. No program called the ISR, so there is nothing to return to.

What is kernel paging?

Paging is a memory management scheme by which computers can store and retrieve data from the secondary memory storage when needed in to primary memory. In this scheme, the operating system retrieves data from secondary storage in same-size blocks called pages. The paging scheme allows the physical address space of a process to be non continuous. Paging allows OS to use secondary storage for data that does not fit entirely into physical memory.

Can structures be passed to the functions by value?

Passing structure by its value to a function is possible, but not a good programming practice. First of all, if we pass the structure by value and the function changes some of those values, then the value change is not reflected in caller function. Also, if the structure is big, then passing the structure by value means copying the whole structure to the function argument stack which can slow the program by a significant amount.

What are the IPCs available?

Pipes, Named Pipes or FIFO, Semaphores, Shared Memory, Message Queue, Sockets

Can we use printf inside ISR?

Printf function in ISR is not supported because printf function is not reentrant, thread safe and uses dynamic memory allocation which takes a lot of time and can affect the speed of an ISR up to a great extent.

What is priority inheritance?

Priority inheritance is a solution to the priority inversion problem. The process waiting for any resource which has a resource lock will have the maximum priority. This is priority inheritance. When one or more high priority jobs are blocked by a job, the original priority assignment is ignored and execution of critical section will be assigned to the job with the highest priority in this elevated scenario. The job returns to the original priority level soon after executing the critical section.

Can we put breakpoint inside ISR?

Putting a break point inside ISR is not a good idea because debugging will take some time and a difference of half or more second will lead to different behavior of hardware. To debug ISR, definitive logs are better.

Difference between RISC and CISC processor.

RISC (Reduced Instruction Set Computer) could carry out a few sets of simple instructions simultaneously. Fewer transistors are used to manufacture RISC, which makes RISC cheaper. RISC has uniform instruction set and those instructions are also fewer in number. Due to the less number of instructions as well as instructions being simple, the RISC computers are faster. RISC emphasise on software rather than hardware. RISC can execute instructions in one machine cycle. CISC (Complex Instruction Set Computer) is capable of executing multiple operations through a single instruction. CISC have rich and complex instruction set and more number of addressing modes. CISC emphasise on hardware rather that software, making it costlier than RISC. It has a small code size, high cycles per second and it is slower compared to RISC.

What type of scheduling is there in RTOS?

RTOS uses pre-emptive scheduling. In pre-emptive scheduling, the higher priority task can interrupt a running process and the interrupted process will be resumed later.

What is RTOS?

Real Time Operating System. In an embedded system, a certain event must be entertained in strictly defined time. To accomplish this the scheduler must be predictable.

What are Semaphores?

Semaphore is actually a variable or abstract data type which controls access to a common resource by multiple processes

What is Top half & bottom half of a kernel?

Sometimes to handle an interrupt, a substantial amount of work has to be done. But it conflicts with the speed need for an interrupt handler. To handle this situation, Linux splits the handler into two parts - Top half and Bottom half. The top half is the routine that actually responds to the interrupt. The bottom half on the other hand is a routine that is scheduled by the upper half to be executed later at a safer time. All interrupts are enabled during execution of the bottom half. The top half saves the device data into the specific buffer, schedules bottom half and exits. The bottom half does the rest. This way the top half can service a new interrupt while the bottom half is working on the previous.

What are the uses of the keyword static?

Static keyword can be used with variables as well as functions. A variable declared static will be of static storage class and within a function, it maintains its value between calls to that function. A variable declared as static within a file, scope of that variable will be within that file, but it can't be accessed by other files. Functions declared static within a module can be accessed by other functions within that module. That is, the scope of the function is localized to the module within which it is declared.

What is the use of volatile keyword?

The C's volatile keyword is a qualifier that tells the compiler not to optimize when applied to a variable. By declaring a variable volatile, we can tell the compiler that the value of the variable may change any moment from outside of the scope of the program. A variable should be declared volatile whenever its value could change unexpectedly and beyond the comprehension of the compiler.In those cases it is required not to optimize the code, doing so may lead to erroneous result and load the variable every time it is used in the program. Volatile keyword is useful for memory-mapped peripheral registers, global variables modified by an interrupt service routine, global variables accessed by multiple tasks within a multi-threaded application.

What is NULL pointer and what is its use?

The NULL is a macro defined in C. Null pointer actually means a pointer that does not point to any valid location. We define a pointer to be null when we want to make sure that the pointer does not point to any valid location and not to use that pointer to change anything. If we don't use null pointer, then we can't verify whether this pointer points to any valid location or not.

Advantages and disadvantages of using macro and inline functions?

The advantage of the macro and inline function is that the overhead for argument passing and stuff is reduced as the function are in-lined. The advantage of macro function is that we can write type insensitive functions. It is also the disadvantage of macro function as macro functions can't do validation check. The macro and inline function also increases the size of the executable.

What is virtual memory?

The mapping of hardware memory addresses to applications, using swapping or paging.

When should we use register modifier?

The register modifier is used when a variable is expected to be heavily used and keeping it in the CPU's registers will make the access faster.

What is void pointer and what is its use?

The void pointer means that it points to a variable that can be of any type. To dereference (get the value pointed to) you must cast to the correct type

Significance of watchdog timer in Embedded Systems

The watchdog timer is a timing device with a predefined time interval. During that interval, some event may occur or else the device generates a time out signal. It is used to reset to the original state whenever some inappropriate events take place which can result in system malfunction. It is usually operated by counter devices.

What are sockets?

Two way pipes

What are the uses of the keyword volatile?

Volatile keyword is used to prevent compiler to optimize a variable which can change unexpectedly beyond compiler's comprehension. Suppose, we have a variable which may be changed from scope out of the program, say by a signal, we do not want the compiler to optimize it. Rather than optimizing that variable, we want the compiler to load the variable every time it is encountered. If we declare a variable volatile, compiler will not cache it in its register.

Can we use any function inside ISR?

We can use function inside ISR as long as that function is not invoked from other portion of the code.

Can include files be nested?

Yes. Include files can be nested any number of times. But you have to make sure that you are not including the same file twice. There is no limit to how many header files that can be included. But the number can be compiler dependent, since including multiple header files may cause your computer to run out of stack memory.

Can a variable be both const and volatile?

Yes. The const keyword make sure that the value of the variable declared as const can't be changed. This statement holds true in the scope of the program. The value can still be changed by outside intervention. So, the use of const with volatile keyword makes perfect sense.

What is interrupt latency?

time span between firing condition being met, and first instruction of ISR being executed