C Functions

Bit Fields allow the packing of data in a structure.

This is especially useful when memory or data storage is at a premium

The maximum length of the field should be less than or equal to the integer word length of the computer.some compilers may allow memory overlap for the fields.Some store the next field in the next word.

C lets us do this in a structure definition by putting :bit length after the variable:

struct pack

{

unsigned int funny_int:9;

}p;

Also, Boolean datatype flags can be stored compactly as a series of bits using the bits fields. Each Boolean flag is stored in a separate bit.

In modular programming, using functions is advisable when a certain code is repeated several times in a program. However, everytime a function is called the control gets transferred to that function and then back to the calling function. This consumes a lot of execution time. One way to save this time is by using macros. Macros substitute a function call by the definition of that function. This saves execution time to a great extent.

The #pragma Directives are used to turn ON or OFF certain features. They vary from compiler to compiler.

Examples of pragmas are:

#pragma startup // you can use this to execute a function at startup of a program

#pragma exit // you can use this to execute a function at exiting of a program

#pragma warn –rvl // used to suppress return value not used warning

#pragma warn –par // used to suppress parameter not used warning

#pragma warn –rch // used to suppress unreachable code warning

A null pointer does not point to any object.

NULL and 0 are interchangeable in pointer contexts.Usage of NULL should be considered a gentle reminder that a pointer is involved.

It is only in pointer contexts that NULL and 0 are equivalent. NULL should not be used when another kind of 0 is required.

A pointer is an address location of another variable. It is a value that designates the address or memory location of some other value (usually value of a variable). The value of a variable can be accessed by a pointer which points to that variable. To do so, the reference operator (&) is pre-appended to the variable that holds the value. A pointer can hold any data type, including functions.

The following declarations are NOT the same:

char *p;

char a[20];

The first declaration allocates memory for a pointer; the second allocates memory for 20 characters.

The following are the differences between exit() and _exit() functions:

- io buffers are flushed by exit() and executes some functions those are registered by atexit().

- _exit() ends the process without invoking the functions which are registered by atexit().

In ANSI, fflush() [returns 0 if buffer successfully deleted / returns EOF on an error] causes the system to empty the buffer associated with the specified output stream.

It undoes the effect of any ungetc() function if the stream is open for input. The stream remains open after the call.

If stream is NULL, the system flushes all open streams.

However, the system automatically deletes buffers when the stream is closed or even when a program ends normally without closing the stream.

► Memcpy() copies specific number of bytes from source to destinatio in RAM, where as strcpy() copies a constant / string into another string.

► Memcpy() works on fixed length of arbitrary data, where as strcpy() works on null-terminated strings and it has no length limitations.

► Memcpy() is used to copy the exact amount of data, whereas strcpy() is used of copy variable-length null terminated strings.

The keyword ‘extern’ indicates the actual storage of the variable is visible , or body of a function is defined elsewhere, commonly in a separate source code. This extends the scope of the variables or functions to be used across the file scope.

Example:

extern int number;

The following are the differences between malloc() and calloc():

- Byte of memory is allocated by malloc(), whereas block of memory is allocated by calloc().

- malloc() takes a single argument, the size of memory, where as calloc takes two parameters, the number of variables to allocate memory and size of bytes of a single variable

- Memory initialization is not performed by malloc() , whereas memory is initialized by calloc().

- malloc(s) returns a pointer with enough storage with s bytes, where as calloc(n,s) returns a pointer with enough contiguous storage each with s bytes.

The following are the differences between strcpy() and memcpy():

- memcpy() copies specific number of bytes from source to destinatio in RAM, where as strcpy() copies a constant / string into another string.

- memcpy() works on fixed length of arbitrary data, where as strcpy() works on null-terminated strings and it has no length limitations.

- memcpy() is used to copy the exact amount of data, whereas strcpy() is used of copy variable-length null terminated strings.

yes we can use any user defined name in place of argc and argv.

Logical error are caused by an incorrect algorithm or by a statement mistypes in such a way

► That it doesn't violet syntax of language.

► Difficult to find.

A recursion function is one which calls itself either directly or indirectly it must halt at a definite point to avoid infinite recursion.

Syntax Error

► These involves validation of syntax of language.

► Compiler prints diagnostic message.

Logical Error

Logical error are caused by an incorrect algorithm or by a statement

► Mistyped in such a way that it doesn't violet syntax of language.

► Difficult to find.

Actual arguments are available in the function call. These arguments are given as constants or variables or expressions to pass the values to the function.

Formal arguments are the arguments available in the function definition. They are preceded by their own data type.