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Programming in C

Programming in C . Pointers and Arrays, malloc( ). Dynamic memory. In Java, objects are created on the heap using reference variables and the new operator. Memory is freed by the garbage collector.

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Programming in C

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  1. Programming in C Pointers and Arrays, malloc( )

  2. Dynamic memory In Java, objects are created on the heap using reference variables and the new operator. Memory is freed by the garbage collector. In C, memory is allocated from the heap using library functions malloc(),calloc(), and realloc(). These functions return a pointer to the allocated heap memory. Memory must be explicitly freed using free() to avoid memory leaks and program crashes. The argument to free( ) is a pointer returned when the memory is allocated.

  3. Dynamic Memory Comparison In C typedef struct person { int age; double gpa; } PERSON; /* memory allocation */ PERSON *pbob = (PERSON *)malloc(sizeof(PERSON));pbob->age = 42;pbob->gpa = 3.5; ... /* explicitly freeing the memory */ free( pbob ); In JAVA public class Person { public int age; public double gpa; } // memory allocation Person bob = new Person( ); bob.age = 42; bob.gpa = 3.5; // bob is eventually freed by // garbage collector In C, note the use ofmalloc( ) instead of newfree( )-> (arrow) rather than . (dot)

  4. Dynamic Memory Functions These functions are used to allocate and free dynamically allocated heap memory and are part of the standard C library void *malloc( size_t nrBytes ); • Returns a pointer to dynamically allocated memory on the heap of size nrBytes, or NULL if the request cannot be satisfied. The memory is uninitialized. void *calloc( int nrElements, size_t nrBytes ); • Same as malloc( ), but the memory is initialized to zero void *realloc( void *p, size_t nrBytes); • Changes the size of the memory pointed to by p to nrBytes. The contents will be unchanged up to the minimum of the old and new size. If the new size is larger, the new space is uninitialized. Returns a pointer to the new memory, or NULL if request cannot be satisfied in which case *p is unchanged. void free( void *p ) • Deallocates the memory pointed to by p which must point to memory previously allocated by calling one of the functions above. Does nothing if p is NULL.

  5. void* and size_t • The void* type is C’s generic pointer. It may point to any kind of variable (but may not be dereferenced). Any other pointer type may be converted to void* and back again without loss of information. void* is often used as parameter types to, and return types from, library functions. • size_t is an unsigned integral type that should be used (rather than int) when expressing the size of “something” (e.g. an int, array, string, or struct). It too is often used as a parameter to, or return type from, library functions. size_t is defined as the type that is returned from the sizeof( ) operator.

  6. Pointers and Arrays • In C, there is a strong relationship between pointers and arrays. • Any array operation that can be accomplished with subscripting (using [ ]) can also be done with pointers • The declaration int a[10]; defines an array of 10 ints which is guaranteed to be a block of consecutive integers named a[0], a[1], ... a[9]. The notation a[ i ] refers the “i-th” element of the array. • If p is defined as a pointer to an int, then the assignmentp = &a[0]; sets p to point to the first element of the array. The assignment x = *p; will copy the contents of a[0] into x.

  7. More Pointers & Arrays • Since the name of an array is a synonym for the address of the first element of the array, the statement p = &a[0]; is equivalent to p = a; • If p points to a particular element of an array, then p + 1 points to the next element of the array and p + n points n elements after p, REGARDLESS of the type of the array. • The meaning a “adding 1 to a pointer” is that p + 1 points to the next element in the array.

  8. More Pointers and Arrays The expression a[ i ] can also be written as *(a + i). The two forms are equivalent. It follows then that &a[ i ] and (a + i) are also equivalent. (a + i) is the address of the i-th element beyond a. On the other hand, if p is a pointer, then it may be used with a subscript. p[ i ] is identical to *(p + i). In short, an array-and-index expression is equivalent to a pointer-and-offset expression.

  9. Pointer Arithmetic • Note in the last slide that no mention was made of the type of the array. Why not? Because it DOESN’T MATTER! • If “a” is an array of ints, then a[ k ] is the k-th int and so is *(a + k). • If “a” is an array of doubles, then a[ k ] is the k-th double and so is *(a + k). • Adding a constant, k, to a pointer actually adds k * sizeof(pointer type) to the value of the pointer. • This is one important reason why the type of a pointer must be specified when it’s defined. • We’ll see more on this later.

  10. ptrAdd.c int main() { char c, *cPtr = &c; int i, *iPtr = &i; double d, *dPtr = &d; printf("\nThe addresses of c, i and d are:\n"); printf("cPtr = %p, iPtr = %p, dPtr = %p\n", cPtr, iPtr, dPtr) ; cPtr = cPtr + 1 ; iPtr = iPtr + 1 ; dPtr = dPtr + 1 ; printf("\nThe values of cPtr, iPtr and dPtr are:\n") ; printf("cPtr = %p, iPtr = %p, dPtr = %p\n\n", cPtr, iPtr, dPtr) ; return 0; }

  11. Printing an Array The code below shows how to use an array name as a pointer void printStrings( char *strings[ ], int size ) { int i; for (i = 0; i < size; i++) printf( “%s\n”, *strings++); }

  12. Strings revisited Recall, in C, a string is represented as an array of characters terminated with a null (\0) character. As we’ve seen, arrays and pointers are closely related. A string constant may be declared as either a char[ ] or char* char hello[ ] = “Hello Bobby”; or equivalently char *hi = “Hello Bob”; A typedef could also be used to simplify coding; typedef char* STRING; STRING hi = “Hello Bob”; In any case, the compiler will calculate the size of the char array required for the string (including the null) and the null terminator will automatically be appended to the end of the string, There are many library functions written to manipulate strings and characters. See appendix section B2 and B3 in K & R for a detailed list of function signatures and purpose.

  13. Arrays of Pointers Since pointers are variable, we can create an array of pointers just like we can create any array of any other type. Although the pointers may point to any type, the most common use of an array of pointers is an array of char* to create an array of strings.

  14. PointerArray.c int main( ) { int *pInt[3]; /* an array of 3 "pointers to int */ int i, weight, height int *pAge = (int *)malloc( sizeof( int ) ); /* store some values */ weight = 185; height = 73; *pAge = 42; /* the pointers in the array can contain the address of local ** variables or malloc'd variables */ pInt[0] = pAge; pInt[1] = &height; pInt[2] = &weight; for (int i = 0; i < 3; i++) printf( "%d\n", *pInt[i]); return 0; }

  15. names: B o b b y \0 J i m \0 H a r o l d \0 Boy’s Names A common use of an array of pointers is to create an array of strings. The declaration below creates an initialized array of strings (char *) for some boy’s names. The diagram below illustrates the memory configuration. char *name[] = { /* let the compiler computer the size */“Bobby”, “Jim”, Harold” }; 0 1 2

  16. Pointers2Pointer.c int main ( ) { /* a double, a pointer to double, ** and a pointer to a pointer to a double */ double gpa = 3.25, *pgpa, **ppgpa; /* make pgpa point to the gpa */ pgpa = &gpa; /* make ppgpa point to pgpa (which points to gpa) */ ppgpa = &pgpa; /* all of these print 3.25 */ printf( "%0.2f, %0.2f, %0.2f", gpa, *pgpa, **ppgpa); return 0; }

  17. char* array vs. char** Because the name of an array is a synonym for the address of the first element of the array, the name of the array may be treated like a pointer to the array. As a result, the name of an array of strings (for example) may be defined as either char *[ ] or char**. For example, the boy’s name array char *name[] = { “Bobby”, “Jim”, Harold” }; may also be defined as char **name = { “Bobby”, “Jim”, Harold” }; In particular, the parameters for main may be written as either int main (int argc, char *argv[ ]) or int main( int argc, char **argv)

  18. Multidimensional Arrays C support multidimensional arrays using the standard [ ] indexing. The definition int a[5][4]; declares a rectangular, 2-d array with 5 rows, and 4 columns. Enough memory to hold 20 integers is allocated. An alternative definition is int *b[5];. This definition allocates enough memory for 5 int pointers for the rows. The pointers and the memory for the rows must be allocated an initialized. An advantage of this definition is that the rows need not be the same length. Brackets may still be used to access the elements.

  19. Command Line Arguments Command line arguments are passed to your program as parameters to main. int main( int argc, char *argv[ ] ) • argc is the number of command line arguments (and hence the size of argv) • argv is an array of strings (char *) which are the command line arguments. Note that argv[0] is always the name of your program. For example, typing myprog hello world 42 at the linux prompt results in • argc = 4 • argv[0] = “myprog” • argv[1] = “hello” • argv[2] = “world” • argv[3] = “42” Note that to use argv[3] as an integer, you must convert if from a string to an int using the library function atoi( ).E.g. int age = atoi( argv[3] );

  20. End of Class 5

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