Parameter passing. Programming in C. Important. Parameter passing... C implements call-by-value parameter passing. UVic SEng 265

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1 Parameter passing Programming in C UVic SEng 265 Daniel M. German Department of Computer Science University of Victoria 1 SEng 265 dmgerman@uvic.ca C implements call-by-value parameter passing int a = 5; int b = 10; int c; c = maxint(a, b); /* actual parameters */... int maxint(int m, int n) /* formal parameters */ if (m > n) return m; else return n; 2 SEng 265 dmgerman@uvic.ca Parameter passing... Important Call-by-value semantics copies actual parameters into formal parameters. Fact: assignments to formal parameters are not propagated to the actual parameters int power2(float f) if (f > /* some maximum number */) return -1; /* cannot square without overflow */ else f = f*f; return 1; float g = 4.0; power2(g); printf("%f\n", g); 3 SEng 265 dmgerman@uvic.ca Remember that: all variables are data all data resides in memory every memory location has an address Pointer variables: is a pointer to a memory location contains the address of another variable (other variable possibly anonymous) A pointer can be used for call-by-reference 4 SEng 265 dmgerman@uvic.ca

2 Addresses, Pointers Notation Compare int x = 1; int y = x; x = 2; printf("y is %d\n", y); int x = 1; int *y = &x; x = 2; printf("*y is %d\n", *y); In other words, x is the same as *&x. & before a variable means get the memory location of this variable (reference) * before a any use of a variable means get the contents of memory location stored in this variable (dereference) Note that * may appear in a variable declaration and in a variable s use; however, it has a slightly different meaning in each case. float f = 30.0; float *g; g = &f; printf("%d %d\n", f, *g); 5 SEng 265 dmgerman@uvic.ca 6 SEng 265 dmgerman@uvic.ca Pointers Example Why pointers? call-by-value works well for passing in parameters, but what if we want values to be modified in the called function? functions can only return a single value in return statements call-by-reference semantics gets around the limitation of returning a single value. Swap function: void swap(int a, int b) int temp = a; a = b; b = temp; int x = 2; int y = 1; swap(x, y); printf("x == %d, y == %d\n", x, y); x == 2, y == 1 7 SEng 265 dmgerman@uvic.ca 8 SEng 265 dmgerman@uvic.ca

3 Example... Invalid pointers Why did the values not get swapped? The integers a and b were swapped within the scope of swap(), but they were copies of the values of the x and y Using pointers void swap (int *a, int *b) int tmp = *a; *a = *b; *b = tmp; swap(&x, &y); As a pointer contains a variable s memory address, we must ensure the address is valid C does not check the validity of a pointer If you point to garbage (or to nothing), you will get garbage, or crash your program On Sun architecture you get this error: segmentation fault int *x; /* x points to what? */ x = NULL; /* garbage */ printf("%d\n", *x); Segmentation fault (core dumped) 9 SEng 265 dmgerman@uvic.ca (NULL is defined in stdio.h ) 10 SEng 265 dmgerman@uvic.ca Notational caveats Pointers and arrays Pointer variables are declared by specifying the type of object they point to: Recall that arrays are an aggregate data type where each data element has the same type: int *a; float *f; char *st[10]; /* array of pointers*/ int grades[10]; struct date_record info[50]; char buffer[100]; The declaration tells the compiler the kind of object the pointer points to all elements in an array occupy contiguous memory locations to get the address of any data element, we use &: 5th element of grades : &grades[4] 1st element of info: &info[0] last element of buffer : &buffer[99] 11 SEng 265 dmgerman@uvic.ca 12 SEng 265 dmgerman@uvic.ca

4 Pointers and arrays... Strings In C, an array variable name without the subscript represents the address of the first element char buffer[100]; char *cursor; cursor = &buffer[0]; cursor = buffer; *cursor = a ; buffer[0] = a ; *buffer = a ; A C string is an array of characters terminated by the null symbol (character value 0 n0, not 0 ) Pointers to character arrays (i.e., strings) are extensively used in C Character arrays must be large enough to store the entire string including the null terminator char buffer[100] = "Hello, World!"; what is contained in the memory allocated for buffer[100] after assigning it the above string? 13 SEng 265 dmgerman@uvic.ca 14 SEng 265 dmgerman@uvic.ca Strings... Functions and pointers In C, we can manipulate pointers in many ways. Assuming char buffer[100] char *cp = buffer same as char *cp = &buffer[0] cp + n same as &buffer[n] *(cp + n) same as buffer[n] ++cp same as cp = cp +1 *++cp same as cp = cp + 1, *cp As seen earlier, functions can accept pointer variables as parameters Functions may also return memory addresses as results int *max_with_ptr(int *a, int *b) if (*a > *b) return a; else return b; 15 SEng 265 dmgerman@uvic.ca 16 SEng 265 dmgerman@uvic.ca

5 Recapping A pointer contains a memory address This memory address can point to: a variable a function (we will not use them in this course) A * in front of a pointer variable indicates a dereference A & in front of a variable, denotes a reference 17 SEng 265 dmgerman@uvic.ca