ECE 103 Engineering Programming Chapter 41 C Pointers, Part 3 Herbert G. Mayer, PSU CS Status 6/4/2014 Initial content copied verbatim from ECE 103 material developed by Professor Phillip PSU ECE

Syllabus Value Parameters Reference Parameters Passing 1-D Arrays to Functions Revisited Returning a Pointer from a Function

2 Pass by Value and Pass by Reference Generally, C parameters are passed by value With the exception of array parameters, which are passed by reference C++ also introduces explicit reference parameter passing via the & specifier; not part of C Consequences of "pass by value":  The function receives copies of the arguments, which are stored in local parameter variables  The function can change the parameter values, but it cannot change the values of the original arguments

3 Sometimes, the value of a passed argument needs to be changeable from inside the function. Example:  To return more than a single value from a function.  To swap the values of two arguments. How can this be done if the C language only supports pass by value? Answer: Pass the memory address of a variable instead of the value contained in that variable!

4 By using pointers as function parameters, the original arguments can be modified indirectly. To declare a parameter that is a pointer: return_dtype fun_name (…, dtype * param_name, …) { /* Function body */ } This allows C to simulate pass by reference.

5 While arguments can be changed via pointers, it is not required to do so. Passing just the pointer to a large data block may improve space and time efficiency. Example: You pass a data structure 1 MB in size to a function. If passed as a normal argument, then a duplicate 1 megabyte block has to be created. If a pointer to the data block is passed instead, then just a few bytes (the address) are transferred.

6 If a pointer is passed to a function, but the argument must not be changed, then use the const modifier in the parameter declaration. Example: void fun1 (char *s) { /* Can change what s points to */ } void fun2 (const char *s) { /* Cannot change what s points to */ }

7 Example: Function definition: void myfun (int a, float *b, float *c, float **d, float **e) { … } ParameterDatatype of parameterExpected argument a intinteger value b pointer to floatmemory address c pointer to floatmemory address d pointer to pointer to floataddress of a variable that contains another address e pointer to pointer to floataddress of a variable that contains another address

8 Invoking the function: void myfun (int a, float *b, float *c, float **d, float **e); int x = 3;/* Simple variable */ float y = 1.0;/* Simple variable */ float *p = &y;/* p points to y */ float **q = &p;/* q points to p */ myfun(x, &y, p, &p, q); /* Invoke */ Matching arguments to parameters: x  int a &y  float *b p  float *c &p  float **d q  float **e

9 Example: /* Non-pointer parameters */ #include #define PI float circ (float R) { return 2*PI* R; } int main (void) { float radius = 0.75; float C; C = circ(radius); printf("%f\n", C); return 0; } /* Pointer parameters */ #include #define PI float circ (float *R) { return 2*PI* (*R); } int main (void) { float radius = 0.75; float C; C = circ(&radius); printf("%f\n", C); return 0; } Expects a value Retrieve value Pass value to functionExpects an address Retrieve value stored at address Pass address to function

10 Example: /* Non-pointer parameters */ #include double mag (double x, double y) { return (sqrt(x*x + y*y)); } double angle (double x, double y) { return (atan2(y,x)); } int main (void) { double m, a; double u = 1.0, v = 1.0; m = mag(u, v); a = angle(u, v); return 0; } /* Pointer parameters */ #include void mang (double x, double y, double *m, double *a) { *m = sqrt(x*x + y*y); *a = atan2(y,x); } int main (void) { double m, a; double u = 1.0, v = 1.0; mang(u, v, &m, &a); return 0; }

11 Example: /* Try to swap values of a and b */ #include /* This will not work */ void swap (int x, int y) { int temp; temp = x; x = y; y = temp; } int main (void) { int a = 1, b = 3; printf("Pre : a=%d b=%d\n", a, b); swap(a, b); printf("Post: a=%d b=%d\n", a, b); return 0; } Actual Output: Pre : a=1 b=3 Post: a=1 b=3

12 Example: /* Try to swap values of a and b */ #include /* This will work */ void swap (int *x, int *y) { int temp; temp = *x; *x = *y; *y = temp; } int main (void) { int a = 1, b = 3; printf("Pre : a=%d b=%d\n", a, b); swap(&a, &b); printf("Post: a=%d b=%d\n", a, b); return 0; } Actual Output: Pre : a=1 b=3 Post: a=3 b=1

13 Passing 1-D Arrays to Functions Revisited If a function expects a 1-D array argument, the corresponding parameter can be declared using either array notation or pointer notation. Example: void fun1 (double A[]) { … } void fun2 (double *A) { … } Both forms work if the argument is a 1-D array. double v[5]; fun1(v); /* Remember that v by itself */ fun2(v); /* is the address of v[0]. */

14 Returning a Pointer from a Function A function can return a value. It can also return a pointer to a value. What does it mean to return a pointer? Answer: A memory address is returned. Declaration: return_datatype * function_name (…) { /* Function body */ }

15 Example: #include /* Capitalizes a string */ char * capstr (char * s) { char *p = s; while (*p) *p++ = toupper((int)*p); return s; /* Address */ } int main (void) { char s[] = "Hello"; printf("Pre : %s\n", s); printf("Post1: %s\n", capstr(s)); printf("Post2: %s\n", s); return 0; } Actual Output: Pre : Hello Post1: HELLO Post2: HELLO

16 Example: #include int * fun1 (int *s) { return s; } int * fun2 (int s) { return &s; } int main (void) { int x; printf("%p\n", &x); printf("%p\n", fun1(&x)); printf("%p\n", fun2(x)); return 0; } Actual Output: 0x22cd64 ← &x 0x22cd64 ← fun1(&x) 0x22cd40 ← fun2(x) fun1 Notes: The parameter *s is local to fun1. It contains the address that was passed to it (i.e., &x ), which is also the address that will be returned. fun2 Notes: The parameter s is local to fun2. When the address &s is returned, it is the address of the local parameter s, and not of the argument that was passed to fun2. When a function call is done, local variables defined in the function will no longer exist, and the memory addresses of those local variables become invalid.