UNDERSTANDING POINTERS (for beginners) by Ted Jensen Version 0.0 This material is hereby placed in the public domain. September 5, 1993 TABLE OF CONTENTS INTRODUCTION; CHAPTER 1: What is a pointer? CHAPTER 2: Pointer types and Arrays CHAPTER 3: Pointers and Strings CHAPTER 4: More on Strings CHAPTER 5: Pointers and Structures CHAPTER 6: Some more on Strings, and Arrays of Strings EPILOG: ================================================================== INTRODUCTION: Over a period of several years of monitoring various telecommunication conferences on C I have noticed that one of the most difficult problems for beginners was the understanding of pointers. After writing dozens of short messages in attempts to clear up various fuzzy aspects of dealing with pointers, I set up a series of messages arranged in "chapters" which I could draw from or email to various individuals who appeared to need help in this area. Recently, I posted all of this material in the FidoNet CECHO conference. It received such a good acceptance, I decided to clean it up a little and submit it for inclusion in Bob Stout's SNIPPETS file. It is my hope that I can find the time to expand on this text in the future. To that end, I am hoping that those who read this and find where it is lacking, or in error, or unclear, would notify me of same so the next version, should there be one, I can correct these deficiencys. It is impossible to acknowledge all those whose messages on pointers in various nets contributed to my knowledge in this area. So, I will just say Thanks to All. I frequent the CECHO on FidoNet via RBBSNet and can be contacted via the echo itself or by email at: RBBSNet address 8:916/1. I can also be reached via Internet email at ted.jensen@spacebbs.com Or Ted Jensen P.O. Box 324 Redwood City, CA 94064 ================================================================== CHAPTER 1: What is a pointer? One of the things beginners in C find most difficult to understand is the concept of pointers. The purpose of this document is to provide an introduction to pointers and their use to these beginners. I have found that often the main reason beginners have a problem with pointers is that they have a weak or minimal feeling for variables, (as they are used in C). Thus we start with a discussion of C variables in general. A variable in a program is something with a name, the value of which can vary. The way the compiler and linker handles this is that it assigns a specific block of memory within the computer to hold the value of that variable. The size of that block depends on the range over which the variable is allowed to vary. For example, on PC's the size of an integer variable is 2 bytes, and that of a long integer is 4 bytes. In C the size of a variable type such as an integer need not be the same on all types of machines. When we declare a variable we inform the compiler of two things, the name of the variable and the type of the variable. For example, we declare a variable of type integer with the name k by writing: int k; On seeing the "int" part of this statement the compiler sets aside 2 bytes (on a PC) of memory to hold the value of the integer. It also sets up a symbol table. And in that table it adds the symbol k and the address in memory where those 2 bytes were set aside. Thus, later if we write: k = 2; at run time we expect that the value 2 will be placed in that memory location reserved for the storage of the value of k. In a sense there are two "values" associated with k, one being the value of the integer stored there (2 in the above example) and the other being the "value" of the memory location where it is stored, i.e. the address of k. Some texts refer to these two values with the nomenclature rvalue (right value, pronounced "are value") and lvalue (left value, pronunced "el value") respectively. The lvalue is the value permitted on the left side of the assignment operator '=' (i.e. the address where the result of evaluation of the right side ends up). The rvalue is that which is on the right side of the assignment statment, the '2' above. Note that rvalues cannot be used on the left side of the assignment statement. Thus: 2 = k; is illegal. Okay, now consider: int j, k; k = 2; j = 7; <-- line 1 k = j; <-- line 2 In the above, the compiler interprets the j in line 1 as the address of the variable j (its lvalue) and creates code to copy the value 7 to that address. In line 2, however, the j is interpreted as its rvalue (since it is on the right hand side of the assignment operator '='). That is, here the j refers to the value _stored_ at the memory location set aside for j, in this case 7. So, the 7 is copied to the address designated by the lvalue of k. In all of these examples, we are using 2 byte integers so all copying of rvalues from one storage location to the other is done by copying 2 bytes. Had we been using long integers, we would be copying 4 bytes. Now, let's say that we have a reason for wanting a variable designed to hold an lvalue (an address). The size required to hold such a value depends on the system. On older desk top computers with 64K of memory total, the address of any point in memory can be contained in 2 bytes. Computers with more memory would require more bytes to hold an address. Some computers, such as the IBM PC might require special handling to hold a segment and offset under certain circumstances. The actual size required is not too important so long as we have a way of informing the compiler that what we want to store is an address. Such a variable is called a "pointer variable" (for reasons which will hopefully become clearer a little later). In C when we define a pointer variable we do so by preceding its name with an asterisk. In C we also give our pointer a type which, in this case, refers to the type of data stored at the address we will be storing in our pointer. For example, consider the variable definition: int *ptr; ptr is the _name_ of our variable (just as 'k' was the name of our integer variable). The '*' informs the compiler that we want a pointer variable, i.e. to set aside however many bytes is required to store an address in memory. The "int" says that we intend to use our pointer variable to store the address of an integer. Such a pointer is said to "point to" an integer. Note, however, that when we wrote "int k;" we did not give k a value. If this definiton was made outside of any function many compilers will initialize it to zero. Simlarly, ptr has no value, that is we haven't stored an address in it in the above definition. In this case, again if the definition is outside of any function, it is intialized to a value #defined by your compiler as NULL. It is called a NULL pointer. While in most cases NULL is #defined as zero, it need not be. That is, different compilers handle this differently. Also note that while zero is an integer, NULL need not be. But, back to using our new variable ptr. Suppose now that we want to store in ptr the address of our integer variable k. To do this we use the unary '&' operator and write: ptr = &k; What the '&' operator does is retrieve the lvalue (address) of k, even though k is on the right hand side of the assignment operator '=', and copies that to the contents of our pointer ptr. Now, ptr is said to "point to" k. Bear with us now, there is only one more operator we need to discuss. The "dereferencing operator" is the asterisk and it is used as follows: *ptr = 7; will copy 7 to the address pointed to by ptr. Thus if ptr "points to" (contains the address of) k, the above statement will set the value of k to 7. That is, when we use the '*' this way we are refering to the value of that which ptr is pointing at, not the value of the pointer itself. Similarly, we could write: printf("%d\n",*ptr); to print to the screen the integer value stored at the address pointed to by "ptr". One way to see how all this stuff fits together would be to run the following program and then review the code and the output carefully. ------------------------------------------------- #include <stdio.h> int j, k; int *ptr; int main(void) { j = 1; k = 2; ptr = &k; printf("\n"); printf("j has the value %d and is stored at %p\n",j,&j); printf("k has the value %d and is stored at %p\n",k,&k); printf("ptr has the value %p and is stored at %p\n",ptr,&ptr); printf("The value of the integer pointed to by ptr is %d\n", *ptr); return 0; } --------------------------------------- To review: A variable is defined by giving it a type and a name (e.g. int k;) A pointer variable is defined by giving it a type and a name (e.g. int *ptr) where the asterisk tells the compiler that the variable named ptr is a pointer variable and the type tells the compiler what type the pointer is to point to (integer in this case). Once a variable is defined, we can get its address by preceding its name with the unary '&' operator, as in &k. We can "dereference" a pointer, i.e. refer to the value of that which it points to, by using the unary '*' operator as in *ptr. An "lvalue" of a variable is the value of its address, i.e. where it is stored in memory. The "rvalue" of a variable ...
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