Want a quick introduction to C?

  • Keep reading for the quick crash-course to C Programming below
  • Then see the [[C Gotchas wiki page|C Programming, Part 3: Common Gotchas]].
  • And learn about [[text I/O|C Programming, Part 2: Text Input And Output]].
  • Kick back relax with Lawrence's intro videos (Also there is a virtual machine-in-a-browser you can play with!)

External resources

Crash course intro to C

Warning new page Please fix typos and formatting mistakes for me and add useful links too.*

How do you write a complete hello world program in C?

#include <stdio.h>
int main(void) { 
    printf("Hello World\n");
    return 0; 
}

Why do we use '#include <stdio.h>'?

We're lazy! We don't want to declare the printf function. It's already done for us inside the file 'stdio.h'. The #include includes the text of the file as part of our file to be compiled.

Specifically, the #include directive takes the file stdio.h (which stands for standard input and output) located somewhere in your operating system, copies the text, and substitutes it where the #include was.

How are C strings represented?

They are represented as characters in memory. The end of the string includes a NULL (0) byte. So "ABC" requires four(4) bytes ['A','B','C','\0']. The only way to find out the length of a C string is to keep reading memory until you find the NULL byte. C characters are always exactly one byte each.

When you write a string literal "ABC" in an expression the string literal evaluates to a char pointer (char *), which points to the first byte/char of the string. This means ptr in the example below will hold the memory address of the first character in the string.

char *ptr = "ABC"

How do you declare a pointer?

A pointer refers to a memory address. The type of the pointer is useful - it tells the compiler how many bytes need to be read/written. You can declare a pointer as follows.

int *ptr1;
char *ptr2;

Due to C's grammar, a int* or any pointer is not actually its own type. You have to precede each pointer variable with an asterisk. As a common gotcha, the following

int* ptr3, ptr4;

Will only declare *ptr3 as a pointer. ptr4 will actually be a regular int variable. To fix this declaration, keep the * preceding to the pointer

int *ptr3, *ptr4;

How do you use a pointer to read/write some memory?

Let's say that we declare a pointer int *ptr. For the sake of discussion, let's say that ptr points to memory address 0x1000. If we want to write to a pointer, we can deference and assign *ptr.

*ptr = 0; // Writes some memory.

What C will do is take the type of the pointer which is an int and writes sizeof(int) bytes from the start of the pointer, meaning that bytes 0x1000, 0x1004, 0x1008, 0x100a will all be zero. The number of bytes written depends on the pointer type. It is the same for all primitive types but structs are a little different.

What is pointer arithmetic?

You can add an integer to a pointer. However the pointer type is used to determine how much to increment the pointer. For char pointers this is trivial because characters are always one byte:

char *ptr = "Hello"; // ptr holds the memory location of 'H'
ptr += 2; //ptr now points to the first'l'

If an int is 4 bytes then ptr+1 points to 4 bytes after whatever ptr is pointing at.

char *ptr = "ABCDEFGH";
int *bna = (int *) ptr;
bna +=1; // Would cause iterate by one integer space (i.e 4 bytes on some systems)
ptr = (char *) bna;
printf("%s", ptr);
/* Notice how only 'EFGH' is printed. Why is that? Well as mentioned above, when performing 'bna+=1' we are increasing the **integer** pointer by 1, (translates to 4 bytes on most systems) which is equivalent to 4 characters (each character is only 1 byte)*/
return 0;

Because pointer arithmetic in C is always automatically scaled by the size of the type that is pointed to, you can't perform pointer arithmetic on void pointers.

You can think of pointer arithmetic in C as essentially doing the following

If I want to do

int *ptr1 = ...;
int *offset = ptr1 + 4;

Think

int *ptr1 = ...;
char *temp_ptr1 = (char*) ptr1;
int *offset = (int*)(temp_ptr1 + sizeof(int)*4);

To get the value. Every time you do pointer arithmetic, take a deep breath and make sure that you are shifting over the number of bytes you think you are shifting over.

What is a void pointer?

A pointer without a type (very similar to a void variable). Void pointers are used when either a datatype you're dealing with is unknown or when you're interfacing C code with other programming languages. You can think of this as a raw pointer, or just a memory address. You cannot directly read or write to it because the void type does not have a size. For Example

void *give_me_space = malloc(10);
char *string = give_me_space;

This does not require a cast because C automatically promotes void* to its appropriate type. Note:

gcc and clang are not total ISO-C compliant, meaning that they will let you do arithmetic on a void pointer. They will treat it as a char pointer but do not do this because it may not work with all compilers!

Does printf call write or does write call printf?

printf calls write. printf includes an internal buffer so, to increase performance printf may not call write everytime you call printf. printf is a C library function. write is a system call and as we know system calls are expensive. On the other hand printf uses a buffer which suits our needs better at that point

How do you print out pointer values? integers? strings?

Use format specifiers "%p" for pointers, "%d" for integers and "%s" for Strings. A full list of all of the format specifiers is found here

Example of integer:

int num1 = 10;
printf("%d", num1); //prints num1

Example of integer pointer:

int *ptr = (int *) malloc(sizeof(int));
*ptr = 10;
printf("%p\n", ptr); //prints the address pointed to by the pointer
printf("%p\n", &ptr); /*prints the address of pointer -- extremely useful
when dealing with double pointers*/
printf("%d", *ptr); //prints the integer content of ptr
return 0;

Example of string:

char *str = (char *) malloc(256 * sizeof(char));
strcpy(str, "Hello there!");
printf("%p\n", str); // print the address in the heap
printf("%s", str);
return 0;

Strings as Pointers & Arrays @ BU

How would you make standard out be saved to a file?

Simplest way: run your program and use shell redirection e.g.

./program > output.txt

#To read the contents of the file,
cat output.txt

More complicated way: close(1) and then use open to re-open the file descriptor. See [[http://cs-education.github.io/sys/#chapter/0/section/3/activity/0]]

What's the difference between a pointer and an array? Give an example of something you can do with one but not the other.

char ary[] = "Hello";
char *ptr = "Hello";

Example

The array name points to the first byte of the array. Both ary and ptr can be printed out:

char ary[] = "Hello";
char *ptr = "Hello";
// Print out address and contents
printf("%p : %s\n", ary, ary);
printf("%p : %s\n", ptr, ptr);

The array is mutable, so we can change its contents (be careful not to write bytes beyond the end of the array though). Fortunately 'World' is no longer than 'Hello"

In this case, the char pointer ptr points to some read only memory (where the statically allocated string literal is stored), so we cannot change those contents.

strcpy(ary, "World"); // OK
strcpy(ptr, "World"); // NOT OK - Segmentation fault (crashes)

We can, however, unlike the array, we change ptr to point to another piece of memory,

ptr = "World"; // OK!
ptr = ary; // OK!
ary = (..anything..) ; // WONT COMPILE
// ary is doomed to always refer to the original array.
printf("%p : %s\n", ptr, ptr);
strcpy(ptr, "World"); // OK because now ptr is pointing to mutable memory (the array)

What to take away from this is that pointers * can point to any type of memory while C arrays [] can only point to memory on the stack. In a more common case, pointers will point to heap memory in which case the memory referred to by the pointer CAN be modified.

sizeof() returns the number of bytes. So using above code, what is sizeof(ary) and sizeof(ptr)?

sizeof(ary): ary is an array. Returns the number of bytes required for the entire array (5 chars + zero byte = 6 bytes) sizeof(ptr): Same as sizeof(char *). Returns the number bytes required for a pointer (e.g. 4 or 8 for a 32 bit or 64 bit machine)

sizeof is a special operator. Really it's something the compiler substitutes in before compiling the program because the size of all types is known at compile time. When you have sizeof(char*) that takes the size of a pointer on your machine (8 bytes for a 64 bit machine and 4 for a 32 bit and so on). When you try sizeof(char[]), the compiler looks at that and substitutes the number of bytes that the entire array contains because the total size of the array is known at compile time.

char str1[] = "will be 11";
char* str2 = "will be 8";
sizeof(str1) //11 because it is an array
sizeof(str2) //8 because it is a pointer

Be careful, using sizeof for the length of a string!

Which of the following code is incorrect or correct and why?

int* f1(int *p) {
    *p = 42;
    return p;
} // This code is correct;

char* f2() {
    char p[] = "Hello";
    return p;
} // Incorrect!

Explanation: An array p is created on the stack for the correct size to hold H,e,l,l,o, and a null byte i.e. (6) bytes. This array is stored on the stack and is invalid after we return from f2.

char* f3() {
    char *p = "Hello";
    return p;
} // OK

Explanation: p is a pointer. It holds the address of the string constant. The string constant is unchanged and valid even after f3 returns.

char* f4() {
    static char p[] = "Hello";
    return p;
} // OK

Explanation: The array is static meaning it exists for the lifetime of the process (static variables are not on the heap or the stack).

How do you look up information C library calls and system calls?

Use the man pages. Note the man pages are organized into sections. Section 2 = System calls. Section 3 = C libraries. Web: Google "man7 open" shell: man -S2 open or man -S3 printf

How do you allocate memory on the heap?

Use malloc. There's also realloc and calloc. Typically used with sizeof. e.g. enough space to hold 10 integers

int *space = malloc(sizeof(int) * 10);

What's wrong with this string copy code?

void mystrcpy(char*dest, char* src) { 
  // void means no return value   
  while( *src ) { dest = src; src ++; dest++; }  
}

In the above code it simply changes the dest pointer to point to source string. Also the nuls bytes are not copied. Here's a better version - 

  while( *src ) { *dest = *src; src ++; dest++; } 
  *dest = *src;

Note it's also usual to see the following kind of implementation, which does everything inside the expression test, including copying the nul byte.

  while( (*dest++ = *src++ )) {};

How do you write a strdup replacement?

// Use strlen+1 to find the zero byte... 
char* mystrdup(char*source) {
   char *p = (char *) malloc ( strlen(source)+1 );
   strcpy(p,source);
   return p;
}

How do you unallocate memory on the heap?

Use free!

int *n = (int *) malloc(sizeof(int));
*n = 10;
//Do some work
free(n);

What is double free error? How can you avoid? What is a dangling pointer? How do you avoid?

A double free error is when you accidentally attempt to free the same allocation twice.

int *p = malloc(sizeof(int));
free(p);

*p = 123; // Oops! - Dangling pointer! Writing to memory we don't own anymore

free(p); // Oops! - Double free!

The fix is firstly to write correct programs! Secondly, it's good programming hygiene to reset pointers once the memory has been freed. This ensures the pointer cant be used incorrectly without the program crashing.

Fix:

p = NULL; // Now you can't use this pointer by mistake

What is an example of buffer overflow?

Famous example: Heart Bleed (performed a memcpy into a buffer that was of insufficient size). Simple example: implement a strcpy and forget to add one to strlen, when determining the size of the memory required.

What is 'typedef' and how do you use it?

Declares an alias for a type. Often used with structs to reduce the visual clutter of having to write 'struct' as part of the type.

typedef float real; 
real gravity = 10;
// Also typedef gives us an abstraction over the underlying type used. 
// For example in the future we only need to change this typedef if we
// wanted our physics library to use doubles instead of floats.

typedef struct link link_t; 
//With structs, include the keyword 'struct' as part of the original types

In this class, we regularly typedef functions. A typedef for a function can be this for example

typedef int (*comparator)(void*,void*);

int greater_than(void* a, void* b){
    return a > b;
}
comparator gt = greater_than;

This declares a function type comparator that accepts two void* params and returns an integer.

Wow that was a lot of C

Don't worry more to come!