iskm/learnxinyminutes-docs
1
0
Fork 0
mirror of https://github.com/adambard/learnxinyminutes-docs.git synced 2024-12-23 09:41:36 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

Update c.html.markdown

Browse Source
This commit is contained in:
Árpád Goretity  2013-08-15 12:30:22 +02:00
parent bc614fda28
commit 24fd870589
1 changed files with 355 additions and 272 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

169
c.html.markdown
View File

@ -5,19 +5,20 @@ language: c
filename: learnc.c
contributors:
- ["Adam Bard", "http://adambard.com/"]
- ["Árpád Goretity", "http://twitter.com/h2co3_ios"]
---
Ah, C. Still the language of modern high-performance computing.
Ah, C. Still **the** language of modern high-performance computing.
C is the lowest-level language most programmers will ever use, but
it more than makes up for it with raw speed. Just be aware of its manual
memory management and C will take you as far as you need to go.
```c
// Single-line comments start with //
// Single-line comments start with // - only available in C99 and later.
/*
Multi-line comments look like this.
Multi-line comments look like this. They work in C89 as well.
*/
// Import headers with #include
@ -25,6 +26,11 @@ Multi-line comments look like this.
#include <stdio.h>
#include <string.h>
// file names between <angle brackets> are headers from the C standard library.
// They are searched for by the preprocessor in the system include paths
// (usually /usr/lib on Unices, can be controlled with the -I<dir> option if you are using GCC or clang.)
// For your
// Declare function signatures in advance in a .h file, or at the top of
// your .c file.
void function_1();
@ -33,7 +39,6 @@ void function_2();
// Your program's entry point is a function called
// main with an integer return type.
int main() {
// print output using printf, for "print formatted"
// %d is an integer, \n is a newline
printf("%d\n", 0); // => Prints 0
@ -76,11 +81,22 @@ unsigned short ux_short;
unsigned int ux_int;
unsigned long long ux_long_long;
// Other than char, which is always 1 byte, these types vary in size depending
// on your machine. sizeof(T) gives you the size of a variable with type T in
// Other than char, which is always 1 byte (but not necessarily 8 bits!),
// these types vary in size depending on your machine and compiler.
// sizeof(T) gives you the size of a variable with type T in
// bytes so you can express the size of these types in a portable way.
// sizeof(obj) yields the size of an actual expression (variable, literal, etc.).
// For example,
printf("%lu\n", sizeof(int)); // => 4 (on machines with 4-byte words)
printf("%zu\n", sizeof(int)); // => 4 (on most machines with 4-byte words)
// It's worth noting that if the argument of the `sizeof` operator is not a type but an expression,
// then its argument is not evaluated except VLAs (see below). Also, `sizeof()` is an operator, not a function,
// furthermore, the value it yields is a compile-time constant (except when used on VLAs, again.)
int a = 1;
size_t size = sizeof(a++); // a++ is not evaluated
printf("sizeof(a++) = %zu where a = %d\n", size, a);
// the above code prints "sizeof(a++) = 4 where a = 1" (on a usual 32-bit architecture)
// Arrays must be initialized with a concrete size.
char my_char_array[20]; // This array occupies 1 * 20 = 20 bytes
@ -99,6 +115,19 @@ my_array[0]; // => 0
my_array[1] = 2;
printf("%d\n", my_array[1]); // => 2
// In C99 (and as an optional feature in C11), variable-length arrays (VLAs) can be declared as well.
// The size of such an array need not be a compile time constant:
printf("Enter the array size: "); // ask the user for an array size
char buf[0x100];
fgets(buf, sizeof buf, stdin);
size_t size = strtoul(buf, NULL, 10); // strtoul parses a string to an unsigned integer
int var_length_array[size]; // declare the VLA
printf("sizeof array = %zu\n", sizeof var_length_array);
// A possible outcome of this program may be:
Enter the array size: 10
sizeof array = 40
// Strings are just arrays of chars terminated by a NUL (0x00) byte,
// represented in strings as the special character '\0'.
// (We don't have to include the NUL byte in string literals; the compiler
@ -109,11 +138,19 @@ printf("%s\n", a_string); // %s formats a string
/*
You may have noticed that a_string is only 16 chars long.
Char #17 is the NUL byte.
Chars #18, 19 and 20 have undefined values.
Chars #18, 19 and 20 are 0 as well - if an initializer list (in this case, the string literal)
has less elements than the array it is initializing, then excess array elements are implicitly
initialized to zero. This is why int ar[10] = { 0 } works as expected intuitively.
*/
printf("%d\n", a_string[16]); // => 0
// So string literals are strings enclosed within double quotes, but if we have characters
// between single quotes, that's a character literal.
// It's of type `int`, and *not* `char` (for hystorical reasons).
int cha = 'a'; // fine
char chb = 'a'; // fine too (implicit conversion from int to char - truncation)
///////////////////////////////////////
// Operators
///////////////////////////////////////
@ -127,7 +164,7 @@ i2 - i1; // => 1
i2 * i1; // => 2
i1 / i2; // => 0 (0.5, but truncated towards 0)
f1 / f2; // => 0.5, plus or minus epsilon
f1 / f2; // => 0.5, plus or minus epsilon - floating-point numbers and calculations are not exact
// Modulo is there as well
11 % 3; // => 2
@ -135,7 +172,7 @@ f1 / f2; // => 0.5, plus or minus epsilon
// Comparison operators are probably familiar, but
// there is no boolean type in c. We use ints instead.
// 0 is false, anything else is true. (The comparison
// operators always return 0 or 1.)
// operators always yield 0 or 1.)
3 == 2; // => 0 (false)
3 != 2; // => 1 (true)
3 > 2; // => 1
@ -152,7 +189,7 @@ f1 / f2; // => 0.5, plus or minus epsilon
0 || 0; // => 0
// Bitwise operators!
~0x0F; // => 0xF0 (bitwise negation)
~0x0F; // => 0xF0 (bitwise negation, "1's complement")
0x0F & 0xF0; // => 0x00 (bitwise AND)
0x0F | 0xF0; // => 0xFF (bitwise OR)
0x04 ^ 0x0F; // => 0x0B (bitwise XOR)
@ -174,7 +211,7 @@ if (0) {
// While loops exist
int ii = 0;
while (ii < 10) {
printf("%d, ", ii++); // ii++ increments ii in-place, after using its value.
printf("%d, ", ii++); // ii++ increments ii in-place, after yielding its value ("postincrement").
} // => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
printf("\n");
@ -182,7 +219,7 @@ printf("\n");
int kk = 0;
do {
printf("%d, ", kk);
} while (++kk < 10); // ++kk increments kk in-place, before using its value
} while (++kk < 10); // ++kk increments kk in-place, and yields the already incremented value ("preincrement")
// => prints "0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "
printf("\n");
@ -200,7 +237,7 @@ printf("\n");
///////////////////////////////////////
// Every value in C has a type, but you can cast one value into another type
// if you want.
// if you want (with some constraints).
int x_hex = 0x01; // You can assign vars with hex literals
@ -210,7 +247,11 @@ printf("%d\n", (short) x_hex); // => Prints 1
printf("%d\n", (char) x_hex); // => Prints 1
// Types will overflow without warning
printf("%d\n", (char) 257); // => 1 (Max char = 255)
printf("%d\n", (unsigned char) 257); // => 1 (Max char = 255 if char is 8 bits long)
// printf("%d\n", (unsigned char) 257); would be undefined behavior - `char' is usually signed
// on most modern systems, and signed integer overflow invokes UB.
// Also, for determining the maximal value of a `char`, a `signed char` and an `unisigned char`,
// respectively, use the CHAR_MAX, SCHAR_MAX and UCHAR_MAX macros from <limits.h>
// Integral types can be cast to floating-point types, and vice-versa.
printf("%f\n", (float)100); // %f formats a float
@ -226,20 +267,21 @@ printf("%d\n", (char)100.0);
// of your variables, then mess with them.
int x = 0;
printf("%p\n", &x); // Use & to retrieve the address of a variable
// (%p formats a pointer)
printf("%p\n", (void *)&x); // Use & to retrieve the address of a variable
// (%p formats an object pointer of type void *)
// => Prints some address in memory;
// Pointers start with * in their declaration
int *px, not_a_pointer; // px is a pointer to an int
px = &x; // Stores the address of x in px
printf("%p\n", px); // => Prints some address in memory
printf("%d, %d\n", (int)sizeof(px), (int)sizeof(not_a_pointer));
// => Prints "8, 4" on 64-bit system
printf("%p\n", (void *)px); // => Prints some address in memory
printf("%zu, %zu\n", sizeof(px), sizeof(not_a_pointer));
// => Prints "8, 4" on a typical 64-bit system
// To retreive the value at the address a pointer is pointing to,
// put * in front to de-reference it.
// Note: yes, it may be confusing that '*' is used for _both_ declaring a pointer and dereferencing it.
printf("%d\n", *px); // => Prints 0, the value of x, which is what px is pointing to the address of
// You can also change the value the pointer is pointing to.
@ -258,38 +300,55 @@ for (xx=0; xx<20; xx++) {
// Declare a pointer of type int and initialize it to point to x_array
int* x_ptr = x_array;
// x_ptr now points to the first element in the array (the integer 20).
// This works because arrays are actually just pointers to their first element.
// This works because arrays often decay into pointers to their first element.
// For example, when an array is passed to a function or is assigned to a pointer,
// it decays into (implicitly converted to) a pointer.
// Exceptions: when the array is the argument of the `&` (address-od) operator:
int arr[10];
int (*ptr_to_arr)[10] = &arr; // &arr is NOT of type `int *`! It's of type "pointer to array" (of ten `int`s).
// or when the array is a string literal used for initializing a char array:
char arr[] = "foobarbazquirk";
// or when it's the argument of the `sizeof` or `alignof` operator:
int arr[10];
int *ptr = arr; // equivalent with int *ptr = &arr[0];
printf("%zu %zu\n", sizeof arr, sizeof ptr); // probably prints "40, 4" or "40, 8"
// Arrays are pointers to their first element
printf("%d\n", *(x_ptr)); // => Prints 20
printf("%d\n", x_array[0]); // => Prints 20
// Pointers are incremented and decremented based on their type
// (this is called pointer arithmetic)
printf("%d\n", *(x_ptr + 1)); // => Prints 19
printf("%d\n", x_array[1]); // => Prints 19
// You can also dynamically allocate contiguous blocks of memory with the
// standard library function malloc, which takes one integer argument
// representing the number of bytes to allocate from the heap.
int* my_ptr = (int*) malloc(sizeof(int) * 20);
// standard library function malloc, which takes one argument of type size_t
// representing the number of bytes to allocate (usually from the heap, although this
// may not be true on e. g. embedded systems - the C standard says nothing about it).
int *my_ptr = malloc(sizeof(*my_ptr) * 20);
for (xx = 0; xx < 20; xx++) {
*(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx would also work here
*(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx would also work here, and it's also more readable
} // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
// Dereferencing memory that you haven't allocated gives
// unpredictable results
printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what?
// "unpredictable results" - the program is said to invoke "undefined behavior"
printf("%d\n", *(my_ptr + 21)); // => Prints who-knows-what? It may even crash.
// When you're done with a malloc'd block of memory, you need to free it,
// or else no one else can use it until your program terminates
// (this is called a "memory leak"):
free(my_ptr);
// Strings can be char arrays, but are usually represented as char
// pointers:
char* my_str = "This is my very own string";
// Strings are arrays of char, but they are usually represented as a
// pointer-to-char (which is a pointer to the first element of the array).
// It's good practice to use `const char *' when referring to a string literal,
// since string literals shall not be modified (i. e. "foo"[0] = 'a' is ILLEGAL.)
const char *my_str = "This is my very own string literal";
printf("%c\n", *my_str); // => 'T'
// This is not the case if the string is an array (potentially initialized with a string literal)
// that resides in writable memory, as in:
char foo[] = "foo";
foo[0] = 'a'; // this is legal, foo now contains "aoo"
function_1();
} // end main function
@ -300,7 +359,8 @@ function_1();
// Function declaration syntax:
// <return type> <function name>(<args>)
int add_two_ints(int x1, int x2){
int add_two_ints(int x1, int x2)
{
return x1 + x2; // Use return to return a value
}
@ -312,9 +372,11 @@ Example: in-place string reversal
*/
// A void function returns no value
void str_reverse(char* str_in){
void str_reverse(char *str_in)
{
char tmp;
int ii=0, len = strlen(str_in); // Strlen is part of the c standard library
int ii = 0;
size_t len = strlen(str_in); // `strlen()` is part of the c standard library
for (ii = 0; ii < len / 2; ii++) {
tmp = str_in[ii];
str_in[ii] = str_in[len - ii - 1]; // ii-th char from end
@ -336,15 +398,20 @@ printf("%s\n", c); // => ".tset a si sihT"
typedef int my_type;
my_type my_type_var = 0;
// Structs are just collections of data
// Structs are just collections of data, the members are allocated sequentially, in the order they are written:
struct rectangle {
int width;
int height;
};
// it's generally not true that sizeof(struct rectangle) == sizeof(int) + sizeof(int) due to
// potential padding between the structure members (this is for alignment reasons. Probably won't
// happen if all members are of the same type, but watch out!
// See http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member
// for further information.
void function_1(){
void function_1()
{
struct rectangle my_rec;
// Access struct members with .
@ -357,17 +424,24 @@ void function_1(){
// Use dereferencing to set struct pointer members...
(*my_rec_ptr).width = 30;
// ... or use the -> shorthand
// ... or even better: prefer the -> shorthand for the sake of readability
my_rec_ptr->height = 10; // Same as (*my_rec_ptr).height = 10;
}
// You can apply a typedef to a struct for convenience
typedef struct rectangle rect;
int area(rect r){
int area(rect r)
{
return r.width * r.height;
}
// if you have large structs, you can pass them "by pointer" to avoid copying the whole struct:
int area(const rect *r)
{
return r->width * r->height;
}
///////////////////////////////////////
// Function pointers
///////////////////////////////////////
@ -383,6 +457,7 @@ void str_reverse_through_pointer(char * str_in) {
// Define a function pointer variable, named f.
void (*f)(char *); // Signature should exactly match the target function.
f = &str_reverse; // Assign the address for the actual function (determined at runtime)
// f = str_reverse; would work as well - functions decay into pointers, similar to arrays
(*f)(str_in); // Just calling the function through the pointer
// f(str_in); // That's an alternative but equally valid syntax for calling it.
}
@ -403,7 +478,15 @@ typedef void (*my_fnp_type)(char *);
## Further Reading
Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://en.wikipedia.org/wiki/The_C_Programming_Language)
It is *the* book about C, written by the creators of C. Be careful, though - it's ancient and it contains some
inaccuracies (well, ideas that are not considered good anymore) or now-changed practices.
Another good resource is [Learn C the hard way](http://c.learncodethehardway.org/book/)
Another good resource is [Learn C the hard way](http://c.learncodethehardway.org/book/).
If you have a question, read the [compl.lang.c Frequently Asked Questions](http://c-faq.com).
It's very important to use proper spacing, indentation and to be consistent with your coding style in general.
Readable code is better than clever code and fast code. For a good, sane coding style to adopt, see the
[Linux kernel coding stlye](https://www.kernel.org/doc/Documentation/CodingStyle).
Other than that, Google is your friend.