Number Literals

Integers

0b11111111

binary

0B11111111

binary

0377

octal

255

decimal

0xff

hexadecimal

0xFF

hexadecimal

Real Numbers

88.0f / 88.1234567f

single precision float ( f suffix )

88.0 / 88.123456789012345

double precision float ( no f suffix )

Signage

42 / +42

positive

-42

negative

Binary notation 0b... / 0B... is available on GCC and most but not all C compilers.

Variables

Decl­aring

int x;

A variable.

char x = 'C';

A variable & initialising it.

float x, y, z;

Multiple variables of the same type.

const int x = 88;

A constant variable: can't assign to after declar­ation (compiler enforced.)

Naming

johnny5IsAlive;

Alphanumeric, not a keyword, begins with a letter.

2001 ASpaceOddysey;

Doesn't begin with a letter.

while ;

Reserved keyword.

how exciting! ;

Non-alphanumeric.

iamaverylongvariablenameohmygoshyesiam ;

Longer than 31 characters (C89 & C90 only)

Primitive Variable Types

*applicable but not limited to most ARM, AVR, x86 & x64 installations

[class] [qualifier] [unsigned] type/void name;

by ascending arithmetic conversion

Inte­gers

Type

Bytes

Value Range

char

1

unsigned OR signed

unsigned char

1

0 to 2 8-1

signed char

1

-2 7 to 2 7-1

int

2 / 4

unsigned OR signed

unsigned int

2 / 4

0 to 2 16-1 OR 2 31-1

signed int

2 / 4

-2 15 to 2 15-1 OR -2 31 to 2 32-1

short

2

unsigned OR signed

unsigned short

2

0 to 2 16-1

signed short

2

-2 15 to 2 15-1

long

4 / 8

unsigned OR signed

unsigned long

4 / 8

0 to 2 32-1 OR 2 64-1

signed long

4 / 8

-2 31 to 2 31-1 OR -2 63 to 2 63-1

long long

8

unsigned OR signed

unsigned long long

8

0 to 2 64-1

signed long long

8

-2 63 to 2 63-1

Floats

Type

Bytes

Value Range (Norma­lized)

float

4

±1.2×10 -38 to ±3.4×10 38

double

8 / 4

±2.3×10 -308 to ±1.7×10 308 OR alias to float for AVR.

long double

ARM: 8, AVR: 4, x86: 10, x64: 16

Qualifiers

const type

Flags variable as read-only (compiler can optimise.)

volatile type

Flags variable as unpredictable (compiler cannot optimise.)

Storage Classes

register

Quick access required. May be stored in RAM OR a register. Maximum size is register size.

static

Retained when out of scope. static global variables are confined to the scope of the compiled object file they were declared in.

extern

Variable is declared by another file.

Typecasting

(type)a

Returns a as data type.

char x = 1, y = 2; float z = (float) x / y;

Extended Variable Types

[class] [quali­fier] type name;

by ascending arithmetic conver­sion

From the stdint.h Library

Type

Bytes

Value Range

int8_t

1

-2 7 to 2 7-1

uint8_t

1

0 to 2 8-1

int16_t

2

-2 15 to 2 15-1

uint16_t

2

0 to 2 16-1

int32_t

4

-2 31 to 2 31-1

uint32_t

4

0 to 2 32-1

int64_t

8

-2 63 to 2 63-1

uint64_t

8

0 to 2 64-1

From the stdbo­ol.h Library

Type

Bytes

Value Range

bool

1

true / false or 0 / 1

Structures

Defi­ning
struct strctName{ type x; type y; };
A structure type strctName with two members, x and y. Note trailing semicolon
struct item{ struct item *next; };
A structure with a recursive structure pointer inside. Useful for linked lists.
Declaring
struct strctName varName;
A variable varName as structure type strctName.
struct strctName *ptrName;
A strctName structure type pointer, ptrName.
struct strctName{ type a; type b; } varName;
Shorthand for defining strctName and declaring varName as that structure type.
struct strctName varName = { a, b };
A variable varName as structure type strctName and initialising its members.
Accessing
varName.x
Member x of structure varName.
ptrName->x
Value of structure pointer ptrName member x.
Bit Fields
struct{char a:4, b:4} x;
Declares x with two members a and b, both four bits in size (0 to 15.)
Array members can't be assigned bit fields.

类型定义

Defining

typedef unsigned short uint16;

Abbreviating a longer type name to uint16.

typedef struct structName{int a, b;}newType;

Creating a newType from a structure.

typedef enum typeName{false, true}bool;

Creating an enumerated bool type.

Declaring

uint16 x = 65535;

Variable x as type uint16.

newType y = {0, 0};

Structure y as type newType.

Unions

Defining

union uName{int x; char y[8];}

A union type uName with two members, x & y. Size is same as biggest member size.

Declaring

union uN vName;

A variable vName as union type uN.

Accessing

vName.y[int]

Members cannot store values concur­rently. Setting y will corrupt x.

Enumer­ation

Defining

enum bool { false, true };

A custom data type bool with two possible states: false or true.

Declaring

enum bool varName;

A variable varName of data type bool.

Assigning

varName = true;

Variable varName can only be assigned values of either false or true.

Evaluating

if(varName == false)

Testing the value of varName.

Pointers

Declaring

type *x;

Pointers have a data type like normal variables.

void *v;

They can also have an incomplete type. Operators other than assignment cannot be applied as the length of the type is unknown.

struct type *y;

A data structure pointer.

type z[];

An array/­string name can be used as a pointer to the first array element.

Accessing

x

A memory address.

*x

Value stored at that address.

y->a

Value stored in structure pointer y member a.

&varName

Memory address of normal variable varName.

*(type *)v

Dereferencing a void pointer as a type pointer.

Arrays

Decl­aring
type name[int];
You set array length.
type name[int] = {x, y, z};
You set array length and initialise elements.
type name[int] = {x};
You set array length and initialise all elements to x.
type name[] = {x, y, z};
Compiler sets array length based on initial elements.
Size cannot be changed after declaration.
Dimensions
name[int]
One dimension array.
name[int][int]
Two dimens­ional array.
Accessing
name[int]
Value of element int in array name.
*(name + int)
Same as name[­int].
Elements are contiguously numbered ascending from 0.
&name[int]
Memory address of element int in array name.
name + int
Same as &­nam­e[i­nt].
Elements are stored in contiguous memory.
Measuring
sizeof(array) / sizeof(arrayType)
Returns length of array. (Unsafe)
sizeof(array) / sizeof(array[0])
Returns length of array. (Safe)

Strings

'A' character

Single quotes.

"AB" string

Double quotes.

\0

Null termin­ator.

Strings are char arrays.

char name[4] = "Ash";

is equivalent to

char name[4] = {'A', 's', 'h', '\0'};

int i; for(i = 0; name[i]; i++){}

\0 evaluates as false.

Escape Characters

\a
alarm (bell/beep)
\b
backspace
\f
formfeed
\n
newline
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\'
single quote
\"
double quote
\?
question mark
\nnn
Any octal ANSI character code.
\xhh
Any hexadecimal ANSI character code.

Functions

Declaring
type/void funcName([args...]){ [return var;] }
Function names follow the same restrictions as variable names but must also be unique.
type/void
Return value type ( void if none.)
funcName()
Function name and argument parenthesis.
args...
Argument types & names ( void if none.)
{}
Function content delimi­ters.
return var;
Value to return to function call origin. Skip for void type functions. Functions exit immediately after a return.
By Value vs By Pointer
void f(type x); f(y);
Passing variable y to function f argument x (by value.)
void f(type *x); f(array);
Passing an array/string to function f argument x (by pointer.)
void f(type *x); f(structure);
Passing a structure to function f argument x (by pointer.)
void f(type *x); f(&y);
Passing variable y to function f argument x (by pointer.)
type f(){ return x; }
Returning by value.
type f(){ type x; return &x; }
Returning a variable by pointer.
type f(){ static type x[]; return &x; }
Returning an array/string/structure by pointer. The static qualifier is necessary otherwise x won't exist after the function exits.
Passing by pointer allows you to change the originating variable within the function.
Scope
int f(){ int i = 0; } i++;
i is declared inside f(), it doesn't exist outside that function.
Prototyping
type funcName(args...);
Place before declaring or referencing respective function (usually before main.)
type funcName([args...])
Same type, name and args... as respective function.
;
Semicolon instead of function delimiters.

main()

int main(int argc, char *argv[]){return int;}

Anatomy

int main

Program entry point.

int argc

# of command line arguments.

char *argv[]

Command line arguments in an array of strings. #1 is always the program filename.

return int;

Exit status ( inte­ger) returned to the OS upon program exit.

Command Line Arguments

app two 3

Three arguments, "ap­p", "tw­o" and "3".

app "two 3"

Two arguments, "ap­p" and "two 3".

Condit­ional (Branc­hing)

if, else if, else
if(a) b;
Evaluates b if a is true.
if(a){ b; c; }
Evaluates b and c if a is true.
if(a){ b; }else{ c; }
Evaluates b if a is true, c otherwise.
if(a){ b; }else if(c){ d; }else{ e; }
Evaluates b if a is true, otherwise d if c is true, otherwise e.
switch, case, break
switch(a){ case b: c; }
Evaluates c if a equals b.
switch(a){ default: b; }
Evaluates b if a matches no other case.
switch(a){ case b: case c: d; }
Evaluates d if a equals either b or c.
switch(a){ case b: c; case d: e; default: f; }
Evaluates c, e and f if a equals b, e and f if a equals d, otherwise f.
switch(a){ case b: c; break; case d: e; break; default: f; }
Evaluates c if a equals b, e if a equals d and e otherwise.

Iterative (Looping)

while
int x = 0; while(x < 10){ x += 2; }
Loop skipped if test condition initially false.
int x = 0;
Declare and initialise integer x.
while()
Loop keyword and condition parenthesis.
x < 10
Test condition.
{}
Loop delimiters.
x += 2;
Loop contents.
do while
char c = 'A'; do { c++; } while(c != 'Z');
Always runs through loop at least once.
char c = 'A';
Declare and initialise character c.
do
Loop keyword.
{}
Loop delimiters.
c++;
Loop contents.
while();
Loop keyword and condition parenthesis. Note semicolon.
c != 'Z'
Test condition.
for
int i; for(i = 0; n[i] != '\0'; i++){} (C89)
OR
for(int i = 0; n[i] != '\0'; i++){} (C99+)
Compact increment/decrement based loop.
int i;
Declares integer i.
for()
Loop keyword.
i = 0;
Initialises integer i. Semicolon.
n[i] != '\0';
Test condition. Semicolon.
i++
Increments i. No semicolon.
{}
Loop delimiters.
continue
int i=0; while(i<10){ i++; continue; i--;}
Skips rest of loop contents and restarts at the beginning of the loop.
break
int i=0; while(1){ if(x==10){break;} i++; }
Skips rest of loop contents and exits loop.

控制台输入/输出

#include <stdio.h>

Characters

getchar()

Returns a single character's ANSI code from the input stream buffer as an integer. (safe)

putchar(int)

Prints a single character from an ANSI code integer to the output stream buffer.

Strings

gets(strName)

Reads a line from the input stream into a string variable. (Unsafe, removed in C11.)

Alternative

fgets(strName, length, stdin);

Reads a line from the input stream into a string variable. (Safe)

puts("string")

Prints a string to the output stream.

Formatted Data

scanf("%d", &x)

Read value/s (type defined by format string) into variable/s (type must match) from the input stream. Stops reading at the first whitespace. & prefix not required for arrays (including strings.) (unsafe)

print­f("I love %c %d!", 'C', 99)

Prints data (formats defined by the format string) as a string to the output stream.

Alternative

fgets(strName, length, stdin); sscanf(strName, "%d", &x);

Uses fgets to limit the input length, then uses sscanf to read the resulting string in place of scanf. (safe)

文件输入/输出

#include <stdio.h>

Opening

FILE *fptr = fopen(filename, mode);

FILE *fptr

Declares fptr as a FILE type pointer (stores stream location instead of memory location.)

fopen()

Returns a stream location pointer if successful, 0 otherwise.

filename

String containing file's directory path & name.

mode

String specifying the file access mode.

Modes

"r" / "rb"

Read existing text/binary file.

"w" / "wb"

Write new/over existing text/binary file.

"a" / "ab"

Write new/append to existing text/binary file.

"r+" / "r+b" / "rb+"

Read and write existing text/binary file.

"w+" / "w+b" / "wb+"

Read and write new/over existing text/binary file.

"a+" / "a+b" / "ab+"

Read and write new/append to existing text/binary file.

Closing

fclose(fptr);

Flushes buffers and closes stream. Returns 0 if successful, EOF otherwise.

Random Access

ftell(fptr)

Return current file position as a long integer.

fseek(fptr, offset, origin);

Sets current file position. Returns false is successful, true otherwise. The offset is a long integer type.

Origins

SEEK_SET

Beginning of file.

SEEK_CUR

Current position in file.

SEEK_END

End of file.

Utilities

feof(fptr)

Tests end-of-file indicator.

rename(strOldName, strNewName)

Renames a file.

remove(strName)

Deletes a file.

Characters

fgetc(fptr)

Returns character read or EOF if unsucc­essful. (safe)

fputc(int c, fptr)

Returns character written or EOF if unsucc­essful.

Strings

fgets(char *s, int n, fptr)

Reads n-1 characters from file fptr into string s. Stops at EOF and \n. (safe)

fputs(char *s, fptr)

Writes string s to file fptr. Returns non-ne­gative on success, EOF otherwise.

Formatted Data

fscanf(fptr, format, [...])

Same as scanf with additional file pointer parameter. (unsafe)

fprintf(fptr, format, [...])

Same as printf with additional file pointer parameter.

Alternative

fgets(strName, length, fptr); sscanf(strName, "%d", &x);

Uses fgets to limit the input length, then uses sscanf to read the resulting string in place of scanf. (safe)

Binary

fread(void *ptr, sizeof(element), number, fptr)

Reads a number of elements from fptr to array *ptr. (safe)

fwrite(void *ptr, sizeof(element), number, fptr)

Writes a number of elements to file fptr from array *ptr.

Placeh­older Types (f/printf And f/scanf)

printf("%d%d...", arg1, arg2...);

Type

Example

Description

%d or %i

-42

Signed decimal integer.

%u

42

Unsigned decimal integer.

%o

52

Unsigned octal integer.

%x or %X

2a or 2A

Unsigned hexadecimal integer.

%f or %F

1.21

Signed decimal float.

%e or %E

1.21e+9 or 1.21E+9

Signed decimal w/ scientific notation.

%g or %G

1.21e+9 or 1.21E+9

Shortest representation of %f/ %F or %e/ %E.

%a or %A

0x1.207c8ap+30 or 0X1.207C8AP+30

Signed hexadecimal float.

%c

a

A character.

%s

A String.

A character string.

%p

 

A pointer.

%%

%

A percent character.

%n

No output, saves # of characters printed so far. Respective printf argument must be an integer pointer.

Placeh­older Formatting (f/printf And f/scanf)

%[Flags][Width][.Precision][Length]Type
Flags
-
Left justify instead of default right justify.
+
Sign for both positive numbers and negative.
#
Precede with 0, 0x or 0X for %o, %x and %X tokens.
space
Left pad with spaces.
0
Left pad with zeroes.
Width
integer
Minimum number of characters to print: invokes padding if necessary. Will not truncate.
*
Width specified by a preceding argument in printf.
Precision
.integer
Minimum # of digits to print for %d, %i, %o, %u, %x, %X. Left pads with zeroes. Will not truncate. Skips values of 0.
 
Minimum # of digits to print after decimal point for %a, %A, %e, %E, %f, %F (default of 6.)
 
Minimum # of significant digits to print for %g & %G.
 
Maximum # of characters to print from %s (a string.)
.
If no integer is given, default of 0.
.*
Precision specified by a preceding argument in printf.
Length
hh
Display a char as int.
h
Display a short as int.
l
Display a long integer.
ll
Display a long long integer.
L
Display a long double float.
z
Display a size_t integer.
j
Display a intmax_t integer.
t
Display a ptrdiff_t integer.

Prepro­cessor Directives

#include <inbuilt.h>
Replaces line with contents of a standard C header file.
#include "./custom.h"
Replaces line with contents of a custom header file. Note dir path prefix & quotations.
#define NAME value
Replaces all occurrences of NAME with value.

Comments

// We're single-line comments!
// Nothing compiled after // on these lines.

/* I'm a multi-line comment!
   Nothing compiled between
   these delimiters. */

C Reserved Keywords

_Alignas
break
float
signed
_Alignof
case
for
sizeof
_Atomic
char
goto
static
_Bool
const
if
struct
_Complex
continue
inline
switch
_Generic
default
int
typedef
_Imaginary
do
long
union
_Noreturn
double
register
unsigned
_Static_assert
else
restrict
void
_Thread_local
enum
return
volatile
auto
extern
short
while
_A-Z...
__...

C / POSIX Reserved Keywords

E[0-9]...
E[A-Z]...
is[a-z]...
to[a-z]...
LC_[A-Z]...
SIG[A-Z]...
SIG_[A-Z]...
str[a-z]...
mem[a-z]...
wcs[a-z]...
..._t

Header Reserved Keywords

Name
Reserved By Library
d_...
dirent.h
l_...
fcntl.h
F_...
fcntl.h
O_...
fcntl.h
S_...
fcntl.h
gr_...
grp.h
..._MAX
limits.h
pw_...
pwd.h
sa_...
signal.h
SA_...
signal.h
st_...
sys/stat.h
S_...
sys/stat.h
tms_...
sys/times.h
c_...
termios.h
V...
termios.h
I...
termios.h
O...
termios.h
TC...
termios.h
B[0-9]...
termios.h

Heap Space

#include <stdlib.h>

Allocating

malloc();

Returns a memory location if succes­sful, NULL otherwise.

type *x; x = malloc(sizeof(type));

Memory for a variable.

type *y; y = malloc(sizeof(type) * length );

Memory for an array/string.

struct type *z; z = malloc(sizeof(struct type));

Memory for a structure.

Deallocating

free(ptrName);

Removes the memory allocated to ptrName.

Reallocating

realloc(ptrName, size);

Attempts to resize the memory block assigned to ptrName.

标准库

#include <stdlib.h>

Randomicity

rand()

Returns a (predictable) random integer between 0 and RAND_MAX based on the randomiser seed.

RAND_MAX

The maximum value rand() can generate.

srand(unsigned integer);

Seeds the randomiser with a positive integer.

(unsigned) time(NULL)

Returns the computer's tick-tock value. Updates every second.

Sorting

qsort(array, length, sizeof(type), compFunc);

qsort()

Sort using the QuickSort algorithm.

array

Array/string name.

length

Length of the array/string.

sizeof(type)

Byte size of each element.

compFunc

Comparison function name.

compFunc

int compFunc( const void *a, const void b* ){ return( *(int *)a - *(int *)b); }

int compFunc()

Function name unimportant but must return an integer.

const void *a, const void *b

Argument names unimportant but must identical otherwise.

return( *(int *)a - *(int *)b);

Negative result swaps b for a, positive result swaps a for b, a result of 0 doesn't swap.

The Character Type Library

#include <ctype.h>
tolower(char)
Lowercase char.
toupper(char)
Uppercase char.
isalpha(char)
True if char is a letter of the alphabet, false otherwise.
islower(char)
True if char is a lowercase letter of the alphabet, false otherwise.
isupper(char)
True if char is an uppercase letter of the alphabet, false otherwise.
isnumber(char)
True if char is numerical ( 0 to 9) and false otherwise.
isblank
True if char is a whitespace character ( ' ', '\t', '\n') and false otherwise.

字符串库

#include <string.h>

strlen(a)

Returns # of char in string a as an integer. Excludes \0. (unsafe)

strcpy(a, b)

Copies strings. Copies string b over string a up to and including \0. (unsafe)

strcat(a, b)

Concat­enates strings. Copies string b over string a up to and including \0, starting at the position of \0 in string a. (unsafe)

strcmp(a, b)

Compares strings. Returns false if string a equals string b, true otherwise. Ignores characters after \0. (unsafe)

strstr(a, b)

Searches for string b inside string a. Returns a pointer if succes­sful, NULL otherwise. (unsafe)

Alternatives

strncpy(a, b, n)

Copies strings. Copies n characters from string b over string a up to and including \0. (safe)

strncat(a, b, n)

Concat­enates strings. Copies n characters from string b over string a up to and including \0, starting at the position of \0 in string a. (safe)

strncmp(a, b, n)

Compares first n characters of two strings. Returns false if string a equals string b, true otherwise. Ignores characters after \0. (safe)

时间库

#include <time.h>

Variable Types

time_t

Stores the calendar time.

struct tm *x;

Stores a time & date breakdown.

tm structure members:

int tm_sec

Seconds, 0 to 59.

int tm_min

Minutes, 0 to 59.

int tm_hour

Hours, 0 to 23.

int tm_mday

Day of the month, 1 to 31.

int tm_mon

Month, 0 to 11.

int tm_year

Years since 1900.

int tm_wday

Day of the week, 0 to 6.

int tm_yday

Day of the year, 0 to 365.

int tm_isdst

Daylight saving time.

Functions

time(NULL)

Returns unix epoch time (seconds since 1/Jan/1970.)

time(&time_t);

Stores the current time in a time_t variable.

ctime(&time_t)

Returns a time_t variable as a string.

x = localtime( &time_t);

Breaks time_t down into struct tm members.

Unary Operators

by descending evaluation preced­ence
+a
Sum of 0 (zero) and a. (0 + a)
-a
Difference of 0 (zero) and a. (0 - a)
!a
Complement (logical NOT) of a. (~a)
~a
Binary ones complement (bitwise NOT) of a. (~a)
++a
Increment of a by 1. (a = a + 1)
--a
Decrement of a by 1. (a = a - 1)
a++
Returns a then increments a by 1. (a = a + 1)
a--
Returns a then decrements a by 1. (a = a - 1)
(type)a
Typecasts a as type.
&a;
Memory location of a.
sizeof(a)
Memory size of a (or type) in bytes.

二进制运算符

by descending evaluation preced­ence

a * b;

Product of a and b. (a × b)

a / b;

Quotient of dividend a and divisor b. Ensure divisor is non-zero. (a ÷ b)

a % b;

Remainder of integers dividend a and divisor b.

a + b;

Sum of a and b.

a - b;

Difference of a and b.

a << b;

Left bitwise shift of a by b places. (a × 2 b)

a >> b;

Right bitwise shift of a by b places. (a × 2 -b)

a < b;

Less than. True if a is less than b and false otherwise.

a <= b;

Less than or equal to. True if a is less than or equal to b and false otherwise. (a ≤ b)

a > b;

Greater than. True if a is greater than than b and false otherwise.

a >= b;

Greater than or equal to. True if a is greater than or equal to b and false otherwise. (a ≥ b)

a == b;

Equality. True if a is equal to b and false otherwise. (a ⇔ b)

a != b;

Inequality. True if a is not equal to b and false otherwise. (a ≠ b)

a & b;

Bitwise AND of a and b. (a ⋂ b)

a ^ b;

Bitwise exclusive-OR of a and b. (a ⊕ b)

a | b;

Bitwise inclusive-OR of a and b. (a ⋃ b)

a && b;

Logical AND. True if both a and b are non-zero. (Logical AND) (a ⋂ b)

a || b;

Logical OR. True if either a or b are non-zero. (Logical OR) (a ⋃ b)

三元&分配运算符

by descending evaluation precedence

x ? a : b;

Evaluates a if x evaluates as true or b otherwise. (if(x){ a; } else { b; })

x = a;

Assigns value of a to x.

a *= b;

Assigns product of a and b to a. (a = a × b)

a /= b;

Assigns quotient of dividend a and divisor b to a. (a = a ÷ b)

a %= b;

Assigns remainder of integers dividend a and divisor b to a. (a = a mod b)

a += b;

Assigns sum of a and b to a. (a = a + b)

a -= b;

Assigns difference of a and b to a. (a = a - b)

a <<= b;

Assigns left bitwise shift of a by b places to a. (a = a × 2 b)

a >>= b;

Assigns right bitwise shift of a by b places to a. (a = a × 2 -b)

a &= b;

Assigns bitwise AND of a and b to a. (a = a ⋂ b)

a ^= b;

Assigns bitwise exclus­ive-OR of a and b to a. (a = a ⊕ b)

a |= b;

Assigns bitwise inclus­ive-OR of a and b to a. (a = a ⋃ b)