C

C - Programmer Sheet

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.
Console Input/Output

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)
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.
Doesn't begin with a letter.
Do not use reserved keyword.
File Input/Output

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.
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.
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.
Safe functions are those that let you specify the length of the input. Unsafe functions do not, and carry the risk of memory overfl
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.
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.
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.
Type Definitions

Defining

typedef unsigned short uint16; Abbreviating a longer type name to uint16.
ypedef 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.

Accessing

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

Evaluating

if(varName == false) Testing the value of varName.
Pointers

Defining

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.
A pointer is a variable that holds a memory location.
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)
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
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
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.
Strings must include a char element for \0.
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.
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.
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.)
{} unction 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.

Scope

int f(){ int i = 0; }
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.
The Standard Library

#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.
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".
main is the first function called when the program executes.
The Character Type Library
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.
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.
The String Library

#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)
Safe functions are those that let you specify the length of the input. Unsafe functions do not, and carry the risk of memory overflow.
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.
The Time Library

#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.
Binary Operators

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)
Ternary & Assignment Operators

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 × 2b)
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)