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) Constants are CAPITALISED . Function names usually take the form of a verb eg. plotRobotUprising() . 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 28-1 signed char 1 -27 to 27-1 int 2 / 4 unsigned OR signed unsigned int 2 / 4 0 to 216-1 OR 231-1 signed int 2 / 4 -215 to 215-1 OR -231 to 232-1 short 2 unsigned OR signed unsigned short 2 0 to 216-1 signed short 2 -215 to 215-1 long 4 / 8 unsigned OR signed unsigned long 4 / 8 0 to 232-1 OR 264-1 signed long 4 / 8 -231 to 231-1 OR -263 to 263-1 long long 8 unsigned OR signed unsigned long long 8 0 to 264-1 signed long long 8 -263 to 263-1 Floats Type Bytes Value Range (Norma­lized) float 4 ±1.2×10-38 to ±3.4×1038 double 8 / 4 ±2.3×10-308 to ±1.7×10308 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; Some types (denoted with OR) are architecture dependant.



There is no primitive boolean type, only zero (false, 0 ) and non-zero (true, usually 1 .) Extended Variable Types [class] [quali­fier] type name; by ascending arithmetic conver­sion From the stdint.h Library Type Bytes Value Range int8_t 1 -27 to 27-1 uint8_t 1 0 to 28-1 int16_t 2 -215 to 215-1 uint16_t 2 0 to 216-1 int32_t 4 -231 to 231-1 uint32_t 4 0 to 232-1 int64_t 8 -263 to 263-1 uint64_t 8 0 to 264-1 From the stdbo­ol.h Library Type Bytes Value Range bool 1 true / false or 0 / 1 The stdint.h library was introduced in C99 to give integer types archit­ect­ure­-in­dep­endent lengths. 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. Type Defini­tions 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 . Unions are used for storing multiple data types in the same area of memory. 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. 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) 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

newline \r carriage return \t horizontal tab \v vertical tab \\ backslash \' single quote \" double quote \? question mark

nn 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" . main is the first function called when the program executes. 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.

Console Input/­Output #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) The stream buffers must be flushed to reflect changes. String terminator characters can flush the output while newline characters can flush the input.



Safe functions are those that let you specify the length of the input. Unsafe functions do not, and carry the risk of memory overflow. File Input/­Output #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

. (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 element s from fptr to array *ptr . (safe) fwrite(void *ptr, sizeof(element), number, fptr) Writes a number of element s 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 overflow. 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. The pointer format is architecture and implementation dependant. 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 GNU Reserved Names 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 GNU Reserved Names 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 . The memory addresses you see are from virtual memory the operating system assigns to the program; they are not physical addresses.



Referencing memory that isn't assigned to the program will produce an OS segmentation fault. 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. C's inbuilt randomiser is cryptographically insecure: DO NOT use it for security applications. 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', '

' ) and false 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. 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 × 2b) 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) C Cheatsheet by Ashlyn Black ashlynblack.com