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C Reference Cheat Sheet by

C Syntax, Data Types, Structure & Common Library Functions

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.
 
2001ASpaceOddysey;
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
 
\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"
.
 
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 
\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 
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

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
.
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', '\n'
) 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 â‹‚ 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 â‹‚ 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 â‹‚ 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
                       
 

Metadata

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  • Published: 28th January, 2015
  • Last Updated: 12th May, 2016
  • Rated: 5 out of 5 stars based on 19 ratings

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Comments

Wow, this is awesome! Great cheat sheet, really well put together!

Ashlyn Black Ashlyn Black, 13:04 28 Jan 15

Thanks, Dave!

very nice. Few things- there's no such things as pass by reference in c, even pointers are passed by value. Also mention which sting/I/O functions are safe or not.

Ashlyn Black Ashlyn Black, 19:28 2 Feb 15

Good catch on the unsafe functions; I have tagged them as such and also included their safer alternatives.

I'm not quite clear on how passing a pointer to data differs from passing a reference to data? Most of the documentation I've seen so far calls the process of retrieving a value through the use of a pointer "dereferencing"?

Added enumeration.

Added literals.

Fixed declaration error in Enumeration.

Fixed include error in Preprocessing.

Placeholder types are incomplete: what about for example long integer "ul"

Ashlyn Black Ashlyn Black, 11:03 17 Apr 15

If you look under "Placeholder Formatting" you will find it. 'u' is the placeholder type, 'l' is the length modifier. :)

Re: Brian, 19:28 31 Jan 15

dereferencing != pass by reference. Commentor is correct, C has no pass by reference semantics. Better to phrase as 'pass by indirection.'

In C you receive a copy of the thing you pass. When this thing happens to be a pointer, one may indirectly modify the 'pointed at' thing.

In C++ when you pass by reference you receive an alias to the original thing, not a copy. If non-const, when you modify the alias, you modify the thing itself (contrast with C --you never modified the thing itself, only what it pointed at).

Ashlyn Black Ashlyn Black, 14:48 17 Apr 15

Yep, I fixed that up. :)

In float/double, in addition to range, please also add the precision: how many decimal digits they can store. 7 and 15, IIRC.

Ashlyn Black Ashlyn Black, 01:37 5 Dec 15

As far as I know, the precision isn't fixed. That's why it's call "Floating Point." c:

strncpy is somewhat unsafe: if the source string is too long to fit in the destination, the result is not NUL-terminated and is not a valid C string; attempting to do things with it later (e.g. print it) will overrun. Try this out:

#include <stdio.h>
typedef struct test {
char first[8];
char second[8];
} TEST;
int main (void)
{
TEST s;
strncpy(s.first, "01234567", 8);
strncpy(s.second, "private", 8);
printf(s.first);
return 0;
}

In Naming, '_' is allowed along with AlphaNumeric. This is missing

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