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C Programming Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Data types and Variables

Arrays
 
 
 
<t­ype­>
 
<array_name>[<index>]
 
= expres­sion
Pointers
 
 
 
<ty­pe> 
*p
Declares p a pointer.
 
p =
 
&var
Declares pointer to the variable var.
 
<ty­pe> 
**pp
Declares pp a pointer to a pointer.
 
pp =
 
&p
Declares pointer to the pointer p.
Structures
 
 
 
struct tag_name { 
   <type> <element1>; 
   <type> <element2>; 
}
Structures allow a programmer to have a collection of elements of different types repres­enting something.

Formatting

Escape Characters
 
 
\a
Alert bell
 
\b
Backspace
 
\n
Newline
 
*\*
Backslash
 
\"
Double quote
 
\?
Question Mark
Conversion Specifiers
 
 
%c
char
 
%s
string
 
%d
int
 
%u
unsigned int
 
%ld
long int
 
%o
octal
 
%x
hexade­cimal
 
%d
double
Formatted I/O
 
 
%5.2f
Width of the printed field.
ie. 
'123.5'
becomes 
'  12­3.50'
.
 
%04d
Fills unused space with zeros.
ie. 
21
becomes 
0021
.
 
%-f
Aligns the output to the right.
 
%[aeiou]
Remove all characters but vowels.
 
%[^aeiou]
Remove all the vowels.
 
%d*%d*%d
Eliminate unnece­ssary charac­ters.
ie. 
1/1/2001
can be stored in three integers as 
1
,
1
and 
2001
.
Example:
int integer = 1; 
printf("This is an integer: %d", integer);

Dynamic memory allocation

void* malloc(int size)
Allocates size contiguous bytes of memory and returns a void pointer to the first byte allocated.
 
void* calloc(int items, int size)
Allocates iitems x size contiguous cleared (set to 0) bytes of memory and returns a void pointer to the first byte allocated.
 
void* calloc(void* ptr, int new_size)
Resizes allocated memory being pointed at by ptr to be size bytes and returns a void pointer to the first byte allocated.
 
void free( void* ptr)
Frees memory that is pointed at by ptr.

Linked lists

struct node {   int x;
  struct node* next;
};
...
struct node* head;
A structure pointing to other nodes.
The first element is assigned to a pointer head.
 
struct node* ptr = head;
while(ptr!= NULL){
  printf("%d\n", ptr->x);
  ptr = ptr->next;
}
Traversing the list.
 
while(head != NULL){
  ptr = head->next;{
  free(head);{
  head = ptr;{
}
Deleting an element.
You need to free the elements in the right order.
 
if(ptr == NULL){
  ptr = malloc(sizeof(struct node));
  ptr->x = 4;
  ptr->next = NULL;
  head = ptr;
}
Adding an element.
You need to cycle to the end first.
A linked list is a dynamic data structure consisting of a sequence of records where each element contains a link to the next record. They can be linked singularly, doubly or circularly.
Every node has a payloead and a link to the next node.
The end is indicated by a NULL pointer.
It needs a pointer to the first item in the list.
  

Basics of C

General functions
 
 
 
char
 
getchar()
Obtains character from input stream
 
int
 
sizeof(void var)
Returns size in bytes of
Mathematical functions
 
 
 
double
 
sqrt(double x)
Square root of x
 
double
 
pow(double x, double y)
x raised to the power of y
 
double
 
abs(double x)
Absolute value of x
 
double
 
ceil(double x)
Rounds x to the smallest int no less than x
 
double
 
floor(double x)
Rounds x to the largest int not greater than x
Command line Arguments
 
 
 
int main (int argc, char* argv[]){ 
     /*code*/
 }
 
int main (int argc, char* argv[]){ 
     /*code*/
 }

Pre-pr­oce­ssing

Pre-processor identifiers
 
 
_LINE_
Current line being compiled.
 
_FILE_
Name of source file.
 
_DATE_
Date of compil­ation (mm dd yy).
 
_TIME_
Time of compil­ation (hh:mm:ss)
Macros
 
 
#include <so­me_­lib.h>
The contents of 
#include
 are read and merged into the file.
 
#define VAR VALUE
Define a variable.
 
#ifdef DEBUG 
  expression 
#endif
Define a variable.
 
#ifdef DEBUG 
  expression 
#endif
Condit­ional compil­ation can be turned on by both setting 
#define DEBUG 1
or by 
-D
in the command line.
 
#ifdef condition 
  #error "Error message" 
#endif
Prints text as error message.
Functi­on-like macros are pre-pr­ocessed and have no type checking and are not checked for compil­ation errors, but are executed faster than normal C functions.

Strings

int
 
printf(char out)
Prints formattedoutput to stdout
 
int
 
scanf(char *input)
Reads formatted input from stdin
 
int
 
puts(char *input)
Writes a string to stdout up to but not including the null character. A newline character is appended to the output.
 
char*
 
fgets(char *str, int n, FILE *stream)
Reads a line from the specified stream and stores it into the string pointed to by str. It stops when either (n-1) characters are read, the newline character is read, or the end-of­-file is reached, whichever comes first.
 
char*
 
strcpy(char dest, char src)
Pass a string to another string variable.
 
int
 
strcat(char *dest, char *src)
Appends the string pointed to by src to the end of the string pointed to by dest
 
char*
 
strlen(const char *str)
Computes the length of the string str up to, but not including the termin­ating null character.

Sockets

int socket(int domain, int type, int protocol)
Creates a socket.
 
int close(int sockid)
Closes a socket.
 
int bind(int sockid, struct sockaddr* addr, int addrlen)
Selects the port which is going to be used and reserves it for use by the socket.
It can be skipped for TCP and UDP sockets.
 
int listen(int sockid, int backlog)
Listens for connections.
It's only used by a TCP server.
 
int accept(int sockid, struct sockaddr* client­Addr, int* addrlen)
Establ­ishes a connection for a TCP server.
addrlen should be set to 
sizeof­(cl­ien­tAddr)
.
 
int connect(int sockid, struct sockaddr* server­Addr, int addrlen)
Establ­ishes a connection for a TCP client.
addrlen should be set to 
sizeof­(cl­ien­tAddr)
.
 
int send(int sockid, void* msg, int len, int flags)
Sends a message to a TCP client­/server with length 
len
.
 
int recv(int sockid, void* buffer, int len, int flags)
Receives a message from a TCP client­/server with length 
len
.
 
int sendto(int sockid, void* msg, int len, int flags, struct sockaddr* foreign, int addrlen)
Sends a message to a UDP client­/server with length 
len
.
 
int recvfrom(int sockid, void* msg, int len, int flags, struct sockaddr* foreign, int addrlen)
Receives a message from a UDPcli­ent­/server with length 
len
.