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C What you Know*
•^ Objective: To introduce some of the features of C.This assumes that you are familiar with C++ orjava and concentrates on the features that are notused in C++. •^ C does not have the OOP features of C++, nordoes it have as rich a library^ – no classes^ – I/O, string operations, and dynamic memory
management done through library calls – Not much support for higher level data structures (e.g.STL) – Commonly lots of pointer manipulation
Topics
• Getting Started • pointers • character arrays and string functions • structs • I/O • dynamic memory (
malloc
and
free
Pointers
•^ To declare a pointer put
*^ in front of the variable
name.^ – int
i;^
/*^ declare
an^
int^
*ip;
declare
a^ pointer
to^
an^ int
*name;
declare
a^ pointer
to^
/*^
a^ character
•^ Use
&^ to get the address of a variable and
*^ to
dereference a pointer^ – ip
=^ &i;
assign
the
pointer
variable
/*^ ip
the
address
of^
i^ */
=^ *ip;
assign
i^ the
value
of^
what
/*^
ip^ points
to.
Pointers and Arrays
•^ Arrays are related to pointers. An array variable isa pointer to the first location in the array.^ – declare static character array of size 10
name[10];
- can initialize character arrays^ • char
name[10]
=^ “Fred”;
– char *anotherName; – anotherName = Name; • You can use pointer arithmetic to access arrayelements – access 4
th^ element of name
+^ 4);
Library Support for Strings
string.h
-^ strlen
( const char *s );
- returns length of s (not including \0) • int^
strcmp
( const char *s1,^ const char *s2 );
- return 0 if s1 == s2, <0 if s1 < s2, >0 if s1 > s2 • strcat
( char *dest, const char *src );
- appends src to the end of dest – char D[20]; D[0] = ‘\0’; – strcat(D,”Jeremy”); strcat(D,” “); strcat(D,”Johnson”); • strncat
( char *dest,^ const char *src, int n )
-^ appends upto n characters of src to the end of dest
Library Support for Strings
strcpy
( char *dest, const char *src );
- copy the string pointed to by src (including
‘\0’
) to a
character array pointed to by dest. – dest must already point to a character array withsufficient space to hold src. – no error checking for insufficient space
strncpy
( char *dest,
const char *src, int n );
- copy at most n characters from the string pointed to bysrc to a character array pointed to by dest. – This should be used to prevent overflow errors.
Arrays of Arrays
• char
*names[8];
• names[0]
“start”;^ s
t
a
r
t
\
e^
n^
d^
\
m^
i^
d^
d^
l^
\0e
names
I/O
- #include <stdio.h>• stdin
,^ stdout
,^ stderr
- standard input, standard output, and standard error filestreams• character I/O – int
getchar
- read a single character from stdin – if none available returnEOF (end-of-file) • getchar()
is a macro that uses
getc
getc
( FILE *FP )
- read a single character from the file or stream identified by FP – int^
putchar
( char c )
- write the character c to stdout – if successful return the thecharacter c otherwise EOF. – int^
putc
( char c, FILE *FP )
- write the character c to the file or stream identified by FP
Formatted Output
-^ printf
prints a variable number of arguments using a specified format which is represented by a string^ – int
printf(
const
char
*FORMAT
[,^ ARG,
...]);
– FORMAT
is a string of characters (including special characters such as^ ‘\n’
for newline, and conversion formats for different types (with optional width and precision information). – man^
3 printf
for
more
info
-^ %d
int
-^ %c
character
-^ %s
string
-^ %f, %e, %g
floating point number
-^ %4.2f
(4 indicates width, 2 precision, i.e. xx.xx)
-^ %x
hexadecimal int
-^ %o
octal int
Example
#include <stdio.h> int main() {
typedef struct Employee Employee; struct Employee {
char
*name; double
wage; int^
id;
}; Employee worker; worker.name = "Fred"; worker.id = 1; worker.wage = 15.25; printf( "%s earns $%4.2f per hour\n",
worker.name, worker.wage ); return 0; }
Formatted Input
•^ scanf
is used to read a variable number of
arguments with input format specified by a fomatstring similar to
printf
- int scanf(const char *FORMAT
[, ARG, ...]);
scanf
are the addresses to the
variables passed – this is required since the inputvariables will be modified – There must be sufficient arguments for the number ofspecified conversion types or unpredictable behaviormay result – There must be sufficient space in the arguments (e.g.character arrays) to hold the input or unpredictablebehavior may result
Example
int main() {^ char name[MAXNAMELEN];
/* fixed size array */
/* prompt the user to enter their name and^ read the string.
*/
/* "%s" tells scanf to read a string,^ which it stores in the array
name
.^
*/
/* A string is read until whitespace is^ encountered.
*/
/* The address of the name must be^ passed to scanf.
*/
printf("Enter first name\n"); scanf("%s",name); printf("Hello %s\n",name); return 0; }
free and realloc
•^ When the memory allocated by
malloc
is no
longer in use, it should be freed using
free
memory leak
, which can cause
problems when lots of memory is required. – freeing memory when it is still being used is a commonbug; one that may be difficult to find.
•^ realloc
can be used to obtain additional
memory when the amount requested is insufficient(see
grow
from the text)
*realloc(
void
*APTR,
size_t
NBYTES
free(
void
*APTR
Example (1/2)
#include <stdlib.h> int main() {^ typedef struct Employee Employee;^ struct Employee {
char
*name;
double
wage;
int^
id;
}; Employee *worker;
/* allocate space for an Employee */ worker = (Employee *) malloc( sizeof( Employee ));
/* check for NULL - see emalloc in the text. */ if (worker == NULL) {
printf("malloc failed\n"); exit(1); }
