crash course in c dynamic allocationweb.mit.edu/ingolia/ccc01/dynalloc.pdfdynamic allocation of...
TRANSCRIPT
Dyn
amic
Allo
catio
n of
Mem
ory
malloc
The
mal
loc
func
tion
is u
sed
to a
lloca
te m
emor
y
void *malloc(size_t sz);
sz
Thi
s ar
gum
ent s
peci
fies
the
size
, in
byte
s, o
f
The
fun
ctio
n re
turn
s an
unt
yped
poi
nter
to th
e
the
mem
ory
to b
e al
loca
ted.
allo
cate
d m
emor
y, o
r N
UL
L if
the
mem
ory
cann
otbe
allo
cate
d.
int main(void)
{ return 0;
} printf("|double| = %d\n",
sizeof(double));
printf("|double| = %d\n", 8);
|double| = 8
Dyn
amic
Allo
catio
n of
Mem
ory
sizeof
Alw
ays
use
size
of to
det
erm
ine
the
size
of
data
.
At c
ompi
le ti
me,
a s
izeo
f ex
pres
sion
is e
xpan
ded
into
an
inte
ger.
{ char name[64];
strcpy(name, "Nicholas Ingolia");
return name;
}char *get_name(void)
Dyn
amic
Allo
catio
n of
Mem
ory
Pers
iste
nce
of A
lloca
ted
Mem
ory
The
sto
rage
ass
ocia
ted
with
nam
e di
sapp
ears
whe
n
Thu
s, th
e re
turn
ed p
oint
er is
use
less
and
dan
gero
us.
the
func
tion
retu
rns.
Dyn
amic
Allo
catio
n of
Mem
ory
Pers
iste
nce
of A
lloca
ted
Mem
ory
{ static char val[256];
int idx = keyidx(key);
strcpy(val, keyvals[idx].val);
return val;
}char *lookup(char *key)
Thi
s is
an
acce
ptab
le s
olut
ion.
..
Exc
ept,
the
next
cal
l to
look
up o
verw
rite
s th
e ol
d va
l, w
hich
is a
pro
blem
.
Dyn
amic
Allo
catio
n of
Mem
ory
Pers
iste
nce
of A
lloca
ted
Mem
ory
char *lookup(char *key)
{ char *val;
int idx = keyidx(key);
val = malloc(sizeof(char)
* (strlen(vals[idx]) + 1);
strcpy(val, vals[idx]);
return val;
} Val
will
per
sist
unt
il ex
plic
itly
deal
loca
ted.
A n
ew v
al w
ill b
e cr
eate
d on
eve
ry c
all t
o lo
okup
.
Dyn
amic
Allo
catio
n of
Mem
ory
Allo
catin
g St
ring
s
char *strdup(const char *str)
{ char *newstr;
int len = strlen(str);
newstr = malloc(sizeof(char)
* (len + 1));
strcpy(newstr, str);
return newstr;
} Stri
ngs
need
an
extr
a ch
ar to
hol
d th
e 0
term
inat
or.
Dyn
amic
Allo
catio
n of
Mem
ory
Allo
catin
g A
rray
s
double *normalize(double *d, int nd)
{ int i;
for (i = 0; i < nd; i++)
...
{
double *normalize(double *d, int nd)
{ double dnorm[nd];
double *dnorm;
dnorm = malloc(sizeof(double)*nd);
...
Dyn
amic
Allo
catio
n of
Mem
ory
Pers
iste
nce
of A
lloca
ted
Mem
ory
Allo
cate
d m
emor
y pe
rsis
ts
Thu
s, it
is n
eces
sary
to r
elea
se u
nnee
ded
mem
ory
Failu
re to
rel
ease
una
lloca
ted
mem
ory
caus
es th
e
Free
all
"tem
pora
ry"
allo
cate
d m
emor
y; e
nsur
e th
at
Not
e w
hich
fun
ctio
ns r
etur
n po
inte
rs to
new
mem
ory
Free
allo
cate
d m
emor
y "i
nsid
e" d
ata
stru
ctur
es
prog
ram
to c
onsu
me
syst
em r
esou
rces
.
mem
ory
is f
reed
bef
ore
all p
ossi
ble
retu
rns.
Dyn
amic
Allo
catio
n of
Mem
ory
Rel
easi
ng A
lloca
ted
Mem
ory
The
fre
e fu
nctio
n re
leas
es a
lloca
ted
mem
ory
void free(void *p)
pA
poi
nter
to a
lloca
ted
mem
ory
It is
ver
y im
port
ant t
hat f
ree
only
be
calle
d on
apo
inte
r re
turn
ed b
y m
allo
c or
str
dup.
Free
may
be
calle
d on
ly o
nce
on s
uch
a po
inte
r.
Nei
ther
the
poin
ter
nor
any
othe
r de
rive
d fr
om it
m
ay b
e us
ed a
fter
it is
fre
ed.
Dyn
amic
Allo
catio
n of
Mem
ory
Rel
easi
ng A
lloca
ted
Mem
ory
void foo(const char *str)
{ char *buf;
buf = strdup(str);
munge(buf);
display(buf);
free(buf);
}
Dyn
amic
Allo
catio
n of
Mem
ory
rho[17][37] = 0.666;
rho
...
3.2
8.1
5.7
0.2
0.7
3.4
8.2
5.5
0.1
0.3
3.4
8.2
5.5
0.1
0.3
3.2
8.1
5.7
0.2
0.7
A m
ultid
imen
sion
al a
rray
in C
is a
n ar
ray
of a
rray
s.
Mul
tidim
ensi
onal
Arr
ays
...
...
...
...
double rho[NROWS][NCOLS];
Dyn
amic
al A
lloca
tion
of M
emor
y
double *new_2d(int lx, int ly)
{ double **dx;
int x;
dx = malloc(sizeof(double *)*lx);
for (x = 0; x < lx; x++) {
dx[x] = malloc(sizeof(double)
* ly);
}
return dx;
}
Mul
tidim
ensi
onal
Arr
ays
Dyn
amic
Allo
catio
n of
Mem
ory
Rel
easi
ng M
ultid
imen
sion
al A
rray
s
void free_2d(double **d,
int nx, int ny)
{ int x;
for (x = 0; x < nx; x++) {
free(d[x]);
}
free(d);
} All
allo
cate
d m
emor
y m
ust b
e fr
eed.
Dyn
amic
Allo
catio
n of
Mem
ory
Alte
rnat
e M
ultid
imen
sion
al A
rray
s
Rat
her
than
usi
ng a
n ar
ray
of a
rray
s, it
is p
ossi
ble
rho[
double rho[NROWS * NCOLS];
(17 * NCOLS) + 37] = 0.666;
For
100
row
s, 5
0 co
lum
s:
but t
he a
ctua
l ind
ex m
ust b
e co
mpu
ted.
Thi
s re
quir
es o
nly
a si
ngle
blo
ck o
f m
emor
y,
...
...
...
...
0,49
1,1
1,0
1,49
99,099,1
99,49
0,1
0,0
to u
se a
sin
gle
arra
y of
siz
e (xy)
Mod
ular
Pro
gram
min
g in
CA
dvan
tage
s of
Mod
ular
Pro
gram
min
g
Prog
ram
s ou
ght t
o be
div
ided
into
sm
alle
r m
odul
es
Mod
ules
pro
vide
con
cept
ual b
ound
arie
s
Indi
vidu
al m
odul
es c
an b
e te
sted
, deb
ugge
d, a
nd r
euse
d
Mod
ules
ass
ist i
n al
loca
tion
man
agem
ent
Mod
ular
Pro
gram
min
g in
CIn
terf
aces
and
Im
plem
enta
tions
A C
pro
gram
can
be
brok
en in
to m
ultip
le s
ourc
e fi
les
Usi
ng o
ne s
ourc
e fi
le p
er m
odul
e he
lps
enfo
rce
mod
ule
boun
dari
es
Hea
der
file
s sp
ecif
y th
e in
terf
ace
to a
mod
ule
Inte
rfac
es in
clud
e fu
nctio
n an
d da
ta ty
pe d
ecla
ratio
ns
Sour
ce f
iles
prov
ide
the
impl
emen
tatio
n of
a m
odul
eIm
plem
enta
tions
pro
vide
def
initi
ons
of f
unct
ions
prom
ised
by
the
mod
ule
in th
e in
terf
ace.
Mod
ular
Pro
gram
min
g in
CH
eade
r Fi
les
/* llist.h */
#if !defined(_llist_h)
struct llist_struct;
typedef struct llist_struct llist;
llist *llist_new(void);
void llist_free(llist *);
llist *llist_insert(llist *,
const char *);
llist *llist_remove(llist *,
const char *);
int llist_contains(const llist *,
const char *);
#endif
#define _llist_h 1
Mod
ular
Pro
gram
min
g in
CH
eade
rs
The
C p
roce
ssor
can
be
used
to p
rote
ct a
gain
st
mul
tiple
incl
usio
ns o
f a
head
er f
ile
#if !defined(_header_h)
#define _header_h 1
...
#endif
Priv
ate
Stru
ctur
e D
efin
ition
s
struct llist_struct;
typedef struct llist_struct llist;
llist.c
llist.h
struct llist_struct {
llist *next;
};
Poin
ters
to a
n lli
st c
an b
e us
ed a
nyw
here
.
The
fie
lds
of a
n lli
st c
an b
e ac
cess
ed o
nly
in w
ithin
th
e lli
st.c
sou
rce
file
impl
emen
ting
an ll
ist.
char *elt;
Mod
ular
Pro
gram
min
g in
C
Mod
ular
Pro
gram
min
g in
CIm
mut
abili
ty
prom
ise
not t
o ch
ange
the
argu
men
t.A
con
stan
t poi
nter
in a
fun
ctio
n de
clar
atio
n is
a
char *strdup(const char *str);
The
cal
ling
func
tion
know
s th
at th
e st
r pa
ssed
to
strd
up w
ill n
ot b
e ch
ange
d.
C e
nfor
ces
this
agr
eem
ent,
thou
gh it
can
be
over
ridd
en
str[0] = ’a’;
Mod
ular
Pro
gram
min
g in
CC
ompi
ling
and
Lin
king
A c
ompi
led
obje
ct is
a c
olle
ctio
n of
fun
ctio
ns a
wai
ting
com
plet
ion
by li
nkin
g
llist_new
...
malloc
...
strdup
...
llist_free
... ...
free
llist_new 2718281
strdup 3141592
malloc 1618034
Mod
ular
Pro
gram
min
g in
CC
ompi
latio
n an
d L
inki
ng
Lin
king
ass
embl
es o
bjec
ts in
to a
pro
gram
The
link
er b
uild
s a
tabl
e of
fun
ctio
ns a
nd g
loba
l va
riab
les
prov
ided
by
vari
ous
obje
cts
The
tabl
e is
use
d to
"co
mpl
ete"
the
obje
cts
blllist_new
bl 2718281
...
...
