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---
title: "Allocation dynamique de mémoire"
date: "2021-11-09"
patat:
wrap: true
margins:
left: 10
right: 10
---
# Allocation dynamique de mémoire (1/8)
- La fonction `malloc`{.C} permet d'allouer dynamiquement (pendant l'exécution du programme) une zone de mémoire contiguë.
```C
#include <stdlib.h>
void *malloc(size_t size);
```
- `size`{.C} est la taille de la zone mémoire **en octets**.
- Retourne un pointeur sur la zone mémoire ou `NULL`{.C} en cas d'échec: **toujours vérifier** que la valeur retournée est `!= NULL`{.C}.
# Allocation dynamique de mémoire (2/8)
- On peut allouer et initialiser une `fraction_t`{.C}:
```C
fraction_t *num = malloc(sizeof(fraction_t));
num->num = 1;
num->denom = -1;
```
- La zone mémoire **n'est pas** initialisée.
- Désallouer la mémoire explicitement $\Rightarrow$ **fuites mémoires**.
<!-- - Toujours garder un pointeur sur la mémoire allouée sinon **pointeur pendouillant**. -->
![La représentation mémoire de `fraction_t` et fuites.](figs/pointer_struct_ok.svg){width=100%}
# Allocation dynamique de mémoire (3/8)
- La fonction `free()`{.C} permet de libérer une zone préalablement allouée avec `malloc()`{.C}.
```C
#include <stdlib.h>
void free(void *ptr);
```
- Pour chaque `malloc()`{.C} doit correspondre exactement un `free()`{.C}.
- Si la mémoire n'est pas libérée: **fuite mémoire** (l'ordinateur plante quand il y a plus de mémoire).
- Si la mémoire est **libérée deux fois**: *seg. fault*.
- Pour éviter les mauvaises surprises mettre `ptr`{.C} à `NULL`{.C} après
libération.
# Allocation dynamique de mémoire (4/8)
## Tableaux dynamiques
- Pour allouer un espace mémoire de 50 entiers: un tableau
```C
int *p = malloc(50 * sizeof(int));
for (int i = 0; i < 50; ++i) {
p[i] = 0;
}
```
## Arithmétique de pointeurs
- Parcourir la mémoire différemment qu'avec l'indexation
```C
int *p = malloc(50 * sizeof(int));
// initialize somehow
double a = p[7];
double b = *(p + 7); // on avance de 7 "double"
p[0] == *p; // le pointeur est le premier élément
```
# Allocation dynamique de mémoire (5/8)
## Arithmétique de pointeurs
![L'arithmétique des pointeurs.](figs/pointer_arithmetics.svg){width=100%}
## Quelle est la complexité de l'accès à une case d'un tableau?
. . .
$$
\mathcal{O}(1).
$$
# Allocation dynamique de mémoire (6/8)
## Pointeur de pointeur
- Tout comme une valeur a une adresse, un pointeur a lui-même une adresse:
```C
int a = 2;
int *b = &a;
int **c = &b;
```
- Chaque `*`{.C} ou `&`{.C} rajoute une indirection.
# Allocation dynamique de mémoire (7/8)
## Pointeur de pointeur
![Les références de pointeurs.](figs/double_pointeur.svg){height=100%}
# Allocation dynamique de mémoire (8/8)
- Avec `malloc()`, on peut allouer dynamiquement des tableaux de pointeurs:
```C
int **p = malloc(50 * sizeof(int*));
for (int i = 0; i < 50; ++i) {
p[i] = malloc(70 * sizeof(int));
}
int a = p[5][8]; // on indexe dans chaque dimension
```
- Ceci est une matrice (un tableau de tableau).
# Les *sanitizers*
Problèmes mémoire courants:
* Dépassement de capacité de tableaux.
* Utilisation de mémoire non allouée.
* Fuites mémoire.
* Double libération.
Outils pour leur détection:
* Valgrind (outil externe).
* Sanitizers (ajouts de marqueurs à la compilation).
Ici on utilise les sanitizers (modification de la ligne de compilation):
```bash
gcc -o main main.c -g -fsanitize=address -fsanitize=leak
```
**Attention:** Il faut également faire l'édition des liens avec les sanitizers.
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style="font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:10.58333302px;line-height:1.25;font-family:monospace;-inkscape-font-specification:monospace;letter-spacing:0px;word-spacing:0px;stroke-width:0.26458332"
x="59.490494"
y="155.76122"
id="text1463"><tspan
sodipodi:role="line"
id="tspan1461"
x="59.490494"
y="155.76122"
style="stroke-width:0.26458332">denom</tspan></text>
<text
xml:space="preserve"
style="font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:8.47521019px;line-height:1.25;font-family:monospace;-inkscape-font-specification:monospace;letter-spacing:0px;word-spacing:0px;stroke-width:0.21188024"
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id="text1467"><tspan
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id="tspan1465"
x="0.21260698"
y="181.14244"
style="stroke-width:0.21188024">fraction_t *frac = malloc(sizeof(fraction_t));</tspan><tspan
sodipodi:role="line"
x="0.21260698"
y="191.73645"
style="stroke-width:0.21188024"
id="tspan1471">frac = NULL; // addresse mémoire inaccessible</tspan></text>
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