added 2024

slides/allocation_dynamique.md

+---
+title: "Allocation dynamique de mémoire"
+date: "2024-12-03"
+---
+
+# 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}.
+- Le *type* du retour est `void *`{.C} (un pointeur de type quelconque).
+
+# Allocation dynamique de mémoire (2/8)
+
+- Allocation *sur le tas* d'un type de base:
+
+    ```C 
+    int *val = malloc(sizeof(int)); // réserve 4 octets sur le tas
+    *val = 4; // les 4 octets contiennent la valeur 4
+    ```
+- Presque la même chose que:
+
+    ```C 
+    int val = 4; // ici l'allocation et l'initialisation
+                 // sont faites en une fois mais se trouve sur
+                 // le *tas* (mémoire managée automatiquement)
+    ```
+- Attention:
+    
+    ```C 
+    int *val = 4; // Ca c'est très faux...
+    ```
+
+
+# Allocation dynamique de mémoire (3/9)
+
+- On peut allouer et initialiser une `fraction_t`{.C}:
+
+    ```C
+    fraction_t *frac = malloc(sizeof(fraction_t));
+    frac->num = 1;
+    frac->denom = -1;
+    ```
+- La zone mémoire **n'est pas** initialisée.
+- Désallouer la mémoire explicitement, sinon **fuites mémoires**.
+- Il faut connaître la **taille** des données à allouer.
+
+![La représentation mémoire de `fraction_t` et fuites.](figs/pointer_struct_ok.svg){width=100%}
+
+# Allocation dynamique de mémoire (4/9)
+
+- La fonction `free()`{.C} permet de libérer une zone préalablement allouée avec `malloc()`{.C}.
+
+    ```C
+    #include <stdlib.h>
+    void free(void *ptr);
+    ```
+- A 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 (5/9)
+
+## 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
+    int a = p[7];
+    int b = *(p + 7); // on avance de 7 "int"
+    p[0] == *p; // le pointeur est le premier élément
+    ```
+
+# Allocation dynamique de mémoire (6/9)
+
+## 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 (7/9)
+
+## 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;
+    ```
+- En effet, un pointeur est aussi une variable (une variable qui contient une adresse mémoire).
+
+- Chaque `*`{.C} rajoute une indirection.
+
+# Allocation dynamique de mémoire (8/9)
+
+## Pointeur de pointeur
+
+![Les références de pointeurs.](figs/double_pointeur.svg){height=100%}
+
+# Allocation dynamique de mémoire (9/9)
+
+- 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 tableaux).
+
+# Tableau dynamique en argument d'une fonction
+
+## Implémenter la fonction ci-dessous
+
+```C
+int32_t *p = malloc(50 * sizeof(*p));
+initialize_to(p, 50, -1); // initialise un tableau à -1
+free(p); // ne pas oublier
+```
+
+. . .
+
+```C
+void initialize_to(int32_t *p, size_t size, int32_t val) {
+    for (size_t i = 0; i < size; ++i) {
+        p[i] = val;
+    }
+}
+```
+
+
+# Tableau dynamique retourné d'une fonction
+
+## Implémenter la fonction ci-dessous
+
+```C
+// alloue un tableau de taille 50 et l'initialise à -1
+int32_t *p = initialize_to(50, -1); 
+free(p); // ne pas oublier
+```
+
+. . .
+
+```C
+int32_t *initialize_to(size_t size, int32_t val) {
+    int32_t *p = malloc(size * sizeof(*p));
+    for (size_t i = 0; i < size; ++i) {
+        p[i] = val;
+    }
+    return p;
+}
+```
+
+## Pourquoi on peut retourner un tableau dynamique et pas un statique?
+
+. . .
+
+* Le tableau est alloué sur le **tas** et non sur la **pile**. 
+* La mémoire est gérée manuellement sur le tas, automatiquement sur la pile.
+
+
+
+# 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, modifiez donc vos *Makefile*):
+
+```bash
+gcc -o main main.c -g -fsanitize=address -fsanitize=leak
+```
+
+**Attention:** Il faut également faire l'édition des liens avec les sanitizers.
+
+# Questions
+
+## Que fait le code suivant?
+
+```C
+int *p = malloc(50 * sizeof(int));
+p[10] = 1;
+```
+
+. . .
+
+* On alloue de la place pour 50 entiers.
+* On initialise le 11ème élément du tableau à 1.
+* Les autres éléments sont non-initialisés.
+
+# Questions
+
+## Que fait le code suivant?
+
+```C
+float *p = malloc(50);
+p[20] = 1.3;
+```
+
+. . .
+
+* On déclare un pointeur de floats de taille 50 octets.
+* Mais il ne peut contenir que `50 / 4` floats (un float est composé de 32 bits).
+* On dépasse la capacité de la mémoire allouée: comportement indéfini.
+
+
+# Questions
+
+* Soit le code suivant
+
+```C
+int *p = malloc(50 * sizeof(int));
+for (int i = 0; i < 50; ++i) {
+    p[i] = 0;
+}
+```
+
+* Le réécrire en utilisant uniquement l'arithmétique de pointeurs.
+
+. . .
+
+```C
+int *p = malloc(50 * sizeof(int));
+for (int i = 0; i < 50; ++i) {
+    *(p+i) = 0;
+}
+```
+
+# Questions
+
+## Que valent les expressions suivantes?
+
+```C
+in32_t *p = malloc(50 * sizeof(int32_t));
+sizeof(p);
+sizeof(*p);
+(p + 20);
+*(p + 20);
+p[-1];
+p[50];
+7[p];
+```

slides/figs/pointer_struct_ok.svg

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