Question

我比C更新并做一些练习，在这个例子中，我想构建一个表，它可以包含很多元素，这是一个符号类型，但我不知道怎么写那个部分。我想要使用malloc将堆内存分配给Symbol并插入表中（SymbolTable）。

#include<stdio.h>
#include<stdlib.h> 
#include<stdint.h>
#include<string.h>

typedef struct {

    char *name;
    uint32_t addr;
}Symbol;

typedef struct {
    Symbol* tbl;
    uint32_t len;
    uint32_t cap;
    int mode;
} SymbolTable; /*this is the table i want to mantiply*/

SymbolTable* create_table(int mode) {
    SymbolTable* st = (SymbolTable*)malloc(sizeof(SymbolTable));
    if (st != NULL)
    {
        st->mode = mode;
        st->len = 0;
        return st;
    }
    printf("Memory allocation failed!\n");
    return NULL;
}

void free_table(SymbolTable* table) {
    int i;
    for (i = 0; i < table->len; ++i)
    {
        free(table->tbl[i].name);
        free(&(table->tbl[i]));
    }
    free(table);
}

int add_to_table(SymbolTable* table, const char* name, uint32_t addr) {
    if (addr % 4 != 0)
    {
        printf("Address alignment erron!\n");
        return -1;
    }
    int table_len = table->len;
    if (table->mode == 1)
    {
        int i;
        for (i = 0; i < table_len; ++i)
        {
            if (*((table->tbl[i]).name) == *name)
            {
                printf("Name existed!\n");
                return -1;
            }
            `I don't know how to inset element here`
        }
    }
    return 0;
}

int main()
{
    SymbolTable * st = create_table(0);
    add_to_table(st, "aaa", 4);
    add_to_table(st, "bb", 8);
    write_table(st, stdout);
    free_table(st);
}

Answer 1

首先，我认为您需要在*tbl中分配cap并设置create_table()（如果他在表格中包含最大nb的单元格）。

接下来，在add_to_table()中，尝试malloc(sizeof(struct symbol))如果(len < cap)并为*name分配内存，并设置为您的价值（不要忘记{在\0结束时{1}}。将其分配给*name，不要忘记增加tbl[len]。

尝试将函数分开，例如返回索引的len或-1，或创建和设置新符号的int is_in_table(const char *name)。

我希望我对你有用：）。

Answer 2

您的代码几乎没有问题;另外，我不得不做一些假设，因为你没有在你的问题中指明这些细节：

cap是table->tbl的容量，即我们分配了多少内存
添加新符号时，我们应该复制包含其名称的字符串，而不是仅仅将该指针指定给新的Symbol条目。

你还应该选择一种编码风格并坚持下去（相同与新线上的大括号，T* ptr vs T *ptr等）。最后，我删除了create_table中的演员表;见Do I cast the result of malloc?

以下是您的代码的固定版本;在add_to_table中，如果我们没有足够的内存来添加新内存，我们将Symbol数组的容量加倍（每次调用realloc为另外一个元素添加空间会浪费）。当我们扩展数组的容量时，我们必须注意将每个name指针设置为NULL，因为如果我们不这样做，free_table时会调用cap ; len，我们会尝试free一个未初始化的指针（而在free上调用NULL则完全正常，什么也不做。）

#include <stdio.h>
#include <stdlib.h> 
#include <stdint.h>
#include <string.h>

typedef struct
{
    char* name;
    uint32_t addr;
} Symbol;

typedef struct
{
    Symbol* tbl;
    uint32_t len;
    uint32_t cap;
    int mode;
} SymbolTable; /*this is the table i want to mantiply*/

SymbolTable* create_table(int mode)
{
    SymbolTable* st = malloc(sizeof(SymbolTable));
    if (st != NULL)
    {
        st->tbl = NULL;
        st->mode = mode;
        st->len = 0;
        st->cap = 0;
        return st;
    }
    printf("Memory allocation failed!\n");
    return NULL;
}

void free_table(SymbolTable* table)
{
    int i;
    for (i = 0; i < table->cap; ++i)
    {
        free(table->tbl[i].name);
    }
    free(table->tbl);
    free(table);
}

int add_to_table(SymbolTable* table, const char* name, uint32_t addr)
{
    if (addr % 4 != 0)
    {
        printf("Address alignment erron!\n");
        return -1;
    }
    int table_len = table->len;
    if (table->mode == 1)
    {
        int i;
        for (i = 0; i < table_len; ++i)
        {
            if (!strcmp(table->tbl[i].name, name))
            {
                printf("Name existed!\n");
                return -1;
            }
        }

        if (table_len + 1 > table->cap)
        {
            // allocate more memory
            uint32_t new_cap = table->cap ? table->cap * 2 : 2;
            table->tbl = realloc(table->tbl, new_cap * sizeof(*table->tbl));
            if (table->tbl == NULL)
            {
                // handle the error
            }

            table->cap = new_cap;
            int i;
            for (i = table_len; i < new_cap; ++i)
            {
                table->tbl[i].name = NULL;
            }
        }

        uint32_t name_len = strlen(name);
        table->tbl[table_len].name = malloc(name_len + 1);
        strncpy(table->tbl[table_len].name, name, name_len);
        table->tbl[table_len].name[name_len] = '\0';

        table->tbl[table_len].addr = addr;
        table->len++;
    }

    return 0;
}

int main(void)
{
    SymbolTable*  st = create_table(1);
    add_to_table(st, "aaa", 4);
    add_to_table(st, "bb", 8);
    write_table(st, stdout);
    free_table(st);
}

Answer 3

使用实现自引用结构的概念。给出提示：

typedef struct S{
    char *name;
    uint32_t addr;
    S* next;
}Symbol;

typedef struct {
    Symbol* tbl;
    uint32_t len;
    int mode;
} SymbolTable;

http://www.how2lab.com/programming/c/link-list1.php

self referential struct definition?

可能的实施

#include<stdio.h>
#include<stdlib.h> 
#include<stdint.h>
#include<string.h>
#include <stdio.h> 
typedef struct S{
    char *name;
    uint32_t addr;
    S* next;
}Symbol;

typedef struct {
    Symbol* tbl;
    uint32_t len;
    int mode;
} SymbolTable; /*this is the table I want to maltiply*/

SymbolTable* create_table(int mode) {
    SymbolTable* st = (SymbolTable*)malloc(sizeof(SymbolTable));
    if (st != NULL)
    {
        st->tbl = NULL;
        st->mode = mode;
        st->len = 0;
        return st;
    }
    printf("Memory allocation failed!\n");
    return NULL;
}

void free_table(SymbolTable* table) {
    free(table);
}

int add_to_table(SymbolTable* table,  char* name, uint32_t addr) {
    if (addr % 4 != 0)
    {
        printf("Address alignment erron!\n");
        return -1;
    }
    int table_len = table->len;
    if (table->mode == 1)
    {
        if (table_len == 0)
        {
            Symbol *t = (Symbol*)malloc(sizeof(Symbol));
            t->name = name;
            t->next = NULL;
            table->len++;
            table->tbl = t;
        }
        else
        {
            Symbol *t = table->tbl;
            while (t->next != NULL)
            {
                if (t->name == name)
                {
                    printf("Name existed!\n");
                    return -1;
                }
                t = t->next;
            }   
            if (t->name == name)
            {
                printf("Name existed!\n");
                return -1;
            }
            t->next = (Symbol*)malloc(sizeof(Symbol));
            table->len++;
            t->next->name = name;
            t->next->next = NULL;
        }
    }
    return 0;
}
void write_table(SymbolTable *st)
{
    Symbol *t = st->tbl;
    while (t != NULL)
    {
        printf("%s\n",t->name);
        t = t->next;
    }
    printf("\n");
}
int main()
{
    SymbolTable *st = create_table(0);
    st->mode = 1;// Table mode setting to 1 for importing next value to table.
    // You may implement it in your own way.
    add_to_table(st, "cc", 8);
    st->mode = 1;
    add_to_table(st, "bb", 8);
    st->mode = 1;
    write_table(st);
    free_table(st);
}

如何将元素插入此结构中

3 个答案: