Void * that saves different structs - c

Question

typedef struct s_list
{
void *content;
struct s_list *next;
} t_list;
typedef struct s_env
{
char *key;
char *val;
struct s_env *next;
} t_env;
typedef struct s_heredoc
{
char *term;
char *delimiter;
char *heredoc;
} t_heredoc;
typedef struct s_cmd
{
char *cmd;
char *path;
int istream;
int ostream;
} t_cmd;
typedef struct s_table{
char **paths;
char **reserved;
char *cmdline;
t_list *cmds;
} t_table;
I have a general struct t_table which have a pointer to t_list struct called cmds.
How can i save different structs for example t_env, t_heredoc, t_cmd in void * content; property of t_list struct. How should I cast the void * content to another type (t_env, t_cmd, t_heredoc, etc)
void lexical_analyze(char *cmdline, t_table **table)
{
cmdline = openquotes(cmdline);
cmdline = find_replace(cmdline, (*table)->env);
(*table)->cmds = malloc(sizeof(t_list));
}
After this point i can’t understand how to cast and how to allocate a memory for the other structs for example t_cmd struct

Answer 1

You can assign a pointer to any struct to your context member. But if you want to use that struct later, you will need to recover the original pointer type. That means you must store extra information about type of your content in your struct:
typedef enum {
CT_Env,
CT_Cmd,
CT_HereDoc
} t_context_type;
typedef struct s_list
{
void *content;
t_context_type type;
struct s_list *next;
} t_list;
t_list *add_Env(t_env *env)
{
t_list *list = malloc(sizeof *list);
... Do the list handling...
list->context = env;
list->type = CT_Env;
return list;
}
Then, when you want to access the nodes of your list you can cast to correct type:
void handle_node(t_list *list)
{
switch (list->type)
{
case CT_Env:
{
t_env *env = list->context;
// Access members of t_enf via env pointer
}
break;
case CT_Cmd: // TODO handle t_cmd type
break;
case CT_HereDoc: // TODO handle t_heredoc type
break;
default: // TODO: Error handling
break;
}
}
Of course that code is not complete. It should show how it is supposed to work.