Allocate and free memory of an array from structure

Question

I have following structure:

typedef struct
{
    char *name[10];
    char *msg[100];
} Log;

How I can free the name and msg arrays from Log structure? I know free is used only for dynamic allocation, and dynamic allocation doesn't work in structures. What I can do?

Tryed this but give error:

typedef struct
{
    char *name[] = (char *)malloc(sizeof(char) * 10);
    char *msg[] = (char *)malloc(sizeof(char) * 100);
} Log;

Who can help me? Thanks!

You have tagged this as C++, but the code is most certainly C. Which do you want an answer in? — TartanLlama, Sep 10 '14 at 13:54
Depends. First off, a `char* x[]` is a char-ptr array. `char *name[10]` allocates 10 char ptrs, which is not what you want to do. `typedef struct {char name[10]; char msg[100];} Log;` will allocate these fields as inline to the structure (`sizeof(Log)>=110`). Else you'll need to provide an init and cleanup function for the struct. Personally, in such cases I make the user only interact with handles and never the struct directly. — IdeaHat, Sep 10 '14 at 14:01
`sizeof(char)` is per definitionem 1, so don't write it. Also, [Don't cast the result of malloc (and friends)](http://stackoverflow.com/q/605845). — Deduplicator, Sep 10 '14 at 14:06

haccks · Accepted Answer · 2014-09-10T19:59:08.050

Declaring a structure doesn't allocate memory for its members. Memory is allocated when an instance of a structure is created. So

typedef struct
{
    char *name[10];
    char *msg[100];
} Log;

doesn't allocate memory for name and msg, it just declare Log as a new data (user defined) type. But when you create an instance of it

Log log_flie;

memory is allocated for name and msg. Now you can allocate memory (dynamically) for elements of data member as

for(int i = 0; i < 10; i++)
    log_file.name[i] = malloc(N);  //N is the desired size

Similarly you can do for msg.

To free the dynamically allocated memory, call free as

for(int i = 0; i < 10; i++)
    free(log_file.name[i]);

If you want to allocate memory for name and msg dynamically then do as follows

typedef struct
{
    char **name;
    char **msg;
} Log;

Then

Log log_file;
log_file.name = malloc(10 * sizeof(char *));
log_file.msg = malloc(100 * sizeof(char *));

Allocate and free memory of an array from structure

1 Answers1