Strange characters from char array in structure

Question

Here are my structures (defined in a header file):

typedef struct                          
{
    char            *name;
    char            *value;
} struct_param;

typedef struct                          
{
  char              *UID;               
  int               number;             
  char              *type;              
  char              *name;              
  struct_param      param[10];          
} struct_cmd;

the prototype :

struct_cmd *ParseFile(char buffer[]);

The function in the c file:

struct_cmd *ParseFile(char buffer[])
{
struct_cmd      *cmd;
cmd = malloc(sizeof(struct_cmd));
...
if (mxmlGetFirstChild(node_msgUID) != NULL)
    cmd->UID = node_msgUID->child->value.opaque;

...
printf("Message Type :: %s | Message UID :: %s \n", cmd->type, cmd->UID);
...

return cmd;

}

The printf in ParseFile works perfectly.

Now, from the main function:

int main(int argc, char **argv)
{
    ...
    struct_cmd *mystruct;
    mystruct = malloc(sizeof(struct_cmd));
    mystruct = ParseFile(buf);
    printf("Message Type :: %s | Message UID :: %s \n", mystruct->type, mystruct->UID);
    ...
}

The same printf doesn't work. The function returns the structure, but values are weird... It's not values, but strange characters.

Any idea? Thanks

Answer 1

You are making a shallow copy from the data allocated by Mini-XML to your own struct cmd.

For example, this statement copies a pointer, not the actual characters:

cmd->UID = node_msgUID->child->value.opaque;

cmd->UID still refers to the original memory block allocated by Mini-XML. There's nothing wrong with that, just remember that this memory will be de-allocated once you call mxmlDelete. Which is probably what you are doing somewhere near the end of function ParseFile. I am guessing here, since you did not post all your code.

Possible solutions:

1. Instead of a shallow copy, make a deep copy, e.g. with strdup: cmd->UID = strdup(node_msgUID->child->value.opaque);
2. Do all processing before freeing memory.

Remember, you are programming in plain C, without a garbage collector. Memory management is your responsibility.