How to access structs in Unions and point to them in c - c

Question

I have to implement a contructor function in c based upon this struct:
struct Node {
char name[MAX_NAME_LEN + 1];
NodeType type;
union {
struct {
Entry* entries; // list of directory entries
} dir;
struct {
void* contents; // any binary data of the given length
int length;
} file;
};
};
Now I want to build a constructor function to file, point to the contents and save the length in bytes but somehow I don't know how to do it...
Here is my Attempt:
i KNOW that i have to allocate space for this but how do i make this pointer stuff?
Node* new_file(char* name) {
Node* ptr_file = xmalloc(sizeof(Node));
ptr_file->name;
return NULL;
}

Answer 1

You need to typedef struct { ... } Node for your code to compile.
When using anonymous structs/unions you simply don't give a name for the anonymous member. ptr_file->dir.entries or ptr_file->file.contents.
Should you ditch the internal struct names and make those anonymous as well you would only need to type ptr_file->entries;.
Please note that anonymous structs/unions were added to the C language in the ISO 9899:2011 version of the language, so you need to use a fairly modern compiler to use them.
As a side note, the void* probably doesn't make any sense. What I think that you are trying to do is this:
#include <stdint.h>
typedef struct {
char name[MAX_NAME_LEN + 1];
NodeType type;
union {
struct {
Entry* entries; // list of directory entries
};
struct {
uint8_t contents [sizeof(Entry)]; // use to inspect raw data
int length; // probably not needed
};
};
} Node;

Answer 2

With Node* new_file(char* name), Node is not yet defined. Code needs typedef struct Node Node; or the like.
A big challenge to this task is the many potential errors that could creep in: file name too long, memory allocation failure, fopen open failure, read failure, file too long, ...
int length; should be size_t length; as files may be longer than INT_MAX, yet fit in an allocation.
new_file(char* name) looks like it should read a file. Let's make a helper function as there are various degrees of robustness needed. The below commonly "works" yet is technical UB (seeking to the end of a binary file). Adjust as needed.
Illustrative, untested code:
// The below does _not_ return a null character appended string,
// just the data in the file.
void *alloc_file(const char *file_name, size_t *size) {
*size = 0;
FILE *binary_stream = fopen(file_name, "rb");
if (binary_stream == NULL) {
return NULL;
}
long offset = fseek(binary_stream, SEEK_END);
rewind(binary_stream);
if (offset == -1 || (unsigned long) offset > SIZE_MAX) {
fclose(binary_stream); // Trouble or file too long
return NULL;
}
size_t file_size = (size_t) offset;
void *buf = malloc(file_size) {
fclose(binary_stream);
return NULL;
}
if (fread(buf, 1, file_size, binary_stream) != file_szie) {
fclose(binary_stream);
free(buf);
return NULL;
}
*size = file_size;
fclose(binary_stream);
return buf;
}
Now new_file is easier.
// Better as: Node* new_file(const char* name)
Node* new_file(char* name) {
if (strlen(name) >= MAX_NAME_LEN) {
return NULL // name too long
}
// Allocate to the size of the referenced object, not type.
Node* ptr_file = malloc(sizeof ptr_file[0]);
if (ptr_file == NULL) {
return NULL;
}
strcpy(ptr_file->name, name);
ptr_file->type = NodeType_file; // Some constant (OP has not shown yet).
size_t size = 0;
ptr_file->file.contents = alloc_file(name, &size);
if (ptr_file->file.contents == NULL || size > INT_MAX) {
free(ptr_file->file.contents);
free(ptr_file);
return NULL;
}
ptr_file->length = (int) size;
return ptr_file;
}