Allocating memory to hold an array of structs in C

Antithesis Published at Dev

Antithesis

I'm trying to allocate memory to hold an array of structs

SERVER* topology = malloc(sizeof(struct SERVER*)* 10 );
    for (int i = 0; i < 10; ++i)
    {
        topology[i] = malloc(sizeof(struct SERVER));
    }
    PATH* paths = malloc(sizeof(struct PATH*)*10);
    for (int i = 0; i < 10; ++i)
    {
        paths[i] = malloc(sizeof(struct PATH));
    }

These are my structs

typedef struct{
    int id;
    char ip_addr[MAX_IP + 1];
    int port;
}SERVER;

typedef struct{
    int server1;
    int server2;
    int weight;
}PATH;

And then later in my code I try to free it using

for (int i = 0; i < 10; ++i)
   {
     free(paths[i]);
   }
   free(paths);

   for (int i = 0; i < 10; ++i)
   {
     free(topology[i]);
   }
   free(topology);

I keep getting the following error

error: invalid application of 'sizeof' to an incomplete type 'struct SERVER'
                topology[i] = malloc(sizeof(struct SERVER));
                                     ^     ~~~~~~~~~~~~~~~
:18:42: note: forward declaration of 'struct SERVER'
        SERVER* topology = malloc(sizeof(struct SERVER*)* 10 );
                                                ^
:26:21: error: invalid application of 'sizeof' to an incomplete type 'struct PATH'
                paths[i] = malloc(sizeof(struct PATH));
                                  ^     ~~~~~~~~~~~~~
:23:37: note: forward declaration of 'struct PATH'
        PATH* paths = malloc(sizeof(struct PATH*)*10);

...........

c:97:11: error: passing 'PATH' to parameter of incompatible type 'void *'
         free(paths[i]);
              ^~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/stdlib.h:143:18: note: passing argument to parameter here
void     free(void *);
                    ^
:103:11: error: passing 'SERVER' to parameter of incompatible type 'void *'
         free(topology[i]);
              ^~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/stdlib.h:143:18: note: passing argument to parameter here
void     free(void *);

I'm generally new to C. Any help appreciated.

Kenney

Try this:

typedef struct SERVER {
  ....
} SERVER;

typedef struct PATH {
  ....
} PATH;

You see, you use struct SERVER and struct PATH but you haven't declared these. You did declare a type SERVER and a type PATH using a typedef on an unnamed struct.

Alternatively, you could leave your structs as they are, and use sizeof(PATH*) and sizeof(STRUCT*).

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