I know there are similar posts but I am having trouble getting the hang of it: I have a .c and a .h file:

graph.c

#ifndef graph
#define graph

/*1. standard headers*/
#include <stdbool.h>

/*2. user-def. headers*/
//#include ".."

/*3. global macros*/
//#define GLOB_MACRO … 

/*4. global type decls.*/

struct graph;
typedef struct graph graph_t;

graph_t* graph_create(int n);
void graph_free(graph_t *g);

void graph_insert_edge(graph_t *g, int source, int target, double weight);
void graph_remove_edge(graph_t *g, int source, int target);

bool graph_has_edge(graph_t *g, int source, int target);
double graph_get_weight(graph_t *g, int source, int target);

void graph_print(graph_t *g);

/*5. glob. var. decls.*/
//extern … glob_var; 

/*6. glob. func. decls*/
//[extern] … glob_func(…);

#endif

graph_m.c

 
// Driver Code
int main()
{
    printf("test");
 
    return 0;
}

But I get these compiler errors:

gcc  -MM graph_m.c > .depend
gcc -pedantic -Wall -Wextra -Wvla -std=c11 -ggdb3    -c -o graph_m.o graph_m.c
In file included from graph_m.c:14:
graph.h:15:13: error: expected '{' before ';' token
   15 | struct graph;
      |             ^
graph.h:16:22: warning: useless storage class specifier in empty declaration
   16 | typedef struct graph graph_t;
      |                      ^~~~~~~
graph.h:18:1: error: unknown type name 'graph_t'; use 'struct' keyword to refer to the type
   18 | graph_t* graph_create(int n);
      | ^~~~~~~
      | struct
graph.h:19:17: error: unknown type name 'graph_t'; did you mean 'graph'?
   19 | void graph_free(graph_t *g);
      |                 ^~~~~~~
      |                 graph
graph.h:21:24: error: unknown type name 'graph_t'; did you mean 'graph'?
   21 | void graph_insert_edge(graph_t *g, int source, int target, double weight);
      |                        ^~~~~~~
      |                        graph
graph.h:22:24: error: unknown type name 'graph_t'; did you mean 'graph'?
   22 | void graph_remove_edge(graph_t *g, int source, int target);
      |                        ^~~~~~~
      |                        graph
graph.h:24:21: error: unknown type name 'graph_t'; did you mean 'graph'?
   24 | bool graph_has_edge(graph_t *g, int source, int target);
      |                     ^~~~~~~
      |                     graph
graph.h:25:25: error: unknown type name 'graph_t'; did you mean 'graph'?
   25 | double graph_get_weight(graph_t *g, int source, int target);
      |                  graph
graph_m.c:24:1: warning: unnamed struct/union that defines no instances
   24 | } graph;
      | ^
make: *** [<builtin>: graph_m.o] Error 1
root@8c9bf05a5390:/home/swo3/src/graph# In file included from graph_m.c:14:
graph.h:15:13: error: expected '{' before ';' token
   15 | struct graph;
      |             ^
graph.h:16:22: warning: useless storage class specifier in empty declaration
   16 | typedef struct graph graph_t;
      |                      ^~~~~~~
graph.h:18:1: error: unknown type name 'graph_t'; use 'struct' keyword to refer to the type
   18 | graph_t* graph_create(int n);
      | ^~~~~~~
      | struct
make: *** [<builtin>: graph_m.o] Error 1union that defines no instances?e weight);

So am I getting the order wrong? I have typedef and struct in play, and different filenames for the actual implementation for the header file, where I want to do two datatstructures in the background for graph.h, graph_m.c and another one.

CodePudding user response：

You have macro names clashing with other names.

You start with this:

#define graph

Then later you have this

struct graph;

Since macros do direct token substitution, after preprocessing the above line becomes:

struct ;

Which is invalid.

You need to change your macro name so it doesn't conflict. By convention, macros are typically uppercase, and header guard macros typically end in _H. So change the first two lines to:

#ifndef GRAPH_H
#define GRAPH_H

CodePudding user response：

After #define graph, macro processing replaces struct graph; with struct ;.

Use a different identifier for your preprocessor guard, such as graph_h.