Type name unknown when using struct as type in C?

Question

I cannot for the life of me figure why my code isn't compiling. I am using an up to date gcc version to compile and this code is in the bottom part of a lex file (so it is copied directly into a .c file after using flex on it) which makes me 90% sure that this problem is due to something I am doing wrong in C. Here is the code which is causing problems:

void checkAliases() {
    if (aliasHead==0) {
        printf("No aliases have been created.");
    }
    else {
    struct AliasNode* current = aliasHead;
    printf("Current Aliases: \n");
    while (current!=0) {
        printAlias(current);
        current = current->next;
    }
}

struct AliasNode {
    struct AliasNode* next;
    char* key;
    char* value;
};

struct AliasNode* aliasHead = 0;


void printAlias(struct AliasNode* alias) {
    char* toPrint = alias->key;
    printf(toPrint);
    printf(": ");
    toPrint = alias->value;
    printf(toPrint);
    printf("\n");
}

Here are the errors I am getting:

error: ‘aliasHead’ undeclared (first use in this function)
   if (aliasHead==0) {
       ^
error: unknown type name ‘AliasNode’
   AliasNode* current = aliasHead;

error: request for member ‘next’ in something not a structure or union
    current = current->next;
                     ^

error: unknown type name ‘AliasNode’
   AliasNode* next;
   ^

This C code (along with thousands of lines more that I left out) is generated by this yacc file:

%token CD BYE ALIAS

%%

program: /*empty*/
     | program command
    ;

command:
    cd|bye|alias

cd:     
    CD     {printf("Not a valid directory\n");    }
    ;
bye:    
    BYE    {exit(0); }
     ;
alias:
    ALIAS   {
    if (aliasHead==0) {
        printf("No aliases have been created.");
    }
    else {
    AliasNode* current = aliasHead;
    printf("Current Aliases: \n");
    while (current!=0) {
        printAlias(current);
        current = current->next;
    }


    }
    }
    ;

    %%

    /*global variables*/

/*linked list to store alias - command pairs */
struct AliasNode {
    AliasNode* next;
    char* key;
    char* value;
};

struct AliasNode* aliasHead = 0;

void printAlias(struct AliasNode* alias) {
    char* toPrint = alias->key;
    printf(toPrint);
    printf(": ");
    toPrint = alias->value;
    printf(toPrint);
    printf("\n");
}

I am absolutely lost as to how to fix these errors. Any help at all, even if its just a nudge in the right direction, is greatly appreciated. Thanks in advance!

Edit:

I changed my code in the Yacc file so that now when I put it through Bison, the .c I get out has this:

struct AliasNode {
AliasNode* next;
char* key;
char* value;
};

struct AliasNode* aliasHead = 0;

void printAlias(struct AliasNode* alias) {
    char* toPrint = alias->key;
    printf(toPrint);
    printf(": ");
    toPrint = alias->value;
    printf(toPrint);
    printf("\n");
}

void aliasNoPrompt() {
    if (aliasHead==0) {
        printf("No aliases have been created.");
    }
    else {
        AliasNode* current = aliasHead;
        printf("Current Aliases: \n");
        while (current!=0) {
            printAlias(current);
            current = current->next;
    }
}

However, I am still running into the same errors noted above. Note that from the main, I call aliasNoPrompt().

Answer 1

Put the declaration to the top of your code. C requires code written in top to down order, meaning you should define AliasNode before use it.

struct AliasNode {
    struct AliasNode* next;
    char* key;
    char* value;
};

struct AliasNode* aliasHead = 0;

// ... other code here

Answer 2

Your struct declaration has to be above functions that use it.

---

struct AliasNode {
    struct AliasNode* next;
    char* key;
    char* value;
};

struct AliasNode* aliasHead = 0;

void printAlias(struct AliasNode* alias) {
    char* toPrint = alias->key;
    printf(toPrint);
    printf(": ");
    toPrint = alias->value;
    printf(toPrint);
    printf("\n");
}

void checkAliases() {
if (aliasHead==0) {
        printf("No aliases have been created.");
    }
    else {
    struct AliasNode* current = aliasHead;
    printf("Current Aliases: \n");
    while (current!=0) {
        printAlias(current);
        current = current->next;
    }
}