Linked Lists & Pointer Syntax errors

Question

This program simply takes a file with ASCII lines, puts it into a linked-list stack, and then prints the reversed list to a new file in the same ASCII format.

My struct Code:

typedef struct Node{
    char info[15];
    struct Node *ptr;
};

I'm getting the following errors on Main. Most have to do where I declare the new Node Head... what's wrong with that syntax?:

Errors
    strrev.c:28: error: ‘Node’ undeclared (first use in this function)
    strrev.c:28: error: (Each undeclared identifier is reported only once
    strrev.c:28: error: for each function it appears in.)
    strrev.c:28: error: ‘head’ undeclared (first use in this function)
    strrev.c:34: warning: passing argument 1 of ‘strcpy’ from incompatible pointer type
   /usr/include/string.h:128: note: expected ‘char * __restrict__’ but argument is of         type ‘char **’

Main Code:

int main(int argc, char *argv[])
{
    if (argc != 3) {
        fprintf(stderr, "usage: intrev <input file> <output file>\n");
        exit(1);
    }

    FILE *fp = fopen(argv[1], "r");
    assert(fp != NULL);


    Node *head = malloc(sizeof(Node));
    head->ptr=NULL;

    char str[15];
    while (fgets(str, 15, fp) != NULL){
        struct Node *currNode = malloc(sizeof(Node));
        strcpy(currNode->info, str);
        currNode->ptr = head;
        head=currNode;
    }

    char *outfile = argv[2];
    FILE *outfilestr = fopen(outfile, "w");
    assert(fp != NULL);

    while (head->ptr != NULL){
        fprintf(outfilestr, "%s\n", head->info);
        head = head->ptr;
    }

    fclose(fp);
    fclose(outfilestr);
    return 0;
}

What is char *info[15] supposed to do in your mind? Can you please try and verify your assumptions of how C works on a small program first and move to a more complex one only after you're confident how it works? — Kerrek SB
– Kerrek SB, Commented Oct 2, 2013 at 7:48
But it isn't. It's an array of pointers. In fact, your compiler should have given you a warning about this. Are you sure you've enabled all warnings? — Kerrek SB
– Kerrek SB, Commented Oct 2, 2013 at 7:49
Or, it will when I add it. Once again, the problem seems to occur when I declare a new head node. Coming from Java, I guess I don't see where the problem is coming from. — katiea
– katiea, Commented Oct 2, 2013 at 7:51

unwind · Accepted Answer · 2013-10-02 07:59:25Z

5

You have the wrong syntax for typedef of a structure. You need to place the typedef name after the structure definition:

typedef struct Node  /* <- structure name */
{
    /* ... */
} Node;  /* <- typedef name */

And it's okay to use the same name for both the structure and the type, as both live in different namespaces.

edited Oct 2, 2013 at 7:59

unwind

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answered Oct 2, 2013 at 7:50

Some programmer dude

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8 Comments

xmoex Over a year ago

wouldn't it also legal to omit the structure name?

Gauthier Over a year ago

Or use struct Node as in the while loop. But in that case there's no need for having the typedef at all.

katiea Over a year ago

Holy cow. I.. I'll be honest, maybe because I'm new, but I'm not entirely sure why that worked... Do you know where in K&R I can read up more on this?

Some programmer dude Over a year ago

@xmoex While it's legal, the OP uses a pointer to the structure inside the structure itself, so then need to give the structure a name.

Some programmer dude Over a year ago

@katiea It's how fgets works. It read and leaves the newline in the string, and you need to manually get rid of it (typically str[strlen(str) - 1] = '\0'; will work, be sure that the fgets call didn't fail first though).

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Vorsprung · Accepted Answer · 2013-10-02 07:57:20Z

2

You need to have a Node typedef first

typedef struct Node Node;
typedef struct Node{
char *info[15];
Node *ptr;
};

Or do it in one

typedef struct Node{
    char *info[15];
    struct Node *ptr;
} Node;

edited Oct 2, 2013 at 7:57

answered Oct 2, 2013 at 7:52

Vorsprung

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1 Comment

Gauthier Over a year ago

The declaration of the struct does not need typedef. It is not a type definition (the second code line should be only struct Node{).

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