Array of strings in C

Question

I'm trying to create a structure storing strings and I'm getting an error incompatible types when I try and insert as string into the array. This my first time working with a program in C. Could somebody help spot my problem.

This is my implementation of list.c

struct list *init_list(int num) {
    struct list *p;
    p = malloc(LISTSZ(num));
    if(p == NULL)
        return(NULL);
    p->maxsz = num;
    p->sz = 0;
    return(p);
}

void debug_list(struct list *p) {
    int i;
    fprintf(stderr, "\nDynamic List\n\n");
    fprintf(stderr, "   sz  = %d\n", p->sz);
    fprintf(stderr, "   maxsz   = %d\n", p->maxsz);
    for(i = 0; i < p->maxsz; i++)
        fprintf(stderr,"    %s\n", (p->item[i]));
}

void prt_list(struct list *p) {
    int i;
    for(i = 0; i < p->sz; i++)
        printf("%s\n", (p->item[i]));
}

int ins_list(char *data, struct list **p) {
    struct list *q;
    if((*p)->sz == (*p)->maxsz) {
        q = realloc(*p, LISTSZ((*p)->maxsz + INCRSZ)); // Problem?
        if(q == NULL)
            return(-1);
        q->maxsz += INCRSZ;
        *p = q;
    }
    (*p)->item[(*p)->sz] = data; // incompatible types in assignment
    (*p)->sz ++;
    return(0);
}

This is my implementation of list.h

struct list {
    int sz;
    int maxsz;
    char item[][1024]; // Problem?
};

#define INITSZ 5
#define INCRSZ 5
#define LISTSZ(n) ((size_t)(sizeof(struct list) + ((n)-1)*sizeof(char[1024]))) // Problem?

struct list *init_list(int num);
int ins_list(char *data, struct list **p);
void prt_list(struct list *p);
void debug_list(struct list *p);

Answer 1

You have an array of char, yet you're trying to put a char * into it.

I would guess that strncpy will do what you want. Alternatively, declare item as an array of char *.

Answer 2

struct list {
   int sz;
   int maxsz;
   char *item[1024]; 
};

Answer 3

There more differences between C and C++ than it's commonly admit.

For your error the reason is simple you are trying to assign a pointer (char*) at sz wich is an int. This kind of assignment generate the incompatible type warning.

Second thing you can't do (a least as far as i know) a partially dynamic array as you do it. In your case you should use at least a malloc and the type of item should be char**. However there is a trick to use only one malloc to create a 2D array.

For the realloc nothing hit me ... What is the compilation error ?

However your code doesn't looks like C code :/

You might need to rebuild it form scratch, because you are here making confusion between lists and 2D arrays ...

I can write some examples of codes if you want but you should probably find a C basics tutorial on google.

Good luke :)

Answer 4

At this line:

(*p)->item[(*p)->sz] = data; // incompatible types in assignment

(*p)->item[(*p)->sz] is an array of 1024 char - you can't assign to arrays with = (arrays are "non-modifiable lvalues").

You just need to do a copy. For a length-safe string copy, I prefer to use strncat():

(*p)->item[(*p)->sz][0] = '\0'; /* Truncate existing string to empty */
strncat((*p)->item[(*p)->sz], data, (sizeof (*p)->item[(*p)->sz]) - 1);