Memory Allocating (malloc) a Struct Array

Question

#include <stdlib.h>
#include <stdio.h>
struct CELL {
    char* data;
    struct CELL* next;
} ;

struct CELL* table;

// The line below gives an error
table = malloc(SIZE * sizeof(struct HASHCELL));

I am trying to create an array of struct CELL using malloc.

However, doing so gives "warning: type specifier missing, defaults to 'int' [-Wimplicit-int]". Consequently, it gives "error: redefinition of 'table' with a different type: 'int' vs 'struct CELL * ". I am new to C and I am unsure why this is happening.

Why not do `static struct CELL table[100]` *if you don't need malloc* — An Ant, Feb 27 '21 at 11:30
Is it legal to define a member of a struct as being itself while defining the struct ? — An Ant, Feb 27 '21 at 11:32
@An Ant I have tried not using malloc. But for some reason, when I run my program, I either get a segmentation error or a bus error. I assumed those two errors happened because I did not allocate memory. However, I am not sure as I am new to the language. — user4672, Feb 27 '21 at 11:33
@AnAnt: `stdlib.h` has nothing to do with the redefinition of the user-defined `table`. — Eric Postpischil, Feb 27 '21 at 11:34
@EricPostpischil I see, i though only malloc needed a library header there — An Ant, Feb 27 '21 at 11:35
@AnAnt: The source codecshown does not contain any definition of a structure inside itself. A pointer to a structure is not the structure and may be a member of the structure. — Eric Postpischil, Feb 27 '21 at 11:36
@EricPostpischil isn't `struct CELL { char *data; struct CELL* next; }` the definition ? — An Ant, Feb 27 '21 at 11:38
@EricPostpischil Ah, so are you sying what I was thinking ? that this sort of code confuses the compiler ? — An Ant, Feb 27 '21 at 11:41
@AnAnt: No, it is fine. Why do you think there is any problem with it? — Eric Postpischil, Feb 27 '21 at 11:42
Please [edit] and show a [mcve]. I tried [this code](https://www.godbolt.org/z/bMsP7r) and it compiles fine. — Jabberwocky, Feb 27 '21 at 11:44
@EricPostpischil Because I honestly have no idea how you can define something as a member of itself ... how would that work in the compiler ? — An Ant, Feb 27 '21 at 11:44
@Jabberwocky That is pretty much all I have for now. Thanks for compiling it. I am not sure why it's not working for me. — user4672, Feb 27 '21 at 11:45
@AnAnt: `struct CELL` is not a member of itself. The `next` member is a **pointer** to a `struct CELL`. Structures can contain pointers. This is ordinary C code. — Eric Postpischil, Feb 27 '21 at 11:46
@user4672 please post a [mcve], meaning the _verbatim_ code you compile and tell us what compiler/IDE/OS/environnment/etc. you use. — Jabberwocky, Feb 27 '21 at 11:46
As told, please try to create a [mcve] to show us. And copy-paste the full and complete build output (from the shown code) into the question. And then add comment on the lines where you get the errors. — Some programmer dude, Feb 27 '21 at 11:47
As an educated ***guess*** you probably define the `CELL` structure in a header file, but forget to include that header file in the code where you use it. — Some programmer dude, Feb 27 '21 at 11:50
Voted to reopen so it can be closed as a duplicate. (There ought to be an option to vote to change the close reason.) — Eric Postpischil, Feb 27 '21 at 12:09
@Jabberwocky What happened? We reopened it, then closed as a duplicate, then you reopened it ... again? I'm confused. — Adrian Mole, Feb 27 '21 at 14:25

score 1 · Answer 1 · answered Feb 27 '21 at 13:28

table = malloc(SIZE * sizeof(struct HASHCELL)); is a statement (it is an expression statement). Statements cannot be written outside of functions. Outside of functions, you can only have declarations and preprocessing directives. (In the C standard, declarations are separate from statements.)

When the compiler sees table, it is expect a declaration, but this is not a declaration because there is no type, like int table or struct Cell table. The compiler is applying a default type of int and warning you about this.

Then, because you had an earlier declaration struct CELL *table;, this new “declaration” of int table conflicts with the prior declaration, so the compiler warns you about that.

The solution is to write a function, such as the main function for your program, and put table = malloc(SIZE * sizeof(struct HASHCELL)); inside it.

You may also wish to move struct CELL *table; inside the function; using variables defined outside of functions is generally bad practice.

score -3 · Answer 2 · answered Feb 27 '21 at 11:43

-3

Make sure you closed the semicolon at the end of your structure definition.

struct CELL {
    char *data;
    struct CELL* next;
};

Make sure you include <stdlib.h> standard library where malloc() is defined before you call it.

Cast the return type of malloc to

table = (struct CELL*)malloc(100 * sizeof(struct CELL));

as malloc always returns void * to the allocated memory.

Compile your source code using command line gcc to make sure you don't have any library configuration issues in vs code which may cause the issue.

answered Feb 27 '21 at 11:43

Vahan

166
13

5

C does not require casts of `void *` in assignments; it will automatically be converted to the destination type, and using casts can sometimes obscure errors. – Eric Postpischil Feb 27 '21 at 11:44
1

This is rather a comment than an answer. – Jabberwocky Feb 27 '21 at 11:45
Agreed with the above comments , OP's code already has everything mentioned here, and the cast is implicit . The second part , is probably useful but more of a comment too, and if I might add to it, I'd ask that he compile with all warnings – An Ant Feb 27 '21 at 11:47
2

In C you really [shouldn't cast the result of `malloc`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). – Some programmer dude Feb 27 '21 at 11:48
@Someprogrammerdude brilliant, this is interesting – An Ant Feb 27 '21 at 11:49

Memory Allocating (malloc) a Struct Array

2 Answers2