C语言中的文件描述符

Question

If I'm writing a library that uses a file descriptor for doing stuff, when should I return it from lib_init() for the higher layer code to use and pass to my lib_do_stuff() calls, and when can I leave it as a private "member" in my C library as a static global in .c file?

If I don't think the user of my library should have control or even access to the file descriptor, can I just leave it, much like in C++ it would just be private ?

What are the downsides for doing it either way?

Answer 1

Expanding my suggestion with an example.

Your library needs two (at least) header files: One public that the users of your library includes, and one private that you include only in your library source files.

The public could be something like

#pragma once

// This is all that is needed to declare pointers to the internal structure
typedef struct internal_structure STRUCTURE;

// The public API of your library
STRUCTURE *lib_init(void);
void lib_cleanup(STRUCTURE *s);
...

Then you have the private header file

#pragma once

struct internal_structure
{
    int fd;
    // Other members as needed
    ...
};

// Possible function prototypes of private functions

Then in your library source files you include both the public and the private header files, and use STRUCTURE for the black-box structure:

#include <stdlib.h>
#include "public.h"
#include "private.h"

STRUCTURE *lib_init(void)
{
    STRUCTURE *s = malloc(sizeof *s);
    s->fd = open(...);
    // Other initialization
    ...
    return s;
}

void lib_cleanup(STRUCTURE *s)
{
    // Other cleanup
    ...
    close(s->fd);
    free(s);
}

Then the users of your library includes only the public header file, and uses your well-defined API:

#include "public.h"

int main(void)
{
    STRUCTURE *s = lib_init();
    ...
    lib_cleanup(s);
    return 0;
}

The public functions should all take STRUCTURE * as one of their arguments, typically their first argument, similar to the lib_cleanup function. The function can then use the structure and its members any way they want.