c - 使用 libpq 获取 sizeof(PGconn)

Question

我正在尝试为程序包装 libpq，并且我正在使用的 FFI 工具的一部分尝试获取正在使用的任何结构 sizeof()。在这种情况下，问题在于尝试获取 sizeof(PGconn) 会导致 GCC 出现一堆错误，因为它是一个不完整的类型。有没有办法获得相同的数据，或者我是否需要训练这个 FFI 工具以忽略旨在不透明的类型？此处供引用的是生成的 C 代码和编译器错误:

/* Define on Darwin to activate all library features */
#define _DARWIN_C_SOURCE 1
/* This must be set to 64 on some systems to enable large file support. */
#define _FILE_OFFSET_BITS 64
/* Define on Linux to activate all library features */
#define _GNU_SOURCE 1
/* This must be defined on some systems to enable large file support. */
#define _LARGEFILE_SOURCE 1
/* Define on NetBSD to activate all library features */
#define _NETBSD_SOURCE 1
/* Define to activate features from IEEE Stds 1003.1-2001 */
#define _POSIX_C_SOURCE 200112L
/* Define on FreeBSD to activate all library features */
#define __BSD_VISIBLE 1
#define __XSI_VISIBLE 700
/* Windows: winsock/winsock2 mess */
#define WIN32_LEAN_AND_MEAN

#include <libpq-fe.h>

#include <stdio.h>
#include <stddef.h>   /* for offsetof() */

void dump(char* key, int value) {
    printf("%s: %d\n", key, value);
}


void dump_section_PGconn(void) {
    typedef PGconn platcheck_t;
    typedef struct {
        char c;
        platcheck_t s;
    } platcheck2_t;

    platcheck_t s;
    dump("align", offsetof(platcheck2_t, s));
    dump("size",  sizeof(platcheck_t));
}

int main(int argc, char *argv[]) {
    printf("-+- PGconn\n");
    dump_section_PGconn();
    printf("---\n");
    return 0;
}

错误:

[platform:execute] gcc -c -O3 -pthread -fomit-frame-pointer -Wall -Wno-unused -I/usr/include/postgresql/ /tmp/usession-default-52/platcheck_10.c -o /tmp/usession-default-52/platcheck_10.o
[platform:Error] /tmp/usession-default-52/platcheck_10.c: In function ‘dump_section_PGconn’:
[platform:Error] /tmp/usession-default-52/platcheck_10.c:34: error: field ‘s’ has incomplete type
[platform:Error] /tmp/usession-default-52/platcheck_10.c:37: error: storage size of ‘s’ isn’t known
[platform:Error] /tmp/usession-default-52/platcheck_10.c:39: error: invalid application of ‘sizeof’ to incomplete type ‘platcheck_t’

Answer 1

PGConn 在设计上是不透明的。如果您需要查看它，请包含 libpq-int.h(对于“内部”)。但请考虑重新考虑您的要求。