使用Postgres测试文件是否存在

Question

我需要创建一个函数来测试文件是否存在postgres.我是用C语言做的,但我遇到了问题.代码是:

#include "postgres.h"
#include <string.h>
#include "fmgr.h"
#include <stdio.h>
#include<sys/stat.h>

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

/* by value */

PG_FUNCTION_INFO_V1(file_exists);
Datum
file_exists (PG_FUNCTION_ARGS)
{
   text *fileName= PG_GETARG_TEXT_P(0);

   struct stat buf;
   int i = stat(fileName, &buf);
     /* File found */
     if ( i == 0 )
     {
       PG_RETURN_INT32(1);
     }
     PG_RETURN_INT32(0);

}

我认为问题在于"stat"函数中的第一个参数,因为fileName是一个文本而且这个函数接收一个char.

这是我第一次编写C代码,所以也许一切都是错的.

Answer 1

如果你仔细阅读标题,你会发现server/c.h:

/* ----------------
 *      Variable-length datatypes all share the 'struct varlena' header.
 *...
 */
struct varlena
{
    char        vl_len_[4];     /* Do not touch this field directly! */
    char        vl_dat[1];
};

#define VARHDRSZ        ((int32) sizeof(int32))

/*
 * These widely-used datatypes are just a varlena header and the data bytes.
 * There is no terminating null or anything like that --- the data length is
 * always VARSIZE(ptr) - VARHDRSZ.
 */
typedef struct varlena bytea;
typedef struct varlena text;

所以你有text数据类型的定义.请注意评论中相当重要的部分:

没有终止null或类似的东西

这表明你绝对不想把它fileName->data当作C字符串,除非你喜欢segfaults.你需要一种方法来转换为text你可以交给的以空字符结尾的C字符串stat; 有一个功能:text_to_cstring.

text_to_cstring我能找到的唯一文档就是源代码中的这条评论:

/*
 * text_to_cstring
 *
 * Create a palloc'd, null-terminated C string from a text value.
 *
 * We support being passed a compressed or toasted text value.
 * This is a bit bogus since such values shouldn't really be referred to as
 * "text *", but it seems useful for robustness.  If we didn't handle that
 * case here, we'd need another routine that did, anyway.
 */

还有一个使用它的示例:

char *command;
/*...*/

/* Convert given text object to a C string */
command = text_to_cstring(sql);

/*...*/
pfree(command);

你应该可以做这样的事情:

struct stat buf;
char *fileName = text_to_cstring(PG_GETARG_TEXT_P(0)); 
int i = stat(fileName, &buf);
pfree(fileName);