使用Postgres测试文件是否存在

时间:2012-10-12 19:59:30

标签: c postgresql

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,所以可能一切都都错了。

2 个答案:

答案 0 :(得分:5)

如果您仔细阅读标题,可以在server/c.h中找到:

/* ----------------
 *      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。请注意这个评论中相当重要的部分:

  

没有终止null或类似的东西

这表明您绝对不希望将fileName->data视为C字符串,除非您喜欢segfaults。您需要一种方法将text转换为以Nul结尾的C字符串,您可以将其转交给stat;有一个功能:text_to_cstring

我可以找到的text_to_cstring唯一的文档是this comment in the source

/*
 * 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an example that uses it

char *command;
/*...*/

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

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

你应该可以这样做:

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

答案 1 :(得分:1)

我有同样的问题。 text_to_cstring对我不起作用,然后我意识到问题是我没有包含text_to_cstring func的头文件。这有多愚蠢。所以我不得不在/ usr / include / postgresql / your_version / server / utils /文件夹中搜索,直到我最终找到它然后一切运行良好。尝试

char *filename = text_to_cstring(PG_GETARG_TEXT_PP(0));

但不要忘记包含第一个builtins.h

#include "utils/builtins.h"