libmagic，magic_descriptor始终返回null

如何解决libmagic，magic_descriptor始终返回null

在Centos 8上运行。

完整代码如下。这是编译和执行。嗯，是的，我以root用户身份运行，所以权限应该没有问题。

我只想获得与从“文件”命令获得的输出相同的输出，并且该代码基于来自文件5.33源代码rpm的源代码。哪个应该是最新的和最大的。

[root@ws ~]# gcc a3magic.c -lmagic
[root@ws ~]# ./a.out 
set fd = 0
before 'open' errno = 0
fd = 3
fd errno = 0
    Success
magicfile = /etc/magic:/usr/share/misc/magic
after getpath errno = 2
after load errno = 2
cont    offset  type    opcode  mask    value   desc
magic_check igot=0
magic_check should be okay.
after magic_check errno = 0
input flags = 0
mcookie is okay i guess
after magic_open errno = 0
desript (null)
errno = 0
    Success
cooklabel = (null)

我一直在拔头发。测试文件存在，它只是一个文本文件。在我看来，对magic_ *例程的所有调用均有效，也就是说，我认为我检查了所有返回代码，并且设置例程的返回代码都很好。我假设描述例程的手册页是正确的。

但是返回的描述和magic_file返回的都是null。无奈之下我打印出了errno，但是我完全不相信会设置errno。 （手册页没有说将设置errno，只是说错误会返回空值。我一直在抓住稻草。）

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <magic.h>
# include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
# include <errno.h>


//------------------------------------------------------------------------------
struct magic_set * load(const char *magicfile,int flags)
{
struct magic_set *magic = magic_open(flags);
const char *e;

if (magic == NULL) {
        printf("Can't create magic");
        return NULL;
}
if (magic_load(magic,magicfile) == -1) {
        printf("%s",magic_error(magic));
        magic_close(magic);
        return NULL;
}
if ((e = magic_error(magic)) != NULL)
        printf("%s\n",e);
return magic;
}
//------------------------------------------------------------------------------
int main(void)
{
magic_t mcookie;
int igot;
int opflags = O_RDONLY ;
int flags = 0;
int posixly = 0;
int action = 0;
int e =0;
struct magic_set *msetptr = NULL;


int fd=0;
printf("set fd = %d\n",fd );
//const char *testfile = (char *)"/root/bin/twatch";
const char *testfile = (char *)"/root/URL";
printf("before 'open' errno = %d\n",errno );
fd = open(testfile,opflags );
printf("fd = %d\n",fd );
printf("fd errno = %d\n",errno );
printf("\t%s\n",strerror(errno) );


#ifdef S_IFLNK
posixly = getenv("POSIXLY_CORRECT") != NULL;
flags |=  posixly ? MAGIC_SYMLINK : 0;
#endif

//--------------------------------------
const char *magicfile = NULL;
if (magicfile == NULL)
    magicfile = magic_getpath(magicfile,action);
printf("magicfile = %s\n",magicfile );
printf("after getpath errno = %d\n",errno );

msetptr = load(magicfile,flags);
if( msetptr == NULL )
    printf("no mset ptr\n");
printf("after load errno = %d\n",errno );
//--------------------------------------


//  this looks okay
igot = magic_check(msetptr,magicfile);
printf("magic_check igot=%d\n",igot);
if( igot == 0 ) printf("magic_check should be okay.\n");
if( igot < 0 ) printf("magic_check is bad.\n");
errno = 0;
printf("after magic_check errno = %d\n",errno );
//--------------------------------------

printf("input flags = %d\n",flags );
mcookie = magic_open(flags|MAGIC_CHECK);
//mcookie = magic_open(flags);
if (mcookie == NULL) {
        printf("Can't create magic\n");
        return 1;
        }
printf("mcookie is okay i guess\n");
printf("after magic_open errno = %d\n",errno );
//--------------------------------------


//  this one spits out a lot.
//  so this works
//igot = magic_list( msetptr,magicfile );
//  this has a legit lookinhg value
//printf("magicfile = %s\n",magicfile );
//--------------------------------------

const char *sptr;
sptr = magic_descriptor(mcookie,fd);
printf("desript %s\n",sptr );
printf("errno = %d\n",strerror(errno) );

//--------------------------------------
const char *cooklabel;
cooklabel = magic_file( mcookie,testfile );
printf("cooklabel = %s\n",cooklabel );

//--------------------------------------
return 0;
}