#include <sys/stat.h>
int stat( const char * restrict pathname, struct stat * restrict buf );
int fstat( int fd, struct stat * buf );
int lstat( const char * restrict pathname,struct stat * restrict buf );
int fstatat( int fd, const char * restrict pathname, struct stat *restrict buf,int flag );

返回值：成功，0
　　　　出错，-1

struct stat  {    __dev_t st_dev;            /* Device.  文件系统的设备号*/    unsigned short int __pad1;#ifndef __USE_FILE_OFFSET64    __ino_t st_ino;            /* File serial number. i-node number    */#else    __ino_t __st_ino;            /* 32bit file serial number.    */#endif    __mode_t st_mode;            /* File mode.  */    __nlink_t st_nlink;            /* Link count.  */    __uid_t st_uid;            /* User ID of the file's owner.    */    __gid_t st_gid;            /* Group ID of the file's group.*/    __dev_t st_rdev;            /* Device number, if device. device设备的major和minor号 */    unsigned short int __pad2;#ifndef __USE_FILE_OFFSET64    __off_t st_size;            /* Size of file, in bytes.  */#else    __off64_t st_size;            /* Size of file, in bytes.  */#endif    __blksize_t st_blksize;        /* Optimal block size for I/O. 操作IO时的最优block大小 */#ifndef __USE_FILE_OFFSET64    __blkcnt_t st_blocks;        /* Number 512-byte blocks allocated. */#else    __blkcnt64_t st_blocks;        /* Number 512-byte blocks allocated. */#endif#if defined __USE_MISC || defined __USE_XOPEN2K8    /* Nanosecond resolution timestamps are stored in a format       equivalent to 'struct timespec'.  This is the type used       whenever possible but the Unix namespace rules do not allow the       identifier 'timespec' to appear in the <sys/stat.h> header.       Therefore we have to handle the use of this header in strictly       standard-compliant sources special.  */    struct timespec st_atim;        /* Time of last access.  */    struct timespec st_mtim;        /* Time of last modification.  */    struct timespec st_ctim;        /* Time of last status change.  */# define st_atime st_atim.tv_sec    /* Backward compatibility.  */# define st_mtime st_mtim.tv_sec# define st_ctime st_ctim.tv_sec#else    __time_t st_atime;            /* Time of last access.  */    unsigned long int st_atimensec;    /* Nscecs of last access.  */    __time_t st_mtime;            /* Time of last modification.  */    unsigned long int st_mtimensec;    /* Nsecs of last modification.  */    __time_t st_ctime;            /* Time of last status change.  */    unsigned long int st_ctimensec;    /* Nsecs of last status change.  */#endif#ifndef __USE_FILE_OFFSET64    unsigned long int __unused4;    unsigned long int __unused5;#else    __ino64_t st_ino;            /* File serial number.    */#endif  };

#include <sys/stat.h>#define    __S_ISTYPE(mode, mask)    (((mode) & __S_IFMT) == (mask))#define    S_ISDIR(mode)     __S_ISTYPE((mode), __S_IFDIR)#define    S_ISCHR(mode)     __S_ISTYPE((mode), __S_IFCHR)#define    S_ISBLK(mode)     __S_ISTYPE((mode), __S_IFBLK)#define    S_ISREG(mode)     __S_ISTYPE((mode), __S_IFREG)#ifdef __S_IFIFO# define S_ISFIFO(mode)     __S_ISTYPE((mode), __S_IFIFO)#endif#ifdef __S_IFLNK# define S_ISLNK(mode)     __S_ISTYPE((mode), __S_IFLNK)#endif#if defined __USE_BSD && !defined __S_IFLNK# define S_ISLNK(mode)  0#endif#if (defined __USE_BSD || defined __USE_UNIX98 || defined __USE_XOPEN2K) \    && defined __S_IFSOCK# define S_ISSOCK(mode) __S_ISTYPE((mode), __S_IFSOCK)#elif defined __USE_XOPEN2K# define S_ISSOCK(mode) 0#endif

/* These are from POSIX.1b.  If the objects are not implemented using separate   distinct file types, the macros always will evaluate to zero.  Unlike the   other S_* macros the following three take a pointer to a `struct stat'   object as the argument.  */#ifdef    __USE_POSIX199309# define S_TYPEISMQ(buf) __S_TYPEISMQ(buf)　　　　// 消息队列# define S_TYPEISSEM(buf) __S_TYPEISSEM(buf)　　 // 信号量# define S_TYPEISSHM(buf) __S_TYPEISSHM(buf)    // 共享存储对象#endif

/* lseek test */#include <stdio.h>    // printf#include <stdlib.h>    // exit#include <unistd.h>#include <fcntl.h>#include <errno.h>    // errno#include <string.h>    // strerror#include <sys/stat.h>    // mode,stat函数int main(int args, char *argv[]){    int i;    struct stat fl_stat;        if( args<2 ){        printf("args error.\r\n");        exit(1);    }                for( i=1;i<args;i++ ){        printf("%s:",argv[i]);        if( lstat(argv[i],&fl_stat) != 0 ){            printf("lstat error.\r\n");            exit(1);        }                    if( S_ISREG(fl_stat.st_mode) )            printf("reguler file.\r\n");        else if( S_ISDIR(fl_stat.st_mode) )            printf("directory file.\r\n");        else if( S_ISCHR(fl_stat.st_mode) )            printf("char special file.\r\n");            else if( S_ISBLK(fl_stat.st_mode) )            printf("BLOCK special file.\r\n");            else if( S_ISFIFO(fl_stat.st_mode) )            printf("FIFO file.\r\n");                    else if( S_ISLNK(fl_stat.st_mode) )            printf("symble link file.\r\n");        else if( S_ISSOCK(fl_stat.st_mode) )            printf("socket file.\r\n");        else            printf("unknow file.\r\n");    }                exit(0);}

$./example * /dev/cdrom /dev 
example:reguler file.
example.c:reguler file.
example.o:reguler file.
Makefile:reguler file.
result:reguler file.
/dev/cdrom:symble link file.
/dev:directory file.

登录者ID = login_name
可执行文件exe_file的用户ID，uid  = file_owner_name
1.如果没有设置文件exe_file的SUID属性，则文件执行后，该进程的有效用户ID = 登录者ID（实际用户ID） = login_name，
  文件权限校验时用login_name校验。如果exe_file的other权限里没有x，则不能执行。
2.如果设置文件exe_file的SUID属性，则文件执行后，该进程的有效用户ID = 该文件的UID = file_owner_name，
　文件权限校验时用file_owner_name，如果exe_file的所有者权限里没有x，则不能执行


另一个典型的例子是 /etc/passed,该文件所有者是root，且设置了SUID，则其他用户执行时，有效进程ID=文件所有者ID=root，所以能够执行。

if( 目录的SGID )    新文件的GID=所在目录的GIDelse        新文件的GID=进程的有效GID