C <sys/stat.h> 查找文件类型

Question

I create my own ls command for a project and I use the "st_mode" variable to find the files permissions but I don't use the macros.

Ex: st_mode = 16877

I convert it in octal base:

st_mode = 40755

I keep the three last characters and I get my permissions.

But when I try to find the file's type, I try to use the two first characters but they don't really help me... So I would like to know if I can use the two first characters to find the file's type (Link, folder,...). And if I can't, what I should use to find the file's type

Thanks for your help.

Answer 1

Per the POSIX <sys/stat.h> documentation :

The following macros shall be provided to test whether a file is of the specified type. The value m supplied to the macros is the value of st_mode from a stat structure. The macro shall evaluate to a non-zero value if the test is true; 0 if the test is false.

 S_ISBLK(m) Test for a block special file. S_ISCHR(m) Test for a character special file. S_ISDIR(m) Test for a directory. S_ISFIFO(m) Test for a pipe or FIFO special file. S_ISREG(m) Test for a regular file. S_ISLNK(m) Test for a symbolic link. S_ISSOCK(m) Test for a socket. 

Answer 2

yes you can use both method with predefined macros as below, open the man page of stat() system call, it says

S_IFMT     0170000   bit mask for the file type bit fields

So do & of st_mode and S_IFMT , you will get file type

                struct stat v;
                stat(file,&v);  
                switch (v.st_mode & S_IFMT) // type of file
                {

                        case S_IFBLK:  printf("b");                 break;
                        case S_IFCHR:  printf("c");                 break;
                        case S_IFDIR:  printf("d");                 break;
                        case S_IFIFO:  printf("p");                 break;
                        case S_IFLNK:  printf("l");                 break;
                        case S_IFREG:  printf("-");                 break;
                        case S_IFSOCK: printf("s");                 break;
                        default:       printf("unknown?");          break;
                }

If you don't want to use macros then first find out st_mode value of each type of files & then write the logic. for eg st_mode value of regular file is 10664 , last 3 digits(664) for permission , write the binary of 10664 you will come to know that 15th bit is set so do st.mode >> 15 . Similarly find the st_mode value of different types of file & analyse.

if( ( v.st_mode >> 15 & 1) && ( v.st_mode >> 14 & 1) ) 
                printf("Socket File\n"); 
        else if( ( v.st_mode >> 15 & 1) && ( v.st_mode >> 13 & 1) )
                printf("Symbolic Link File\n");
        else if( v.st_mode >> 15 & 1) 
                printf("Regular File\n");
        else if((v.st_mode >> 14 & 1) && (v.st_mode >> 13 & 1) )
                printf("Block Dev File\n");
        else if(v.st_mode >> 14 & 1) 
                printf("Directory File\n");
        else if(v.st_mode >> 13 & 1)
                printf("Char Dev File\n");
        else if(v.st_mode >> 12 & 1)
                printf("FIFO/PIPE File\n");

I hope it help's .