What is `S_ISREG()`, and what does it do?

Question

I came across the macro S_ISREG() in a C program that retrieves file attributes. Unfortunately, there isn\'t any basic information about this macro online. There ar

Answer 1

It tests the st_mode member of the stat structure retrieved using the stat() function to determine whether the file is a regualar file (i.e. on on disk or mass storage rather than say a directory, socket, symbolic link for example.

struct stat sb;
if( stat( file_path, &sb) != -1) // Check the return value of stat
{
    if( S_ISREG( sb.st_mode ) != 0 )
    {
        printf( "%s is a file", file_path ) ;
    }
    else
    {
        printf( "%s is not a file", file_path ) ;
    }
}

The st_mode member contains 4 bits masked by S_IFMT (0170000). The values of these bits are:

       S_IFSOCK   0140000   socket
       S_IFLNK    0120000   symbolic link
       S_IFREG    0100000   regular file
       S_IFBLK    0060000   block device
       S_IFDIR    0040000   directory
       S_IFCHR    0020000   character device
       S_IFIFO    0010000   FIFO

so the macro S_ISREG mighte be defined thus:

#define S_ISREG( m ) (((m) & S_IFMT) == S_IFREG)

Since it is a macro you can look at its actual definition in the header file sys/stat.h. In the GNU header it is defined this:

#define __S_ISTYPE(mode, mask)  (((mode) & __S_IFMT) == (mask))
...
#define S_ISREG(mode)    __S_ISTYPE((mode), __S_IFREG)

which is essentially the same at my simplified version.