S_ISDIR and S_ISREG undeclared despite including the <sys/stat.h> header

Question

I am using the "S_ISDIR" and "S_ISREG" but am getting an error that they are undeclared. I tried using it in macOS(using S_IFDIR and S_IFREG) and it worked but not in linux terminal.

error: ‘S_ISDIR’ undeclared (first use in this function)  
error: ‘S_ISREG’ undeclared (first use in this function); did you mean ‘S_ISDIR’?  

struct stat s;
if(stat(fileName, &s) == 0 )
{
    if( s.st_mode & S_ISDIR )
    {
        return false;
    }
    else if( s.st_mode & S_ISREG )
    {
        return true;
    }
    else
    {
        return false;
    }
}
else
{
    return false;
}
return false;

Answer 1

You're using the macros incorrectly. They're function-like macros which accept the mode as a parameter:

if( S_ISDIR(s.st_mode) )
{
    return false;
}
else if( S_ISREG(s.st_mode) )
{
    return true;
}
else
{
    return false;
}

Answer 2

You may be wondering why your original code, with S_IFREG and S_IFDIR , worked on one operating system but not another. This is because the S_IFxxx constants are optional in some revisions of the POSIX standard (this is the standard that specifies the contents of sys/stat.h ).

The S_ISxxx function-like macros are required to be available, so it's better to use them when you can, but sometimes using the S_IFxxx constants can make one's code much clearer. They are macros, so you can test for their presence with #ifdef S_IFREG (if S_IFREG is available, it's safe to assume the others are as well).

Some operating systems don't provide the S_IFxxx constants by default, but do if you put #define _XOPEN_SOURCE 700 above all of your #include lines. Linux is not normally one of those operating systems, but it becomes one of those operating systems if you use -ansi or -std=cNN on the compiler command line. (NB for reasons too complicated to get into here, it is usually a mistake to use those switches instead of -std=gnuNN .)