How to read linux file permission programmatically in C/C++

Question

How can I read linux file permission programmatically instead using \"ls\" then parse the result.

Answer 1

This is a function in C, that returns file permissions in the string format "rwxr-r--"

char* permissions(char *file){
    struct stat st;
    char *modeval = malloc(sizeof(char) * 9 + 1);
    if(stat(file, &st) == 0){
        mode_t perm = st.st_mode;
        modeval[0] = (perm & S_IRUSR) ? 'r' : '-';
        modeval[1] = (perm & S_IWUSR) ? 'w' : '-';
        modeval[2] = (perm & S_IXUSR) ? 'x' : '-';
        modeval[3] = (perm & S_IRGRP) ? 'r' : '-';
        modeval[4] = (perm & S_IWGRP) ? 'w' : '-';
        modeval[5] = (perm & S_IXGRP) ? 'x' : '-';
        modeval[6] = (perm & S_IROTH) ? 'r' : '-';
        modeval[7] = (perm & S_IWOTH) ? 'w' : '-';
        modeval[8] = (perm & S_IXOTH) ? 'x' : '-';
        modeval[9] = '\0';
        return modeval;     
    }
    else{
        return strerror(errno);
    }   
}

Answer 2

The stat(2) system call returns a struct stat that contains a st_mode member. This is the mode bits that ls -l displays.

On my system, the man 2 stat page says:

   The following flags are defined for the st_mode field:

   S_IFMT     0170000   bitmask for the file type bitfields
   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
   S_ISUID    0004000   set UID bit
   S_ISGID    0002000   set-group-ID bit (see below)
   S_ISVTX    0001000   sticky bit (see below)
   S_IRWXU    00700     mask for file owner permissions
   S_IRUSR    00400     owner has read permission
   S_IWUSR    00200     owner has write permission
   S_IXUSR    00100     owner has execute permission
   S_IRWXG    00070     mask for group permissions
   S_IRGRP    00040     group has read permission
   S_IWGRP    00020     group has write permission
   S_IXGRP    00010     group has execute permission
   S_IRWXO    00007     mask for permissions for others (not in group)
   S_IROTH    00004     others have read permission
   S_IWOTH    00002     others have write permission
   S_IXOTH    00001     others have execute permission