char buf[500];

sprintf (buf,"./%s",fname); //hope buf is large enough to hold string without overflowing cause there ain't a lot of out-of-bounds checking here

The open system call takes three arguments:  the file name, the flags, and the file modes should a file be created.

open flags:

O_RDONLY  read only
O_WRONLY  write only
O_RDWR    read & write
O_CREAT   create file if it doesn't eixt
O_EXCL    prevent creation it already exists
O_APPEND  writes new information to the end of the file

for reading:

fd = open ("fname.txt",O_RDONLY);

for writing:

fd = open ("fname,txt",O_WRONLY | O_CREAT, 0644)  or maybe 0700

0644 = user-rw, group-r, world-r.    = user-rwx, group-nothing, world-nothing

Anytime O_CREAT appears as a flag for open, a file mode parameter must be given.

In the C-language, any integer beginning with a leading 0 is treated as octal.  So, in C, 12 and 012 are not the same number.  12 is 12, but 012 is ten.

In the C language, any integer beginning with 0x is treated as hexadecimal.  0xff is 255.


File permission bits:


user  read    write    execute
       400     200      100

Group   40      20       10

World    4       2        1

If I want to give myself read/write access and everyone else read access, that is 0644.  If it's a program, the typical mode is 0755.


Me, personally, I tend to give my files 0700.  I get everything.  screw everyone else.

Read on a directory allows someone to get a look at the file list.
Write on a directory allows someone to add or delete from the directory.
Execute on a directory allows someone to use that directory at all!  (Use it in a pathname).

On a linux system, the root has read/write to everything, no matter what the file permissions are.


There are three special permissions as well:

set-user-id    4000
set-group-id   2000
sticky bit     1000

set-user-id (and also set-group-id) is DANGEROUS.  Be very careful with this one.

set-user-id on an executable program, changes your userid to the owner of the program during its execution.
set-group-id on an executable changes your groupid to the group of the program during its execution.