intro.2

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.\" SCCSID: @(#)intro.2	3.1	11/23/87
.TH intro 2
.SH Name
intro \- introduction to system calls 
.SH Syntax
.B #include <errno.h>
.SH Description
.NXR "intro(2) keyword"
.NXR "system call" "introduction"
.NXAM "system call" "\fIspecific system calls\fP"
Section 2 describes the ULTRIX system calls, which are the
entries into the ULTRIX kernel.  In this section, reference
pages with the extension 2yp are specific to the
Yellow Pages (YP) service.  Those pages ending in 2nfs are specific 
to the Network File System (NFS) service.
.PP
Additionally, some Section 2 reference pages contain an ENVIRONMENT 
section that describes differences between the POSIX or SYSTEM V 
environment and the ULTRIX operating system.
.SH Environmental Compatibility
.NXR "system call" "specifying System V environment"
.NXR "system call" "specifying POSIX environment"
Some system calls contain System V and POSIX features that
are compatible with ULTRIX programs.  These
features are provided for applications that are being
ported from System V or POSIX.
Occasionally, the System V and POSIX features conflict with features 
present in the ULTRIX system.  For example, a function
performed under the ULTRIX operating system can produce
different results in the System V or POSIX environment.  If
conflicts exist, the ENVIRONMENT section of the reference page highlights these
differences.
.PP
Neither the System V compatibility features nor the POSIX
compatibility features are not contained in the standard C
runtime library. To use the compatibility features, you must set your 
programming environment to System V
or POSIX 
when you compile or link your programs.  To set the System V or
POSIX
environment, do either of the following:
.IP 1. 5
Use the 
.I \-Y
option for the 
.PN cc 
command. For example, the following demonstrates compiling a program in
the System V environment first, and then in the POSIX environment:
.EX
% cc \-YSYSTEM_FIVE program.c
% cc \-YPOSIX program.c
.EE
.IP 2. 5
Globally set the environment variable PROG_ENV to
SYSTEM_FIVE or to POSIX.  
.sp
If you are using the C shell,
execute the following line or include it 
in your 
.PN .login 
file:
.EX
setenv PROG_ENV SYSTEM_FIVE  
.EE
Replace ``SYSTEM_FIVE'' with ``POSIX'' if you are using the POSIX 
environment.
.sp
If you are using the Bourne or the System V shell, execute the
following line or include it in your 
.PN .profile 
file:
.EX
PROG_ENV=POSIX ; export PROG_ENV
.EE
Replace ``POSIX'' with ``SYSTEM_FIVE'' if you are using the System V
environment.
.LE
.PP
In each instance, the 
.PN cc
command defines a preprocessor symbol, either SYSTEM_FIVE or POSIX.  
When the SYSTEM_FIVE symbol is defined, the C preprocessor,
.PN cpp ,
selects the System V data structures and symbol definitions. When the
POSIX symbol is defined,
.PN cpp
selects the POSIX data structures and symbol
definitions. 
.PP
In addition, if 
.PN cc 
invokes the 
.PN ld
linker, it resolves references to routines by searching the System V version of the
Standard C library (
.PN libcV.a
)  or the POSIX version of the Standard C library (
.PN libcP.a
) before it searches
.PN libc.a .
The linker searches 
.PN libcV.a
when the SYSTEM_FIVE symbol is defined. It searches
.PN libcP.a
when POSIX is defined.
.PP
In the System V environment, if you specify the
.I \-lm 
option on either the 
.PN cc
or the 
.PN ld  
command line, the linker includes the System V math library, 
instead of the ULTRIX math library, in your program.
.SH Return Values
Most system calls have one or more return values.
An error condition is indicated by an otherwise impossible return
value.  This value is usually \-1.
When a function returns an error condition, it also stores an error number in 
the external variable 
.PN errno .
This variable is not cleared on successful calls.  Thus, you should
test
.PN errno 
only after an error has occurred.
.PP
All return codes and values from
functions are of type 
.I int
unless otherwise noted.
.PP
For a list of the errors and their
names as given in <errno.h>, see the
.MS errno 2 
reference page.
.SH Definitions
The following terms are used in Section 2:
.TP 5
Descriptor
.br
An integer assigned by the system when a file is referenced
by 
.PN open ,
.PN dup ,
.PN pipe ,
or a socket is referenced by 
.PN socket   
or 
.PN socketpair 
The descriptor uniquely identifies an access path to that file or socket from
a given process or any of its children.
.TP 5
Directory
.br
A directory is a special type of file that contains
references to other files, called links.
By convention, a directory
contains at least two links called 
dot (.) and dot-dot (..).  Dot refers to the directory itself 
and dot-dot refers to its parent directory.
.TP 5
Effective User Id, Effective Group Id, and Access Groups
.br
Access to system resources is governed by the 
the effective user ID, the effective group ID, and the
group access list.
.IP 
The effective user ID and effective group ID are initially the
process's real user ID and real group ID respectively.  Either
can be modified through execution of a set-user-ID or set-group-ID
file, or possibly by one of its ancestors.  For more information, see
.MS execve 2 .
.IP
The group access list is an additional set of group IDs
used only in determining resource accessibility.  Access checks
are performed as defined under the term File Access Permissions.
.TP
File Access Permissions
.br
Every file in the file system has a set of access permissions.
These permissions are used in determining whether a process
may perform a requested operation on the file, such as opening
a file for writing.  Access permissions are established at the
time a file is created.  They can be changed with the 
.PN chmod 
call. 
.IP
File access is separated into three types: read, write, and
execute.  Directory files use the execute
permission to control whether or not the directory can be searched.
.IP
File access permissions are interpreted by the system as
they apply to three different classes of users: the owner
of the file, those users in the file's group, and anyone else.
Every file has an independent set of access permissions for
each of these classes.  When an access check is made, the system
decides if permission should be granted by checking the access
information applicable to the caller.
.IP
Read, write, and execute/search permissions on
a file are granted to a process in the following instances:
.RS 10
.IP \(bu 5
The process's effective user ID is that of the superuser.
.IP \(bu 5
The process's effective user ID matches the user ID of the owner
of the file and the owner permissions allow the access.
.IP \(bu 5
The process's effective user ID does not match the user ID of the
owner of the file, but either the process's effective
group ID matches the group ID
of the file or the group ID of the file is in
the process's group access list
and the group permissions allow the access.
.IP \(bu 5
Neither the effective user ID nor the effective group ID
and group access list of the process
match the corresponding user ID and group ID of the file,
but the permissions for other users allow access.
.RE
.IP
Read, write, and execute/search permissions on a file are not
granted, as follows:
.RS 10
.IP \(bu 5
If the process is trying to execute an image and the file
system is mounted no execute, execute permission is denied.  
.IP \(bu 5
If the process's effective UID is not root, the
process is attempting to access a character or block special
device, and the file system is mounted with nodev, 
access is denied.
.IP \(bu 5
If the process's effective UID is not root, the process is trying
to execute an image with the
.I setuid
or
.I setgid
bit
set in the file's permissions, and the file system is mounted
nosuid, execute permission is denied.
.RE
.TP 5
File Name
.br
Names consisting of up to {FILENAME_MAX} characters can be used to name
an ordinary file, special file, or directory.
.IP
These characters can be selected from the set of all ASCII characters
excluding null (0) and the ASCII code for backslash (\\\).  
The parity bit (bit 8) must be 0.
.IP
Avoid using asterisks (*), question marks (?), or brackets ([ ])
as part of filenames because of the special meaning attached to these 
characters by the shell.
.TP 5
Message Operation Permissions
.br
In the 
.MS msgop 2 
and 
.MS msgctl 2
system call descriptions, the permission required for an
operation is specified by a token. The token argument is
the type of permission needed and it is interpreted as follows:
.EX
00400    Read by user
00200    Write by user
00060    Read, Write by group
00006    Read, Write by others
.EE
Read and write permissions are granted to a process if
one or more of the following are true:
.RS 10
.IP \(bu 5
The effective user ID of the process is superuser.
.IP \(bu 5
The effective user ID of the process matches
.I msg_perm.[c]uid
in the data structure associated with
msqid and the appropriate bit of the user portion
(0600) of
.I msg_perm.mode
is set.
.IP \(bu 5
The effective user ID of the process does not match
.I msg_perm.[c]uid, 
but
the effective group ID of the process matches
.I msg_perm.[c]gid
and the appropriate bit of the group portion (060)
of
.I msg_perm.mode
is set.
.IP \(bu 5
The effective user ID of the process does not match
.I msg_perm.[c]uid
and the effective group ID of the process does not match
.I msg_perm.[c]gid, 
but
the appropriate bit of the other portion (06) of
.I msg_perm.mode
is set.
.RE
.IP
If none of the previous conditions are true, the read and write
permissions are denied.
.TP 5
Message Queue Identifier
.br
A message queue identifier (msqid) is a unique positive integer
created by a 
.PN msgget 
system call.  Each msqid has a message queue and a data
structure associated with it.  The data structure is referred
to as
.I msqid_ds
and contains the following members:
.EX 0
struct  ipc_perm msg_perm; /*operation permission struct*/
ushort  msg_qnum;          /*number of msgs on q*/
ushort  msg_qbytes;        /*max number of bytes on q*/
ushort  msg_lspid;         /*pid of last msgsnd operation*/
ushort  msg_lrpid;         /*pid of last msgrcv operation*/
time_t  msg_stime;         /*last msgsnd time*/
time_t  msg_rtime;         /*last msgrcv time*/
time_t  msg_ctime;         /*last change time*/
                           /*Times measured in secs since*/
                           /*00:00:00 GMT, Jan.1, 1970*/
.EE
The
.I msg_perm
structure is an 
.I ipc_perm 
structure that specifies the message operation
permission.  The 
.I msg_perm 
structure includes the following
members:
.EX
ushort  cuid;     /*creator user id*/
ushort  cgid;     /*creator group id*/
ushort  uid;      /*user id*/
ushort  gid;      /*group id*/
ushort  mode;     /*r/w permission*/
.EE
The
.I msg_qnum
member is the number of message currently on the queue.
The
.I msg_qbytes
member is the maximum number of bytes allowed on the queue.
The
.I msg_lspid
member is the process ID of the last process that performed a
.PN msgrcv 
operation.
The
.I msg_lrpid
member is the process ID of the last process that performed a
.PN msgop
operation.
The
.I msg_stime
member is the time of the last
.PN msgop 
operation, 
.I msg_rtime
is the time of the last
.PN msgrcv