intro(2)intro(2)Nameintro - introduction to system calls
Syntax
#include <errno.h>
Description
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.
Additionally, some Section 2 reference pages contain an ENVIRONMENT
section that describes differences between the POSIX or SYSTEM V envi‐
ronment and the ULTRIX operating system.
Environmental Compatibility
Some system calls contain System V and POSIX features that are compati‐
ble 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 sys‐
tem. For example, a function performed under the ULTRIX operating sys‐
tem can produce different results in the System V or POSIX environment.
If conflicts exist, the ENVIRONMENT section of the reference page high‐
lights these differences.
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:
1. Use the -Y option for the command. For example, the following
demonstrates compiling a program in the System V environment
first, and then in the POSIX environment:
% cc -YSYSTEM_FIVE program.c
% cc -YPOSIX program.c
2. Globally set the environment variable PROG_ENV to SYSTEM_FIVE or
to POSIX.
If you are using the C shell, execute the following line or
include it in your file:
setenv PROG_ENV SYSTEM_FIVE
Replace ``SYSTEM_FIVE'' with ``POSIX'' if you are using the POSIX
environment.
If you are using the Bourne or the System V shell, execute the
following line or include it in your file:
PROG_ENV=POSIX ; export PROG_ENV
Replace ``POSIX'' with ``SYSTEM_FIVE'' if you are using the System
V environment.
In each instance, the command defines a preprocessor symbol, either
SYSTEM_FIVE or POSIX. When the SYSTEM_FIVE symbol is defined, the C
preprocessor, selects the System V data structures and symbol defini‐
tions. When the POSIX symbol is defined, selects the POSIX data struc‐
tures and symbol definitions.
In addition, if invokes the linker, it resolves references to routines
by searching the System V version of the Standard C library ( ) or the
POSIX version of the Standard C library ( ) before it searches The
linker searches when the SYSTEM_FIVE symbol is defined. It searches
when POSIX is defined.
In the System V environment, if you specify the -lm option on either
the or the command line, the linker includes the System V math library,
instead of the ULTRIX math library, in your program.
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 This variable is not cleared
on successful calls. Thus, you should test only after an error has
occurred.
All return codes and values from functions are of type int unless oth‐
erwise noted.
For a list of the errors and their names as given in <errno.h>, see the
reference page.
Definitions
The following terms are used in Section 2:
Descriptor
An integer assigned by the system when a file is referenced by or
a socket is referenced by or The descriptor uniquely identifies an
access path to that file or socket from a given process or any of
its children.
Directory
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.
Effective User Id, Effective Group Id, and Access Groups
Access to system resources is governed by the the effective user
ID, the effective group ID, and the group access list.
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
The group access list is an additional set of group IDs used only
in determining resource accessibility. Access checks are per‐
formed as defined under the term File Access Permissions.
File Access Permissions
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 call.
File access is separated into three types: read, write, and exe‐
cute. Directory files use the execute permission to control
whether or not the directory can be searched.
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 per‐
mission should be granted by checking the access information
applicable to the caller.
Read, write, and execute/search permissions on a file are granted
to a process in the following instances:
· The process's effective user ID is that of the supe‐
ruser.
· The process's effective user ID matches the user ID of
the owner of the file and the owner permissions allow
the access.
· 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.
· Neither the effective user ID nor the effective group ID
and group access list of the process match the corre‐
sponding user ID and group ID of the file, but the per‐
missions for other users allow access.
Read, write, and execute/search permissions on a file are not
granted, as follows:
· If the process is trying to execute an image and the
file system is mounted no execute, execute permission is
denied.
· 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.
· If the process's effective UID is not root, the process
is trying to execute an image with the setuid or setgid
bit set in the file's permissions, and the file system
is mounted nosuid, execute permission is denied.
File Name
Names consisting of up to {FILENAME_MAX} characters can be used to
name an ordinary file, special file, or directory.
These characters can be selected from the set of all ASCII charac‐
ters excluding null (0) and the ASCII code for backslash (\. The
parity bit (bit 8) must be 0.
Avoid using asterisks (*), question marks (?), or brackets ([ ])
as part of filenames because of the special meaning attached to
these characters by the shell.
Message Operation Permissions
In the and 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:
00400 Read by user
00200 Write by user
00060 Read, Write by group
00006 Read, Write by others
Read and write permissions are granted to a process if one or more
of the following are true:
· The effective user ID of the process is superuser.
· The effective user ID of the process matches
msg_perm.[c]uid in the data structure associated with
msqid and the appropriate bit of the user portion (0600)
of msg_perm.mode is set.
· The effective user ID of the process does not match
msg_perm.[c]uid, but the effective group ID of the
process matches msg_perm.[c]gid and the appropriate bit
of the group portion (060) of msg_perm.mode is set.
· The effective user ID of the process does not match
msg_perm.[c]uid and the effective group ID of the
process does not match msg_perm.[c]gid, but the appro‐
priate bit of the other portion (06) of msg_perm.mode is
set.
If none of the previous conditions are true, the read and write
permissions are denied.
Message Queue Identifier
A message queue identifier (msqid) is a unique positive integer
created by a system call. Each msqid has a message queue and a
data structure associated with it. The data structure is referred
to as msqid_ds and contains the following members:
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*/
The msg_perm structure is an ipc_perm structure that specifies the
message operation permission. The msg_perm structure includes the
following members:
ushort cuid; /*creator user id*/
ushort cgid; /*creator group id*/
ushort uid; /*user id*/
ushort gid; /*group id*/
ushort mode; /*r/w permission*/
The msg_qnum member is the number of message currently on the
queue. The msg_qbytes member is the maximum number of bytes
allowed on the queue. The msg_lspid member is the process ID of
the last process that performed a operation. The msg_lrpid member
is the process ID of the last process that performed a operation.
The msg_stime member is the time of the last operation, msg_rtime
is the time of the last operation, and msg_ctime is the time of
the last operation that changed a member of the above structure.
Parent process ID
A new process is created by a currently active process. For fur‐
ther information, see The parent process ID of a process is the
process ID of its creator.
Pathname
A pathname is a null-terminated character string containing an
optional slash (/), followed by zero or more directory names sepa‐
rated by slashes. This sequence can optionally be followed by
another slash and a filename. The total length of a pathname must
be less than {PATHNAME_MAX} characters.
If a pathname begins with a slash, the path search begins at the
directory. Otherwise, the search begins from the current working
directory. A slash by itself names the directory. A null path‐
name refers to the current directory.
Process ID
Each active process in the system is uniquely identified by a pos‐
itive integer called a process ID. The range of this ID is from 0
to {PROC_MAX}.
Process Group ID
Each active process is a member of a process group that is identi‐
fied by a positive integer called the process group ID. This is
the process ID of the group leader. This grouping permits the
signaling of related processes. For more information, see and the
job control mechanisms described in
Real User ID and Real Group ID
Each user on the system is identified by a positive integer called
the real user ID.
Each user is also a member of one or more groups. One of these
groups is distinguished from others and used in implementing
accounting facilities. The positive integer corresponding to this
group is called the real group ID.
All processes have a real user ID and real group ID. These are
initialized from the equivalent attributes of the parent process.
Root Directory and Current Working Directory
Each process has associated with it a concept of a root directory
and a current working directory for the purpose of resolving path
name searches. A process's root directory does not need to be the
root directory of the root file system.
Semaphore Identifier
A semaphore identifier (semid) is a unique positive integer cre‐
ated by a system call. Each semid has a set of semaphores and a
data structure associated with it. The data structure is referred
to as semid_ds and contains the following members:
struct ipc_perm sem_perm; /*operation permission struct*/
ushort sem_nsems; /*number of sems in set */
time_t sem_otime; /*last operation time*/
time_t sem_ctime; /*last change time*/
/*Times measured in secs since*/
/*00:00:00 GMT, Jan. 1, 1970*/
The sem_perm is an ipc_perm structure that specifies the semaphore
operation permission. This structure includes the following mem‐
bers:
ushort cuid; /*creator user id*/
ushort cgid; /*creator group id*/
ushort uid; /*user id*/
ushort gid; /*group id*/
ushort mode; /*r/a permission*/
The value of sem_nsems is equal to the number of semaphores in the
set. Each semaphore in the set is referenced by a positive inte‐
ger referred to as a sem_num. The sem_num values run sequentially
from 0 to the value of sem_nsems minus 1. The sem_otime member is
the time of the last operation, and sem_ctime is the time of the
last operation that changed a member of the above structure.
A semaphore is a data structure that contains the following mem‐
bers:
ushort semval; /*semaphore value*/
short sempid; /*pid of last operation*/
ushort semncnt; /*# awaiting semval > cval*/
ushort semzcnt; /*# awaiting semval = 0*/
The semval member is a non-negative integer. The sempid member is
equal to the process ID of the last process that performed a sema‐
phore operation on this semaphore. The semncnt member is a count
of the number of processes that are currently suspended awaiting
this semaphore's semval to become greater than its current value.
The semzcnt member is a count of the number of processes that are
currently suspended awaiting this semaphore's semval to become
zero.
Semaphore Operation Permissions
In the and system call descriptions, the permission required for
an operation is specified as {token}. The token argument is the
type of permission needed and it is interpreted as follows:
00400 Read by user
00200 Alter by user
00060 Read, Alter by group
00006 Read, Alter by others
Read and alter permissions on a semid are granted to a process if
one or more of the following are true:
· The effective user ID of the process is superuser.
· The effective user ID of the process matches
sem_perm.[c]uid in the data structure associated with
semid and the appropriate bit of the user portion (0600)
of sem_perm.mode is set.
· The effective user ID of the process does not match
sem_perm.[c]uid, but the effective group ID of the
process matches sem_perm.[c]gid and the appropriate bit
of the group portion (060) of sem_perm.mode is set.
· The effective user ID of the process does not match
sem_perm.[c]uid and the effective group ID of the
process does not match sem_perm.[c]gid, but the appro‐
priate bit of the other portion (06) of sem_perm.mode is
set.
If none of the previous conditions are true, the read and alter
permissions are denied.
Session
Each process group is a member of a session. A process is consid‐
ered to be a member of the session of which its process group is a
member. Typically there is one session per login.
Shared Memory Identifier
A shared memory identifier (shmid) is a unique positive integer
created by a system call. Each shmid has a segment of memory
(referred to as a shared memory segment) and a data structure
associated with it. The data structure is referred to as and con‐
tains the following members:
struct ipc_perm shm_perm; /*operation permission struct*/
int shm_segsz; /*size of segment*/
ushort shm_cpid; /*creator pid*/
ushort shm_lpid; /*pid of last operation*/
short shm_nattch; /*number of current attaches*/
time_t shm_atime; /*last attach time*/
time_t shm_dtime; /*last detach time*/
time_t shm_ctime; /*last change time*/
/*Times measured in secs since*/
/*00:00:00 GMT, Jan. 1, 1970*/
The shm_perm member is an ipc_perm structure that specifies the
shared memory operation permission. This structure includes the
following members:
ushort cuid; /*creator user id*/
ushort cgid; /*creator group id*/
ushort uid; /*user id*/
ushort gid; /*group id*/
ushort mode; /*r/w permission*/
The shm_segz member specifies the size of the shared memory seg‐
ment. The shm_cpid member is the process ID of the process that
created the shared memory identifier. The shm_lpid member is the
process ID of the last process that performed a operation. The
shm_nattch member is the number of processes that currently have
this segment attached. The shm_atime member is the time of the
last operation, shm_dtime is the time of the last operation, and
shm_ctime is the time of the last operation that changed one of
the members of the above structure.
Shared Memory Operation Permissions
In the and system call descriptions, the permission required for
an operation is given as {token}. The token argument is the type
of permission needed and it is interpreted as follows:
00400 Read by user
00200 Write by user
00060 Read, Write by group
00006 Read, Write by others
Read and write permissions on a shmid are granted to a process if
one or more of the following are true:
· The effective user ID of the process is superuser.
· The effective user ID of the process matches
shm_perm.[c]uid in the data structure associated with
shmid and the appropriate bit of the user portion (0600)
of shm_perm.mode is set.
· The effective user ID of the process does not match
shm_perm.[c]uid, but the effective group ID of the
process matches shm_perm.[c]gid and the appropriate bit
of the group portion (060) of shm_perm.mode is set.
· The effective user ID of the process does not match
shm_perm.[c]uid and the effective group ID of the
process does not match shm_perm.[c]gid, but the appro‐
priate bit of the other portion (06) of shm_perm.mode is
set.
If none of the previous conditions are true, the read and write
permissions are denied.
Sockets and Address Families
A socket is an endpoint for communication between processes. Each
socket has queues for sending and receiving data.
Sockets are typed according to their communications properties.
These properties determine whether messages sent and received at a
socket require the name of the partner, if communication is reli‐
able, and if the format is used in naming message recipients.
Each instance of the system supports some collection of socket
types. See for more information about the types available and
their properties.
Each instance of the system supports some number of sets of commu‐
nications protocols. Each protocol set supports addresses of a
certain format. An Address Family is the set of addresses for a
specific group of protocols. Each socket has an address chosen
from the address family in which the socket was created.
Special Processes
Those processes that have a process ID of 0, 1, and 2 are consid‐
ered special processes. Process 0 is the scheduler. Process 1 is
the initialization process init, and is the ancestor of every
other process in the system. It controls the process structure.
Process 2 is the paging daemon.
Superuser
A process is recognized as a superuser process and is granted spe‐
cial privileges if its effective user ID is 0.
tty Group ID
Each active process can be a member of a terminal group that is
identified by a positive integer called the tty group ID. This
grouping is used to arbitrate between multiple jobs contending for
the same terminal. For more information, see and
See Alsocc(1), csh(1), tty(4), intro(3), perror(3)intro(2)
