intro(2)
NAME
intro − introduction to system calls
SYNTAX
#include <errno.h>
DESCRIPTION
Section 2 describes the ULTRIX system calls, which are the entries into the ULTRIX kernel. Certain distinctions of purpose are made in the headings. Pages ending in (2yp) are specific to the Yellow Pages (YP) service. Pages ending in (2nfs) are specific to the Network File System (NFS) service.
SYSTEM V COMPATIBILITY
Some system calls contain System V compatibility features that are available to general ULTRIX-32 programs. This compatibility sometimes conflict with features already present in ULTRIX; the function performed may be slightly different in the System V environment. These features are provided for applications that are being ported from System V.
The descriptions in this section include an ENVIRONMENT section that describes any differences in function between System V and the standard C runtime library.
The System V compatibility features are not contained in the standard C runtime library. To get System V specific behavior, you must specify that the System V environment is to be used in compiling and linking programs. There are two ways to do this:
Use the −Y option for the cc() command.
Globally set the environment variable PROG_ENV to SYSTEM_FIVE. If you are using the C shell, execute the following line or include it in your .login file:
setenv PROG_ENV SYSTEM_FIVE
If you are using the Bourne shell, execute the following line or include it in your .profile file:
PROG_ENV=SYSTEM_FIVE ; export PROG_ENV
In both cases, the cc command defines the preprocessor symbol SYSTEM_FIVE, so that the C preprocessor, /lib/cpp, will select the System V version of various data structures and symbol definitions.
In addition, if cc invokes ld, the library libcV.a (the System V version of the Standard C library) is searched before libc.a to resolve references to the System-V-specific routines. Also, if the −lm option is specified on either the cc(1) or the ld() command line, then the System V version of the math library will be used instead of the ULTRIX math library.
RETURN VALUE
Most of the system calls have one or more return values. An error condition is indicated by an otherwise impossible return value. This is almost always −1; the individual descriptions specify the details.
All return codes and values from functions are of type integer unless otherwise noted. An error number is also made available in the external variable errno which is not cleared on successful calls. Thus errno should be tested only after an error has occurred.
For a list of the errors and their names as given in <errno.h>, see errno(.).
DEFINITIONS
Process ID
Each active process in the system is uniquely identified by a positive integer called a process ID. The range of this ID is from 0 to {PROC_MAX}.
Parent process ID
A new process is created by a currently active process. For further information, see fork(.). The parent process ID of a process is the process ID of its creator.
Process Group ID
Each active process is a member of a process group that is identified 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 further information, see killpg() and the job control mechanisms of csh(.).
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 further information, see csh() and tty(.).
Real User ID and Real Group ID
Each user on the system is identified by a positive integer termed 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 distinguished group is termed the real group ID.
All processes have a real user ID and real group ID. These are initialized from the equivalent attributes of the process which created it.
Effective User Id, Effective Group Id, and Access Groups
Access to system resources is governed by three values: 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 may be modified through execution of a set-user-ID or set-group-ID file, possibly by one its ancestors. For further information, see execve(.).
The group access list is an additional set of group ID’s used only in determining resource accessibility. Access checks are performed as described below in “File Access Permissions”.
Superuser
A process is recognized as a superuser process and is granted special privileges if its effective user ID is 0.
Special Processes
The processes with a process ID’s of 0, 1, and 2 are special. Process 0 is the scheduler. Process 1 is the initialization process init, and is the ancestor of every other process in the system. It is used to control the process structure. Process 2 is the paging daemon.
Descriptor
An integer assigned by the system when a file is referenced by open(,), dup(,), or pipe() or a socket is referenced by socket() or socketpair() which uniquely identifies an access path to that file or socket from a given process or any of its children.
File Name
Names consisting of up to {FILENAME_MAX} characters may be used to name an ordinary file, special file, or directory.
These characters may be selected from the set of all ASCII character excluding 0 (null) and the ASCII code for / (slash). (The parity bit, bit 8, must be 0.)
Note that it is generally unwise to use *, ?, or [ ] as part of file names because of the special meaning attached to these characters by the shell.
Path Name
A path name is a null-terminated character string starting with an optional slash (/), followed by zero or more directory names separated by slashes, optionally followed by a file name. The total length of a path name must be less than {PATHNAME_MAX} characters.
If a path name begins with a slash, the path search begins at the root directory. Otherwise, the search begins from the current working directory. A slash by itself names the root directory. A null pathname refers to the current directory.
Directory
A directory is a special type of file which contains entries which are references to other files. Directory entries are called links. By convention, a directory contains at least two links, . and .., referred to as dot and dot-dot respectively. Dot refers to the directory itself and dot-dot refers to its parent directory.
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 need not be the root directory of the root file system.
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 may be changed at some later time through the chmod() call.
File access is broken down according to whether a file may be read, written, or executed. Directory files use the execute permission to control if the directory may 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, 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.
Read, write, and execute/search permissions on a file are granted to a process if:
The process’s effective user ID is that of the superuser.
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, and 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 corresponding user ID and group ID of the file, but the permissions for other users allow access.
If the process is trying to “exec” an image and the file system is mounted “no exec”, execute permission will be 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 will be denied.
If the process’s effective UID is not root, the process is trying to execute an image with setuid or setgid, bit set in the file’s permissions, and the file system is mounted “nosuid”, execute permission will be denied.
Otherwise, permission is 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 include whether messages sent and received at a socket require the name of the partner, whether communication is reliable, and whether the format is used in naming message recipients.
Each instance of the system supports some collection of socket types. Consult socket() for more information about the types available and their properties.
Each instance of the system supports some number of sets of communications 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.
Message Queue Identifier
A message queue identifier ( msqid ) is a unique positive integer created by a msgget() 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 is an ipc_perm structure that specifies the message operation permission (see below). 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 msg_qnum is the number of message currently on the queue. The msg_qbytes is the maximum number of bytes allowed on the queue. The msg_lspid is the process id of the last process that performed a msgrcv() operation. The msg_lrpid is the process id of the last process that performed a msgop operation. The msg_stime is the time of the last msgop() operation, msg_rtime is the time of the last msgrcv operation, and msg_ctime is the time of the last msgctl() operation that changed a member of the above structure.
Message Operation Permissions
In the msgop() and msgctl() system call descriptions, the permission required for an operation is given as “{token}”, where token is the type of permission needed, 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 and 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 and the appropriate bit of the other portion (06) of msg_perm.mode is set.
Otherwise, the corresponding permissions are denied.
Semaphore Identifier
A semaphore identifier (semid) is a unique positive integer created by a semget() 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 members:
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 integer 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 is the time of the last semop() operation, and sem_ctime is the time of the last semctl() operation that changed a member of the above structure.
A semaphore is a data structure that contains the following members:
ushort semval; /*semaphore value*/
short sempid; /*pid of last operation*/
ushort semncnt; /*# awaiting semval > cval*/
ushort semzcnt; /*# awaiting semval = 0*/
The semval is a non-negative integer. The sempid is equal to the process ID of the last process that performed a semaphore operation on this semaphore. The semncnt 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 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 semop() and semctl() system call descriptions, the permission required for an operation is given as “{token}”, where token is the type of permission needed 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 and 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 and the appropriate bit of the other portion (06) of sem_perm.mode is set.
Otherwise, the corresponding permissions are denied.
Shared Memory Identifier
A shared memory identifier (shmid) is a unique positive integer created by a shmget() 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 shmid_ds and contains 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 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 specifies the size of the shared memory segment. The shm_cpid is the process id of the process that created the shared memory identifier. The shm_lpid is the process id of the last process that performed a shmop() operation. The shm_nattch is the number of processes that currently have this segment attached. The shm_atime is the time of the last shmat operation, shm_dtime is the time of the last shmdt operation, and shm_ctime is the time of the last shmctl() operation that changed one of the members of the above structure.
Shared Memory Operation Permissions
In the shmop() and shmctl() system call descriptions, the permission required for an operation is given as “{token}”, where token is the type of permission needed, 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 and 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 and the appropriate bit of the other portion (06) of shm_perm.mode is set.
Otherwise, the corresponding permissions are denied.