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Creates a temporary mailbox that can be used to read and write data between a parent and child process. The channels through which the processes communicate are called a pipe.
#include <unistd.h>int pipe (int array_fdscptr[2]); (ISO POSIX-1)
int pipe (int array_fdscptr[2], ...); (HP C EXTENSION)
array_fdscptr
An array of file descriptors. A pipe is implemented as an array of file descriptors associated with a mailbox. These mailbox descriptors are special in that these are the only file descriptors which, when passed to the isapipe function, will return 1.The file descriptors are allocated in the following way:
- The first available file descriptor is assigned to writing, and the next available file descriptor is assigned to reading.
- The file descriptors are then placed in the array in reverse order; element 0 contains the file descriptor for reading, and element 1 contains the file descriptor for writing.
...
Represents three optional, positional arguments, flag, bufsize, and bufquota:flag
An optional argument used as a bitmask.If either the O_NDELAY or O_NONBLOCK bit is set, the I/O operations to the mailbox through array_fdscptr file descriptors terminate immediately, rather than waiting for another process.
If, for example, the O_NDELAY bit is set and the child issues a read request to the mailbox before the parent has put any data into it, the read terminates immediately with 0 status. If neither the O_NDELAY nor O_NONBLOCK bit is set, the child will be waiting on the read until the parent writes any data into the mailbox. This is the default behavior if no flag argument is specified.
The values of O_NDELAY and O_NONBLOCK are defined in the <fcntl.h> header file. Any other bits in the flag argument are ignored. You must specify this argument if the second optional, positional argument bufsize is specified. If the flag argument is needed only to allow specification of the bufsize argument, specify flag as 0.
bufsize
Optional argument of type int that specifies the size of the mailbox, in bytes. Specify a value from 512 to 65535.If you specify 0 or omit this argument, the operating system creates a mailbox with a default size of 512 bytes.
If you specify a value less than 0 or larger than 65535, the results are unpredictable.
If you do specify this argument, be sure to precede it with a flag argument.
The DECC$PIPE_BUFFER_SIZE feature logical can also be used to specify the size of the mailbox. If bufsize is supplied, it takes precedence over the value of DECC$PIPE_BUFFER_SIZE. Otherwise, the value of DECC$PIPE_BUFFER_SIZE is used.
If neither bufsize nor DECC$PIPE_BUFFER_SIZE is specified, the default buffer size of 512 is used.
bufquota
Optional argument of type int that specifies the buffer quota of the pipe's mailbox. Specify a value from 512 to 2147483647.OpenVMS Version 7.3-2 added this argument. In previous OpenVMS versions, the buffer quota was equal to the buffer size.
The DECC$PIPE_BUFFER_QUOTA feature logical can also be used to specify the buffer quota. If the optional bufquota argument of the pipe function is supplied, it takes precedence over the value of DECC$PIPE_BUFFER_QUOTA. Otherwise, the value of DECC$PIPE_BUFFER_QUOTA is used.
If neither bufquota nor DECC$PIPE_BUFFER_QUOTA is specified, then the buffer quota defaults to the buffer size.
The mailbox used for the pipe is a temporary mailbox. The mailbox is not deleted until all processes that have open channels to that mailbox close those channels. The last process that closes a pipe writes a message to the mailbox, indicating the end-of-file.The mailbox is created by using the $CREMBX system service, specifying the following characteristics:
- A maximum message length of 512 characters
- A buffer quota of 512 characters
- A protection mask granting all privileges to USER and GROUP and no privileges to SYSTEM or WORLD
The buffer quota of 512 characters implies that you cannot write more than 512 characters to the mailbox before all or part of the mailbox is read. Since a mailbox record is slightly larger than the data part of the message that it contains, not all of the 512 characters can be used for message data. You can increase the size of the buffer by specifying an alternative size using the optional, third argument to the pipe function. A pipe under the OpenVMS system is a stream-oriented file with no carriage-control attributes. It is fully buffered by default in the HP C RTL. A mailbox used as a pipe is different than a mailbox created by the application. A mailbox created by the application defaults to a record-oriented file with carriage return, carriage control. Additionally, writing a zero-length record to a mailbox writes an EOF, as does each close of the mailbox. For a pipe, only the last close of a pipe writes an EOF.
The pipe is created by the parent process before vfork and an exec function are called. By calling pipe first, the child inherits the open file descriptors for the pipe. You can then use the getname function to return the name of the mailbox associated with the pipe, if this information is desired. The mailbox name returned by getname has the format _MBAnnnn: (ALPHA ONLY) or _MBAnnnnn: (INTEGRITY SERVERS ONLY) , where nnnn or nnnnn is a unique number.
Both the parent and the child need to know in advance which file descriptors will be allocated for the pipe. This information cannot be retrieved at run time. Therefore, it is important to understand how file descriptors are used in any HP C for OpenVMS program. For more information about file descriptors, see Chapter 2.
File descriptors 0, 1, and 2 are open in a HP C for OpenVMS program for stdin (SYS$INPUT), stdout (SYS$OUTPUT), and stderr (SYS$ERROR), respectively. Therefore, if no other files are open when pipe is called, pipe assigns file descriptor 3 for writing and file descriptor 4 for reading. In the array returned by pipe , 4 is placed in element 0 and 3 is placed in element 1.
If other files have been opened, pipe assigns the first available file descriptor for writing and the next available file descriptor for reading. In this case, the pipe does not necessarily use adjacent file descriptors. For example, assume that two files have been opened and assigned to file descriptors 3 and 4 and the first file is then closed. If pipe is called at this point, file descriptor 3 is assigned for writing and file descriptor 5 is assigned for reading. Element 0 of the array will contain 5 and element 1 will contain 3.
In large applications that do large amounts of I/O, it gets more difficult to predict which file descriptors are going to be assigned to a pipe; and, unless the child knows which file descriptors are being used, it will not be able to read and write successfully from and to the pipe.
One way to be sure that the correct file descriptors are being used is to use the following procedure:
- Choose two descriptor numbers that will be known to both the parent and the child. The numbers should be high enough to account for any I/O that might be done before the pipe is created.
- Call pipe in the parent at some point before calling an exec function.
- In the parent, use dup2 to assign the file descriptors returned by pipe to the file descriptors you chose. This now reserves those file descriptors for the pipe; any subsequent I/O will not interfere with the pipe.
You can read and write through the pipe using the UNIX I/O functions read and write , specifying the appropriate file descriptors. As an alternative, you can issue fdopen calls to associate file pointers with these file descriptors so that you can use the Standard I/O functions ( fread and fwrite ).
Two separate file descriptors are used for reading from and writing to the pipe, but only one mailbox is used so some I/O synchronization is required. For example, assume that the parent writes a message to the pipe. If the parent is the first process to read from the pipe, then it will read its own message back as shown in Figure REF-1.
Figure REF-1 Reading and Writing to a Pipe
0 Indicates success. - 1 Indicates an error.
Provides users with a mechanism for multiplexing input/output over a set of file descriptors that reference open streams.
#include <poll.h>int poll (struct pollfd filedes [], nfds_t nfds, int timeout);
filedes
Points to an array of pollfd structures, one for each file descriptor of interest. Each pollfd structure includes the following members:int fd -- The file descriptor
int events -- The requested conditions
int revents -- The reported conditionsnfds
The number of pollfd structures in the filedes array.timeout
The maximum length of time (in milliseconds) to wait for at least one of the specified events to occur.
The poll function provides users with a mechanism for multiplexing input/output over a set of file descriptors that reference open streams. For each member of the array pointed to by filedes, poll examines the given file descriptor for the event(s) specified in events. The poll function identifies those streams on which an application can send or receive messages, or on which certain events have occurred.The filedes parameter specifies the file descriptor to be examined and the events of interest for each file descriptor. It is a pointer to an array of pollfd structures. The fd member of each pollfd structure specifies an open file descriptor. The poll function uses the events member to determine what conditions to report for this file descriptor. If one or more of these conditions is true, the poll function sets the associated revents member.
The events and revents members of each pollfd structure are bitmasks. The calling process sets the events bitmask, and poll sets the revents bitmasks. These bitmasks contain inclusive ORed combinations of condition options. The following condition options are defined:
POLLERR -- An error has occurred on the file descriptor. This option is only valid in the revents bitmask; it is not used in the events member.
For STREAMS devices, if an error occurs on the file descriptor and the device is also disconnected, poll returns POLLERR; POLLERR takes precedence over POLLHUP.
POLLHUP -- The device has been disconnected. This event is mutually exclusive with POLLOUT; a stream can never be writable if a hangup occurred. However, this event and POLLIN, POLLRDNORM, POLLRDBAND or POLLPRI are not mutually exclusive. This option is only valid in the revents bitmask; it is ignored in the events member.
POLLIN -- Data other than high-priority data may be read without blocking. This option is set in revents even if the message is of zero length. In revents, this option is mutually exclusive with POLLPRI.
POLLNVAL -- The value specified for fd is invalid. This option is only valid in the revents member; it is ignored in the events member.
POLLOUT -- Normal (priority band equals 0) data may be written without blocking.
POLLPRI -- High-priority data may be received without blocking. This option is set in revents even if the message is of zero length. In revents, this option is mutually exclusive with POLLIN.
POLLRDBAND -- Data from a nonzero priority band may be read without blocking. This option is set in revents even if the message is of zero length.
POLLRDNORM -- Normal data (priority band equals 0) may be read without blocking. This option is set in revents even if the message is of zero length.
POLLWRBAND -- Priority data (priority band greater than 0) may be written. This event only examines bands that have been written to at least once.
POLLWRNORM -- Same as POLLOUT.The poll function ignores any pollfd structure whose fd member is less than 0 (zero). If the fd member of all pollfd structures is less than 0, the poll function will return 0 and have no other results.
The conditions indicated by POLLNORM and POLLOUT are true if and only if at least one byte of data can be read or written without blocking. There are two exceptions: regular files, which always poll true for POLLNORM and POLLOUT, and pipes, when the rules for the operation specify to return zero in order to indicate end-of-file.
The condition options POLLERR, POLLHUP, and POLLNVAL are always set in revents if the conditions they indicate are true for the specified file descriptor, whether or not these options are set in events.
For each call to the poll function, the set of reportable conditions for each file descriptor consists of those conditions that are always reported, together with any further conditions for which options are set in events. If any reportable condition is true for any file descriptor, the poll function will return with options set in revents for each true condition for that file descriptor.
If no reportable condition is true for any of the file descriptors, the poll function waits up to timeout milliseconds for a reportable condition to become true. If, in that time interval, a reportable condition becomes true for any of the file descriptors, poll reports the condition in the file descriptor's associated revents member and returns. If no reportable condition becomes true, poll returns without setting any revents bitmasks.
If the timeout parameter is a value of - 1, the poll function does not return until at least one specified event has occurred. If the value of the timeout parameter is 0 (zero), the poll function does not wait for an event to occur but returns immediately, even if no specified event has occurred.
The behavior of the poll function is not affected by whether the O_NONBLOCK option is set on any of the specified file descriptors.
The poll function supports regular files, terminal and pseudo-terminal devices, STREAMS-based files, FIFOs, and pipes. The behavior of poll on elements of file descriptors that refer to other types of files is unspecified.
For sockets, a file descriptor for a socket that is listening for connections indicates it is ready for reading after connections are available. A file descriptor for a socket that is connecting asynchronously indicates it is ready for writing after a connection is established.
n Upon successful completion, a nonnegative value is returned, indicating the number of file descriptors for which poll has set the revents bitmask. 0 poll has timed out and has not set any of the revents bitmasks. - 1 An error occurred. errno is set to indicate the error:
- EAGAIN -- Allocation of internal data structures failed. A later call to the poll function might complete successfully.
- EINTR -- A signal was intercepted during the poll function, and the signal handler was installed with an indication that functions are not to be restarted.
- EINVAL -- The nfds parameter is greater than OPEN_MAX, or one of the fd members refers to a stream or multiplexer that is linked (directly or indirectly) downstream from a multiplexer.
Initiates a pipe to a process.
#include <stdio.h>FILE *popen (const char *command, const char *type);
command
A pointer to a null-terminated string containing a shell command line.type
A pointer to a null-terminated string containing an I/O mode. Because open files are shared, you can use a type r command as an input filter and a type w command as an output filter. Specify one of the following values for the type argument:
- r ---the calling program can read from the standard output of the command by reading from the returned file stream.
- w ---the calling program can write to the standard input of the command by writing to the returned file stream.
The popen function creates a pipe between the calling program and a shell command awaiting execution. It returns a pointer to a FILE structure for the stream.The popen function uses the value of the DECC$PIPE_BUFFER_SIZE feature logical to set the buffer size of the mailbox it creates for the pipe. You can specify a DECC$PIPE_BUFFER_SIZE value of 512 to 65024 bytes. If DECC$PIPE_BUFFER_SIZE is not specified, the default buffer size of 512 is used.
See also fflush , pclose , and setvbuf .
x A pointer to the FILE structure for the opened stream. NULL Indicates an error. Unable to create files or processes.
Returns the first argument raised to the power of the second argument.
#include <math.h>double pow (double x, double y);
float powf (float x, float y); (INTEGRITY SERVERS, ALPHA)
long double powl (long double x, long double y); (INTEGRITY SERVERS, ALPHA)
x
A floating-point base to be raised to an exponent y.y
The exponent to which the base x is to be raised.
The pow functions raise a floating-point base x to a floating-point exponent y. The value of pow (x,y) is computed as e**(y ln(x)) for positive x.If x is 0 and y is negative, ±HUGE_VAL is returned and errno is set to ERANGE or EDOM.
x The result of the first argument raised to the power of the second. 1.0 The base is 0 and the exponent is 0. HUGE_VAL The result overflowed; errno is set to ERANGE. ±HUGE_VAL The base is 0 and the exponent is negative; errno is set to ERANGE or EDOM.
#include <stdio.h> #include <math.h> #include <errno.h> main() { double x; errno = 0; x = pow(-3.0, 2.0); printf("%d, %f\n", errno, x); }This example program outputs the following:
0, 9.000000
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