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Pipes are used to communicate with newly-created processes.
Currently pipes are only available on UNIX systems.
The Magma I/O module is currently undergoing revision, and the
current pipe facilities are a mix of the old and new methods.
A more uniform model will be available in future releases.
Subsections
Given a shell command line C, together with a type indicator T,
open a pipe between the Magma process and the command to be executed.
The standard C library function popen() is used, so
the possible characters allowed in T are the same as those allowed
for that function in the current operating system, and have the
same interpretation. Thus one should give the value "r" for T
so that Magma can read the output from the command, and give the value
"w" for T so that Magma can write into the input of the command.
See the Pipe intrinsic for a method for sending input to, and
receiving output from, a single command.
Important: this function returns a File object, and the I/O
functions for files described previously must be used rather then
those described in the following.
Given a shell command C and an input string S, create a pipe to
the command C, send S into the standard input of C, and return
the output of C as a string. Note that for many commands, S
should finish with a new line character if it consists of only one line.
We write a function which returns the current time as 3 values:
hour, minutes, seconds. The function opens a pipe to the UNIX
command "date" and applies regular expression matching to the
output to extract the relevant fields.
> function GetTime()
> D := POpen("date", "r");
> date := Gets(D);
> _, _, f := Regexp("([0-9][0-9]):([0-9][0-9]):([0-9][0-9])", date);
> h, m, s := Explode(f);
> return h, m, s;
> end function;
> h, m, s := GetTime();
> h, m, s;
14 30 01
> h, m, s := GetTime();
> h, m, s;
14 30 04
When a read request is made on a pipe, the available data is returned.
If no data is currently available, then the process waits until some
does becomes available, and returns that. (It will also return if the
pipe has been closed and hence no more data can be transmitted.)
It does not continue trying to read more data, as it cannot tell whether
or not there is some "on the way".
The upshot of all this is that care must be exercised as reads may
return less data than is expected.
Max: RngIntElt Default: 0
Waits for data to become available for reading from P and then returns
it as a string. If the parameter Max is set to a positive value
then at most that many characters will be read. Note that less than
Max characters may be returned, depending on the amount of
currently available data.
If the pipe has been closed then the special EOF marker string is
returned.
Max: RngIntElt Default: 0
Waits for data to become available for reading from P and then returns
it as a sequence of bytes (integers in the range 0..255).
If the parameter Max is set to a positive value then at most that
many bytes will be read. Note that less than Max bytes may be
returned, depending on the amount of currently available data.
If the pipe has been closed then the empty sequence is returned.
Writes the characters of the string s to the pipe P.
Writes the bytes in the byte sequence Q to the pipe P. Each
byte must be an integer in the range 0..255.
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