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IRIX Base Documentation 1998 November
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IRIX 6.5.2 Base Documentation November 1998.img
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catman
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Tk
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fileevent.z
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fileevent
Wrap
Text File
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1998-10-30
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7KB
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133 lines
ffffiiiilllleeeeeeeevvvveeeennnntttt((((3333TTTTkkkk)))) ffffiiiilllleeeeeeeevvvveeeennnntttt((((3333TTTTkkkk))))
NNNNAAAAMMMMEEEE
fileevent - Execute a script when a file becomes readable or writable
SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS
ffffiiiilllleeeeeeeevvvveeeennnntttt _f_i_l_e_I_d rrrreeeeaaaaddddaaaabbbblllleeee ?_s_c_r_i_p_t?
ffffiiiilllleeeeeeeevvvveeeennnntttt _f_i_l_e_I_d wwwwrrrriiiittttaaaabbbblllleeee ?_s_c_r_i_p_t?
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
This command is used to create _f_i_l_e _e_v_e_n_t _h_a_n_d_l_e_r_s. A file event handler
is a binding between a file and a script, such that the script is
evaluated whenever the file becomes readable or writable. File event
handlers are most commonly used to allow data to be received from a child
process on an event-driven basis, so that the receiver can continue to
interact with the user while waiting for the data to arrive. If an
application invokes ggggeeeettttssss or rrrreeeeaaaadddd when there is no input data available,
the process will block; until the input data arrives, it will not be
able to service other events, so it will appear to the user to ``freeze
up''. With ffffiiiilllleeeeeeeevvvveeeennnntttt, the process can tell when data is present and only
invoke ggggeeeettttssss or rrrreeeeaaaadddd when they won't block.
The _f_i_l_e_I_d argument to ffffiiiilllleeeeeeeevvvveeeennnntttt refers to an open file; it must be
ssssttttddddiiiinnnn, ssssttttddddoooouuuutttt, ssssttttddddeeeerrrrrrrr, or the return value from some previous ooooppppeeeennnn
command. If the _s_c_r_i_p_t argument is specified, then ffffiiiilllleeeeeeeevvvveeeennnntttt creates a
new event handler: _s_c_r_i_p_t will be evaluated whenever the file becomes
readable or writable (depending on the second argument to ffffiiiilllleeeeeeeevvvveeeennnntttt). In
this case ffffiiiilllleeeeeeeevvvveeeennnntttt returns an empty string. The rrrreeeeaaaaddddaaaabbbblllleeee and wwwwrrrriiiittttaaaabbbblllleeee
event handlers for a file are independent, and may be created and deleted
separately. However, there may be at most one rrrreeeeaaaaddddaaaabbbblllleeee and one wwwwrrrriiiittttaaaabbbblllleeee
handler for a file at a given time. If ffffiiiilllleeeeeeeevvvveeeennnntttt is called when the
specified handler already exists, the new script replaces the old one.
If the _s_c_r_i_p_t argument is not specified, ffffiiiilllleeeeeeeevvvveeeennnntttt returns the current
script for _f_i_l_e_I_d, or an empty string if there is none. If the _s_c_r_i_p_t
argument is specified as an empty string then the event handler is
deleted, so that no script will be invoked. A file event handler is also
deleted automatically whenever its file is closed or its interpreter is
deleted.
A file is considered to be readable whenever the ggggeeeettttssss and rrrreeeeaaaadddd commands
can return without blocking. A file is also considered to be readable if
an end-of-file or error condition is present. It is important for _s_c_r_i_p_t
to check for these conditions and handle them appropriately; for
example, if there is no special check for end-of-file, an infinite loop
may occur where _s_c_r_i_p_t reads no data, returns, and is immediately invoked
again.
When using ffffiiiilllleeeeeeeevvvveeeennnntttt for event-driven I/O, it's important to read the
file in the same units that are written from the other end. For example,
suppose that you are using ffffiiiilllleeeeeeeevvvveeeennnntttt to read data generated by a child
process. If the child process is writing whole lines, then you should
use ggggeeeettttssss to read those lines. If the child generates one line at a time
PPPPaaaaggggeeee 1111
ffffiiiilllleeeeeeeevvvveeeennnntttt((((3333TTTTkkkk)))) ffffiiiilllleeeeeeeevvvveeeennnntttt((((3333TTTTkkkk))))
then you shouldn't make more than a single call to ggggeeeettttssss in _s_c_r_i_p_t: the
first call will consume all the available data, so the second call may
block. You can also use rrrreeeeaaaadddd to read the child's data, but only if you
know how many bytes the child is writing at a time: if you try to read
more bytes than the child has written, the rrrreeeeaaaadddd call will block.
A file is considered to be writable if at least one byte of data can be
written to the file without blocking, or if an error condition is
present. Write handlers are probably not very useful without additional
command support. The ppppuuuuttttssss command is dangerous since it write more than
one byte at a time and may thus block. What is really needed is a new
non-blocking form of write that saves any data that couldn't be written
to the file.
The script for a file event is executed at global level (outside the
context of any Tcl procedure). If an error occurs while executing the
script then the ttttkkkkeeeerrrrrrrroooorrrr mechanism is used to report the error. In
addition, the file event handler is deleted if it ever returns an error;
this is done in order to prevent infinite loops due to buggy handlers.
CCCCRRRREEEEDDDDIIIITTTTSSSS
ffffiiiilllleeeeeeeevvvveeeennnntttt is based on the aaaaddddddddiiiinnnnppppuuuutttt command created by Mark Diekhans.
SSSSEEEEEEEE AAAALLLLSSSSOOOO
tkerror
KKKKEEEEYYYYWWWWOOOORRRRDDDDSSSS
asynchronous I/O, event handler, file, readable, script, writable
PPPPaaaaggggeeee 2222