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67 lines
3.3 KiB
Text
67 lines
3.3 KiB
Text
COMMENT(!MOD!zsh/zselect
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Block and return when file descriptors are ready.
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!MOD!)
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The tt(zsh/zselect) module makes available one builtin command:
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startitem()
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findex(zselect)
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cindex(select, system call)
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cindex(file descriptors, waiting for)
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item(tt(zselect) [ tt(-rwe) ] [ tt(-t) var(timeout) ] [ tt(-a) var(array) ] [ tt(-A) var(assoc) ] [ var(fd) ... ])(
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The tt(zselect) builtin is a front-end to the `select' system call, which
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blocks until a file descriptor is ready for reading or writing, or has an
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error condition, with an optional timeout. If this is not available on
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your system, the command prints an error message and returns status 2
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(normal errors return status 1). For more information, see your systems
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documentation for manref(select)(3). Note there is no connection with the
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shell builtin of the same name.
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Arguments and options may be intermingled in any order. Non-option
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arguments are file descriptors, which must be decimal integers. By
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default, file descriptors are to be tested for reading, i.e. tt(zselect)
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will return when data is available to be read from the file descriptor, or
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more precisely, when a read operation from the file descriptor will not
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block. After a tt(-r), tt(-w) and tt(-e), the given file descriptors are
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to be tested for reading, writing, or error conditions. These options and
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an arbitrary list of file descriptors may be given in any order.
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(The presence of an `error condition' is not well defined in the
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documentation for many implementations of the select system call.
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According to recent versions of the POSIX specification, it is really an
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em(exception) condition, of which the only standard example is out-of-band
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data received on a socket. So zsh users are unlikely to find the tt(-e)
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option useful.)
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The option `tt(-t) var(timeout)' specifies a timeout in hundredths of a
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second. This may be zero, in which case the file descriptors will simply
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be polled and tt(zselect) will return immediately. It is possible to call
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zselect with no file descriptors and a non-zero timeout for use as a
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finer-grained replacement for `sleep'; note, however, the return status is
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always 1 for a timeout.
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The option `tt(-a) var(array)' indicates that var(array) should be set to
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indicate the file descriptor+LPAR()s+RPAR() which are ready. If the option
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is not
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given, the array tt(reply) will be used for this purpose. The array will
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contain a string similar to the arguments for tt(zselect). For example,
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example(zselect -t 0 -r 0 -w 1)
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might return immediately with status 0 and tt($reply) containing `tt(-r 0 -w
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1)' to show that both file descriptors are ready for the requested
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operations.
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The option `tt(-A) var(assoc)' indicates that the associative array
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var(assoc) should be set to indicate the file descriptor+LPAR()s+RPAR()
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which are ready. This option overrides the option tt(-a), nor will
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tt(reply) be modified. The keys of tt(assoc) are the file descriptors, and
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the corresponding values are any of the characters `tt(rwe)' to indicate
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the condition.
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The command returns status 0 if some file descriptors are ready for
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reading. If the operation timed out, or a timeout of 0 was given and no
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file descriptors were ready, or there was an error, it returns status 1 and
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the array will not be set (nor modified in any way). If there was an error
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in the select operation the appropriate error message is printed.
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)
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enditem()
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