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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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syscall NUMBER, LIST

Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, raises an exception. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. You can't use a string literal (or other read-only string) as an argument to syscall because Perl has to assume that any string pointer might be written through. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers. This emulates the syswrite function (or vice versa):

require 'syscall.ph';        # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);

Note that Perl supports passing of up to only 14 arguments to your syscall, which in practice should (usually) suffice.

Syscall returns whatever value returned by the system call it calls. If the system call fails, syscall returns -1 and sets $! (errno). Note that some system calls can legitimately return -1. The proper way to handle such calls is to assign $!=0; before the call and check the value of $! if syscall returns -1.

There's a problem with syscall(&SYS_pipe): it returns the file number of the read end of the pipe it creates. There is no way to retrieve the file number of the other end. You can avoid this problem by using pipe instead.

ISBN 9781906966027Perl Language Reference ManualSee the print edition