gustav@tharr.UUCP (Paul Moore) (11/27/90)
There appears to be a problem with syscall() in perl. When perl is working
out whether an argument is a string or an integer, it decides by working out
whether the argument looks like a number. But what about the *string* "1"?
The above is a bit vague, as I have no experience of syscall() under unix,
so I can't say whether this is a real problem. I encountered it while
porting perl to my machine (the Acorn Archimedes - if anyone is interested
in this port, get in touch!), where I made syscall() call the operating
system routines. These include a "print a string" call, and I noticed the
problem when I tried to print "1" - as mentioned above.
However, from what I can gather of unix syscall(), it allows you to call
any system call available - so what about syscall(creat,"1",0666) [or
however you would say it] to create a file "1"?
Gustav.
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E-Mail: pmoore@cix.compulink.co.uk
or: gustav@tharr.uucp (...!ukc!tharr!gustav)
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[ Posted from: tharr - *free* public access to Usenet in the UK 0234 261804 ] tchrist@convex.COM (Tom Christiansen) (11/27/90)
In article <1454@tharr.UUCP> gustav@tharr.UUCP (Paul Moore) writes: >system routines. These include a "print a string" call, and I noticed the >problem when I tried to print "1" - as mentioned above. > >However, from what I can gather of unix syscall(), it allows you to call >any system call available - so what about syscall(creat,"1",0666) [or >however you would say it] to create a file "1"? This is one of those times that perl cares about strings versus numerics. Sometimes you might have to add 0 or concat a null to coerce to the right type, but in practice I find this seldom to be the case. There's a big difference between syscall(&SYS_write,1,1,1) and syscall(&SYS_write,1,"1",1) The first sets $! to indicate a bad address (0x01) for that expected (char*) buffer to write (on my system, where the high bit indicates user space, not system space as on a Vax). The second actually writes a "1" to STDOUT. I course, you would really have uses syswrite() in the above case. For the record, syscall(&SYS_creat,"1",0666) does indeed creat a file named "1", as you would hope. --tom
lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall) (11/28/90)
In article <1454@tharr.UUCP> gustav@tharr.UUCP (Paul Moore) writes:
: There appears to be a problem with syscall() in perl. When perl is working
: out whether an argument is a string or an integer, it decides by working out
: whether the argument looks like a number. But what about the *string* "1"?
No, it doesn't decide by whether it looks like a number, exactly. It decides
by whether the value has ever been evaluated in a numeric context. The
literal string "1" has never been evaluated in a numeric context, so
should be treated as a string. If you have a string value of "1" in a
variable, but have evaluated it as a number (such as by comparing it using ==),
then it will be treated as a number.
Larryworley@compass.uucp (Dale Worley) (11/29/90)
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall) No, it doesn't decide by whether it looks like a number, exactly. It decides by whether the value has ever been evaluated in a numeric context. IMHO, this is a really weird rule. I prefer a more Snobol-like rule where a value knows whether it is a string or a number, and it retains that identity. (You can still use the "numeric value slot" of the value to cache the result of convert-to-numeric.) BTW, can I give "1" a numeric value by executing: "1" == 0 or am I missing something? Dale Worley Compass, Inc. worley@compass.com -- You think the government has trouble fighting a Drug War - wait until they start the War on Cholesterol. -- David Lively
markb@agora.uucp (Mark Biggar) (12/02/90)
In article <10543@jpl-devvax.JPL.NASA.GOV> lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall) writes: >In article <1454@tharr.UUCP> gustav@tharr.UUCP (Paul Moore) writes: >: There appears to be a problem with syscall() in perl. When perl is working >: out whether an argument is a string or an integer, it decides by working out >: whether the argument looks like a number. But what about the *string* "1"? > >No, it doesn't decide by whether it looks like a number, exactly. It decides >by whether the value has ever been evaluated in a numeric context. The >literal string "1" has never been evaluated in a numeric context, so >should be treated as a string. If you have a string value of "1" in a >variable, but have evaluated it as a number (such as by comparing it using ==), >then it will be treated as a number. Note: you can force the variable $x to be a string for syscall by passing $x.'' as the argument, $x+0 can be used to force it to be a number. -- Perl's maternal uncle Mark Biggar
lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall) (12/28/90)
In article <1990Nov28.165453.28250@uvaarpa.Virginia.EDU> worley@compass.uucp writes:
:
: From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
:
: No, it doesn't decide by whether it looks like a number, exactly.
: It decides by whether the value has ever been evaluated in a
: numeric context.
:
: IMHO, this is a really weird rule. I prefer a more Snobol-like rule
: where a value knows whether it is a string or a number, and it retains
: that identity. (You can still use the "numeric value slot" of the
: value to cache the result of convert-to-numeric.)
That's sort of what it does, unless I'm misunderstanding you. Sorry
I didn't explain clearly.
: BTW, can I give "1" a numeric value by executing:
:
: "1" == 0
:
: or am I missing something?
Yes, that gives it a numeric value along with the string value. Doing
1 eq ''
forces the numeric value to also have a string value.
1 .= ''
would force it to have ONLY a string value, and
"1" += 0
forces it to have ONLY a numeric value. (To avoid confusion, innocent
bystanders should note that we're not just talking about literal "1" and 1
here, but the state of a variable containing a string or numeric value.
You can't, in fact, assign to a literal in Perl.)
Larry