identify() seg.faults (PR#4057) - R-devel

Wed, Sep 3, 2003 2:21 AM #

Full_Name: Roger Bivand
Version: 1.7.1
OS: i686-pc-linux-gnu
Submission from: (NULL) (129.177.30.18)


identify() seg.faults when x is an empty list, when the n argument is given a
positive value, avoiding the check for non-positive n (n <- length(x) when x is
the non-existent x component of the empty list, and when neither x nor y are
components of the list.

suggested resolution: add test in plot.c around line 3010

Segmentation fault

but

Error in identify.default(xy) : invalid number of points in identify

I'm not sure, but adding a check against zero-length and/or NULL x and or 
y about line 3010 in src/main/plot.c should catch this.

On Tue, 2 Sep 2003, kjetil brinchmann halvorsen wrote:

Hola!
I will want to identify pixels in an image with the mouse, for 
so getting the image data from the matrix(es), for use in subsequent 
discriminant analysis. But the following bombs R:
(windows XP, rw1071)

str(baboon)

 list()
 - attr(*, "size")= int [1:2] 512 512
 - attr(*, "cellres")= num [1:2] 1 1
 - attr(*, "bbox")= num [1:4] 0 0 512 512
 - attr(*, "channels")= chr "grey"
 - attr(*, "bbcent")= logi FALSE
 - attr(*, "class")= chr "pixmapGrey"
 - attr(*, "grey")= num [1:512, 1:512] 0.537 0.510 0.345 0.259 0.322 
...

class(baboon)

[1] "pixmapGrey"

library(pixmap)
plot(baboon)
identify(baboon, n=1)

... and then R bombs!

What to do?

library(pixmap)

example(pixmap)

....
pixmap> plot(z[1:20, 10:40])

identify(z, n=1)

Program received signal SIGSEGV, Segmentation fault.
do_set (call=0x869ce7c, op=0x8299648, args=0x869ce98, rho=0x8f2496c)
    at eval.c:1308
1308                switch (NAMED(s)) {


The identify.default() function is passing the pixmap object through to
xy.coords(), which returns two empty x and y vectors, which are checked
for length in the R code - xy.coords() treats the pixmap object as a list, 
does:

} else if (is.list(x)) {
    xlab <- paste(ylab, "$x", sep = "")
    ylab <- paste(ylab, "$y", sep = "")
    y <- x[["y"]]
    x <- x[["x"]]
} else {

and y and x are the same length, so returns to identify.default() with 
nothing, which is passed on to the .Internal() undetected. Two 
possibilities - an identify.pixmap() in pixmap, or a test for the 
(package, S4) pixmap class in xy.coords. But as it stands, it's a quick 
way to exit the program. xy.coords() seems to be trusting the user to have 
a list with x and y components, and here it has neither. The list is 
indeed empty, being an S4 class - it just has attributes.