SIGSEGV with C compiler/RTL. Possible scanf() problem.

[Ian Lance Taylor]

TPCgcc@mklab.ph.rhul.ac.uk writes:

> 	I have been experiencing variable corruption / SIGSEGV problems with 
> some pieces of quite simple code.  The exact symptoms are hard to pin down 
> and can vary from gcc version to version and whether or not compiler 
> optimisation is turned on.  Trivial changes to the program can make the 
> symptoms come and go.  The common factor is a particular scanf() statement.  
> Here is an illustration which generates a SIGSEGV when the EOF is read.  I 
> am entering input data "123.456,55<CR><ctrl-D>" from the keyboard.  The 
> results are identical if redirecting stdin from a file.  Can anyone 
> reproduce this error and/or fault my code?


> int main(int argc, char **argv)
> {
>   double offset, caltime;
>   unsigned int sigbits;
>   unsigned char ignore;
>   
>   offset=0.0;
>   while (scanf("%20lf%[,]%u", &caltime, &ignore, &sigbits) != EOF) {
>     caltime=caltime-offset;
>     printf ("caltime=%lf sigbits=%u\n", caltime, sigbits);
>   }
>   return 0;
> }

The %[ conversion in scanf takes a pointer to a buffer, not a pointer to
a single character.  If you type anything other than precisely a single
comma, scanf will clobber the stack, which is probably why you are
seeing a crash when ^D causes the main function to return.

This question is just about the C language, it doesn't seem to have
anything to do with gcc.

Ian