​Hello,

both functions `__shlim` and `__shgetc` subtract the members

named `buf` and `rpos` of the struct they manipulate.

In `__shlim`, this happens in the statement `f->shcnt = f->buf - f->rpos;`.

And in `__shgetc`, in happens inside the `shcnt` macro:

#define shcnt(f) ((f)->shcnt + ((f)->rpos - (f)->buf))

  

In our tests, while running `testsuite` in `libc-testsuite`,

both the `__shlim` and `__shgetc` functions are reached

with `f->buf` non-null and `f->rpos` a null pointer.

This can be made visible on execution platforms other than ours

by adding a statement at the beginning of the functions:

+      if (f->buf && !f->rpos) dprintf (2, "XXX Problem in __shlim\n");

+      if (f->buf && !f->rpos) dprintf (2, "XXX Problem in __shgetc\n");

Then if, running `libc-testsuite`, you see the following, it means that 

$ ./testsuite

fdopen test passed

fcntl test passed

fnmatch test passed

XXX Problem in __shlim

XXX Problem in __shgetc

XXX Problem in __shlim

XXX Problem in __shgetc

XXX Problem in __shlim

XXX Problem in __shgetc

XXX Problem in __shlim

XXX Problem in __shgetc

XXX Problem in __shlim

XXX Problem in __shgetc

XXX Problem in __shlim

XXX Problem in __shgetc

fscanf test passed

(...)

This has been tested on the (tag: v1.2.0) branch of git://git.musl-libc.org/musl

These pointer subtractions are undefined behavior. This is slightly worse

than computing `(char*)0-(char*)0`, which is undefined in C and defined in C++,

because compilers for both C and C++ are unlikely to exploit this one

for optimization. Subtracting between a non-null pointer and a null pointer 

on the other hand is undefined behavior in both languages, and it is

I mention this because similar undefined behaviors that were extremely

unlikely to cause harm have been fixed in musl in recent months,

so that this looks like something you may want to fix too.

Pascal