Fix the pwrite64 system call.

[atrosinenko]

Fixes #4837. An Emscripten implementation of the pwrite64 system call is untested and references the wrong variable. This pull request fixes crash when this syscall is invoked and adds some tests.

The test_unistd_io passes.
There were 100 random tests run and all of them passed:

• asm1.test_simd7
• asm2f.test_pack
• asm3.test_cases
• asm2g.test_reinterpreted_ptrs
• asm3.test_bad_typeid
• asm2g.test_stdbool
• asm2i.test_sizeof
• asm2nn.test_simd_float64x2
• asm2f.test_stack_overflow
• asm2nn.test_sse4_1_full
• asm2.test_vprintf
• asm2nn.test_i64_cmp
• asm2i.test_sscanf_n
• asm2nn.test_asmjs_unknown_emscripten
• asm2nn.test_sscanf
• asm3.test_align64
• asm2f.test_mainenv
• asm3.test_sscanf_n
• asm3.test_mathfuncptr
• asm1.test_simd_int32x4
• asm2.test_i64_2
• asm2.test_em_asm_unused_arguments
• asm2nn.test_exceptions_uncaught_2
• asm2.test_dlfcn_mallocs
• asm2g.test_i64_llabs
• asm2g.test_dylink_printfs
• default.test_fs_mmap
• asm2nn.test_openjpeg
• asm2f.test_sscanf_3
• asm2f.test_isdigit_l
• asm3.test_errar
• asm1.test_embind_2
• asm2nn.test_iswdigit
• asm1.test_negative_zero
• asm1.test_exceptions_convert
• asm3.test_simd16
• asm2.test_strtoll_dec
• asm2f.test_structs
• asm2nn.test_llvm_used
• asm1.test_iswdigit
• asm2g.test_python
• asm2.test_printf_float
• asm2f.test_exceptions_libcxx
• asm2i.test_openjpeg
• asm2i.test_fgetc_unsigned
• asm2g.test_strtod
• default.test_ssr
• asm2g.test_getgep
• asm3.test_math
• asm2.test_phiundef
• asm2.test_funcptr
• asm2.test_exceptions
• asm2f.test_dlfcn_i64
• asm3.test_lua
• asm1.test_libcextra
• asm2nn.test_simd3
• default.test_isdigit_l
• default.test_em_asm_parameter_pack
• asm2f.test_newstruct
• asm2nn.test_isnan
• asm2i.test_closebitcasts
• asm2g.test_math_lgamma
• asm2nn.test_atomic_cxx
• asm2.test_mmap_file
• asm2g.test_pack
• asm2nn.test_assert
• asm1.test_sscanf_4
• asm2i.test_align64
• asm1.test_dylink_jslib
• asm2i.test_mmap
• asm1.test_alloca_stack
• asm2g.test_memmove3
• asm2f.test_intentional_fault
• asm3.test_raytrace
• asm2f.test_polymorph
• asm2g.test_exceptions_convert
• default.test_fs_base
• asm2.test_dlfcn_i64
• asm1.test_bad_typeid
• asm2f.test_exceptions_uncaught
• asm2f.test_zlib
• asm2i.test_uname
• default.test_sbrk
• default.test_literal_negative_zero
• default.test_dylink_global_init
• asm2nn.test_exit_status
• asm2f.test_stat
• asm2.test_unistd_dup
• asm3.test_bitfields
• asm2f.test_stdbool
• asm1.test_stack_overflow_check
• default.test_dylink_global_inits
• asm2nn.test_binaryen
• asm3.test_dead_functions
• asm3.test_dlfcn_longjmp
• asm3.test_strptime_reentrant
• asm2.test_double_i64_conversion
• asm2.test_emptyclass
• asm2f.test_statvfs
• asm2i.test_simd4

[kripken]

Thanks, this looks great, except I have a question about the test. It seems like it should be added 5 characters, but instead the read size after is increased by 6. And also the appended text at the end is \0 + write instead of just write, so that \0 may be the extra character, that I can't figure out where it's from?

[atrosinenko]

I supposed that extra \0 before write is due to offset = 32. It seems logically: if I would specify offset = 0, it skips 0 bytes and writes at the beginning, so when I specify offset = 32, it skips 32 bytes (only 31 is available, so it adds extra \0 -- I didn't noticed any direct notes about that in the pwrite man page, but there are some phrases about lseek + write behaving that way on the lseek man page) and writes the write string, so final read size if 32 + 5.

[kripken]

Oh, I see, yeah. I thought there were 32 characters before, that confused me.

[kripken]

Thanks!