Time To Get Rid Of errno

the syscall ABI isn't really all that better. if you think using syscall(__NR_xxx) is the solution to all of your problems, then you haven't looked beyond the x86 cpu on your desktop ;).

some functions, like ptrace(), have overlap between valid values and errors. in some cases it returns arbitrary data, so you cannot know whether 0xffffffff is because the data was 0xffffffff or -EPERM (on a 32bit system). you simply have to make assumptions that it's always valid based on other syscalls.

glibc doesn't treat *all* negative values as being errors -- it caps it at different values. on x86, it treats [-1,-4095] (or should it be [-4095,-1] ?) as an errno value. that way you aren't limiting yourself to 31bits, but (2^32 - 4096) possible valid values.

further, the convention for returning errno values isn't consistent across architectures. some (most) will normalize into one register, but a few split it -- at least ia64 & mips do. that way there is no confusion whether there was an error.

further further, some syscalls have to deal with raw C calling conventions. namely, some ABIs (like arm, mips, and ppc) require uint64_t to be split on even/odd pairs. so instead of doing:
syscall(SYS_readahead, fd, (uint32_t)(offset), (uint32_t)(offset >> 32), count)
you have to insert a 0 after the fd by hand:
syscall(SYS_readahead, fd, 0, (uint32_t)(offset), (uint32_t)(offset >> 32), count)

further further further, just because you call a specific syscall by name, it does not mean it's going to be the same across architectures. alpha is a pretty big example of this -- there are many syscalls that don't exist like __NR_getpid. instead they named it __NR_getxpid. they made a lot of decisions so as to be compatible with OSF (after all, surely OSF is more important than this toy "linux" project, and will obivously outlive it). or g'luck trying to do something as simple as mmap -- there's __NR_mmap, __NR_mmap2, and arches are not consistent as to how the offsets are used (maybe they're shifted ?).

the syscall(2) man page has a lot of good discussion in it.