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I'm reviewing man(2) pages on NetBSD 9, and have seen that all of the documents (write(2), open(2), pipe(2)) mention the Standard C Library at the top.

My understanding was that system calls were independent of library functions (such as those in libc). I don't see a similar mention in the Linux System call Manual. Does this mean that invoking these methods is calling some wrapper function included in libc, instead of directly calling a kernel function? Is this generally true, or just a feature of NetBSD?

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    You can only do system calls from assembly, not from C, so yes, any of those "system calls" from section 2 of the manual is actually a wrapper implemented in the standard C library (libc), which is linked either statically or dynamically into your program. Commented Jan 16, 2021 at 7:06

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open(), for example, is a C standard library function.

The implementation is dependent on the OS. On Unix-like systems it's typically implemented as a syscall.

So the application calls open(), and that calls syscall() on Unix-like systems.

C has been ported to a lot of platforms, and not all of them have syscall functionality. Their implementation is different.

So we say that things like open() are part of the standard C library, because the library abstracts from the underlying OS implementation.

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    They aren't part of the library defined by the C standard(s) ('highlevel' calls like fopen fwrite fputc are, 'lowlevel' calls like open write pipe aren't), but most C implementations especially on Unix for convenience bundle them in the same library file canonically libc.whatever (OTOH some math routines, even though in standard C, are traditionally in a separate file -lmath) Commented Jan 16, 2021 at 4:56
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    @dave_thompson_085 "Standard C Library" here means libc, the library linked by -lc. I.e. it means the library implementing the majority of C language functionality (and POSIX extensions to that language). It has less to do with actual standards. See e.g. the pipe(2) manual. Commented Jan 16, 2021 at 7:01
  • open(2) does not call the syscall(2) wrapper. (On x86-64, system calls happen through a processor instruction called just syscall, but that's a different thing). Also, on Linux, open(2) is implemented as a wrapper for openat(2) -- the open system call is only kept for binary compatibility, and only on older platforms. Commented Jan 16, 2021 at 7:15

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