I have bit bit of confusion regarding these two so here are my questions;
The Linux man-pages project lists all these functions: https://www.kernel.org/doc/man-pages/
Looking at recvfrom as an example, this function exists both as a Linux system call as well as a C library function. Their documentation seems different but they are both reachable using #include <sys/socket.h>. I don't understand their difference?
I also thought systems calls are defined using hex values which can be implemented in assembly directly, their list is here: https://syscalls.kernelgrok.com/
However I cannot find recvfrom in the above link. I'm a bit confused between Linux system calls vs C lib functions at this point!
Edit: To add to the questions, alot of functions are under (3) but not (2), i.e clean. Does this mean these are done by C runtime directly without relying on system calls and the underlying OS?
First, understand that that C functions and system calls are two completely different things.
System calls not wrapped in the C library must be called through the syscall function. One example of such a call is gettid.
To create a gettid system call wrapper with syscall, do this:
#define _GNU_SOURCE
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
pid_t gettid(void)
{
pid_t tid = (pid_t)syscall(SYS_gettid);
return tid;
}
Here's an excerpt from the NOTES section of the man-page, which explicitly states that this function is not defined within the C library:
NOTES
Glibc does not provide a wrapper for this system call; call it using syscall(2).
recvfrom is a C library wrapper around a system call.
Everything in section (2) is a system call. Everything in section (3) is not. Everything in section (3) (with a few notable exceptions, such as getumask) has a definition in the C library. About half of everything in section (2) does not have a definition (or wrapper) within the C library (with the exception of functions mandated by POSIX, and some other extensions, which all do), as with gettid.
When calling recvfrom in C, the C library calls the kernel to do the syscall.
The syscall function is the function that puts the system call number in the %eax register and uses int $0x80.
The reason you don't see recvfrom in https://syscalls.kernelgrok.com/ is because https://syscalls.kernelgrok.com/ is very, very incomplete.
The reason there are many functions in (3) that you don't see in (2) is because many functions on (3) don't have a system call. They may or may not rely on system calls, they just don't have a system call with that specific name that backs them.
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