sys_sendto的函数实现

Question：

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I am trying to explore the implementation of the "sendto" socket function. I tried to find an implementation in Linux.

In Linux, sendto function corresponds to "sys_sendto" system call.When I tried to follow it, I only found one definition for it which is the following:

asmlinkage long sys_sendto (void) attribute((weak,alias("sys_ni_syscall"))

That's to say sys_sendto is nothing than an alias to sys_ni_syscall, which does nothing more than returning a value. That does not make sense, as far as I am concerned. Where to find sys_snedto implementation? Is it implemented by some Assembly code?

Answers：

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It’s in net/socket.c, line 1747 of v3.10-rc7.

/* *  Send a datagram to a given address. We move the address into kernel *  space and check the user space data area is readable before invoking *  the protocol. */SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len,        unsigned int, flags, struct sockaddr __user *, addr,        int, addr_len){    struct socket *sock;    struct sockaddr_storage address;...

The SYSCALL_DEFINE6 macro is the macro for defining syscalls that take six arguments. It’s defined in include/linux/syscalls.h. It’s pretty complicated, so let’s start by looking at what happens in the no-argument version:

#define SYSCALL_DEFINE0(sname)                                  \        SYSCALL_METADATA(_##sname, 0);                          \        asmlinkage long sys_##sname(void)

A system call like getuid(), defined in kernel/sys.c, is coded like this:

SYSCALL_DEFINE0(getuid){    /* Only we change this so SMP safe */    return from_kuid_munged(current_user_ns(), current_uid());}

The macro expands out to

SYSCALL_METADATA(_##sname, 0);asmlinkage long sys_getuid(void){    /* Only we change this so SMP safe */    return from_kuid_munged(current_user_ns(), current_uid());}

Which is a straightforward definition of the function sys_getuid.

The metadata is optionally compiled into the kernel for tracing. See the source in syscalls.hfor the details.