accept 中文man页面-boardline accept

NAME 名称

accept - 在一个套接字上接收一个连接

SYNOPSIS 概述

#include <sys/types.h>
#include <sys/socket.h>

int accept(int s, struct sockaddr *addr, socklen_t *addrlen);

DESCRIPTION 描述

accept 函数用于基于连接的套接字 (SOCK_STREAM, SOCK_SEQPACKET 和 SOCK_RDM). 它从未完成连接队列中取出第一个连接请求,创建一个和参数 s 属性相同的连接套接字,并为这个套接字分配一个文件描述符, 然后以这个描述符返回.新创建的描述符不再处于倾听状态.原套接字 s 不受此调用的影响.注意任意一个文件描述符标志 (任何可以被 fcntl以参数 F_SETFL 设置的值,比如非阻塞式或者异步状态)不会被 accept. 所继承.

参数 s 是以 socket(2) 创建,用 bind(2) 绑定到一个本地地址,并且在调用了 listen(2). 之后正在侦听一个连接的套接字. 参数 addr 是一个指向结构sockaddr的指针.这个结构体以连接实体地址填充. 所谓的连接实体,就是众所周知的网络层.参数 addr 所传递的真正的地址格式依赖于所使用的套接字族. (参见 socket(2) 和各协议自己的手册页). addrlen 是一个实时参数: 它的大小应该能够足以容纳参数 addr 所指向的结构体;在函数返回时此参数将以字节数表示出返回地址的实际长度.若 addr 使用NULL作为参数,addrlen将也被置为NULL.

如果队列中没有未完成连接套接字,并且套接字没有标记为非阻塞式, accept 将阻塞直到一个连接到达.如果一个套接字被标记为非阻塞式而队列中没有未完成连接套接字, accept 将返回EAGAIN.

使用 select(2) 或者 poll(2). 可以在一个套接字上有连接到来时产生事件.当尝试一个新的连接时套接字读就绪,这样我们就可以调用 accept 为这个连接获得一个新的套接字.此外,你还可以设置套接字在唤醒时接收到信号 SIGIO; 细节请参见 socket(7)

对于那些需要显式确认的协议,比如 DECNet, accept 可以看作仅仅从队列中取出下一个连接而不做确认.当在这个新的文件描述符上进行普通读写操作时暗示了确认,当关闭这个新的套接字时暗示了拒绝.目前在Linux上只有DECNet有这样的含义.

NOTES 注意

当接收到一个 SIGIO 信号或者 select(2) 或 poll(2) 返回读就绪并不总是意味着有新连接在等待,因为连接可能在调用 accept 之前已经被异步网络错误或者其他线程所移除.如果发生这种情况, 那么调用将阻塞并等待下一个连接的到来.为确保 accept 永远不会阻塞,传递的套接字 s 需要置 O_NONBLOCK 标志(参见 socket(7)).

RETURN VALUE 返回值

此调用在发生错误时返回-1.若成功则返回一个非负整数标识这个连接套接字.

ERROR HANDLING 错误处理

Linux accept 将一个待处理网络错误代码通过 accept 传递给新套接字 . 这种处理方式有别于其他的BSD套接字实现.为可靠操作,应用程序必须在调用 accept 之后能够检测这些为协议定义的网络错误,并且以重试解决,就象 EAGAIN 一样.对于TCP/IP这些网络错误是 ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP, 以及 ENETUNREACH.

ERRORS 错误

EAGAIN或者EWOULDBLOCK: 套接字被标记为非阻塞,且当前没有可接收的连接.
EBADF: 描述符非法.
ENOTSOCK: 描述符指向一个文件,而不是一个套接字.
EOPNOTSUPP: 作为参数的套接字不是 SOCK_STREAM. 类型
EFAULT: 参数 addr 不在用户可写地址空间之内.
EPERM: 防火墙规则禁止连接.
ENOBUFS,ENOMEM: 没有足够内存. 这个错误一般来说意味着内存分配受套接字缓冲区所限, 而不是没有系统内存.

另外,新套接字和协议中定义的网络错误也可能被返回. 不同的Linux内核也可能返回下列错误 EMFILE, EINVAL, ENOSR, ENOBUFS, EPERM, ECONNABORTED, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT, ERESTARTSYS.

CONFORMING TO 兼容于

SVr4,4.4BSD( accept 函数首次出现于BSD 4.2). BSD手册页文档定义了五个可能的错误返回值 (EBADF, ENOTSOCK, EOPNOTSUPP, EWOULDBLOCK, EFAULT). SUSv2文档的定义是EAGAIN, EBADF, ECONNABORTED, EFAULT, EINTR, EINVAL, EMFILE, ENFILE, ENOBUFS, ENOMEM, ENOSR, ENOTSOCK, EOPNOTSUPP, EPROTO, EWOULDBLOCK.

Linux accept不继承象 O_NONBLOCK 这样的套接字标志. 这一点有别于其他的BSD套接字实现. 因此,程序应该在accept所返回的套接字上设置所有需要的标志.

NOTE 注意

函数 accept 的第三个参数原来被声明为'int *'(在libc4和libc5以及其他很多系统中, 比如BSD 4.*,SunOS 4, SGI);POSIX 1003.1g草案试图将其改变为 `size_t *',SunOS 5就是这么做的. 后来的POSIX草案和Single Unix Specification以及glibc2使用了 `socklen_t *'. Quoting Linus Torvalds: 引自Linus Torvalds (译注:这个家伙就是Linux的创始人,所以我保留了他老人家的原文, 仅将原文大意附后): I fails: only italicizes a single line _Any_ sane library _must_ have "socklen_t" be the same size as int. Anything else breaks any BSD socket layer stuff. POSIX initially _did_ make it a size_t, and I (and hopefully others, but obviously not too many) complained to them very loudly indeed. Making it a size_t is completely broken, exactly because size_t very seldom is the same size as "int" on 64-bit architectures, for example. And it _has_ to be the same size as "int" because that's what the BSD socket interface is. Anyway, the POSIX people eventually got a clue, and created "socklen_t". They shouldn't have touched it in the first place, but once they did they felt it had to have a named type for some unfathomable reason (probably somebody didn't like losing face over having done the original stupid thing, so they silently just renamed their blunder).

数据类型"socklen_t"和int应该具有相同的长度.否则就会破坏 BSD套接字层的填充.POSIX开始的时候用的是size_t, Linus Torvalds(他希望有更多的人,但显然不是很多) 努力向他们解释使用size_t是完全错误的,因为在64位结构中 size_t和int的长度是不一样的,而这个参数(也就是accept函数的第三参数)的长度必须和int一致,因为这是BSD套接字接口标准.最终POSIX的那帮家伙找到了解决的办法,那就是创造了一个新的类型"socklen_t".Linux Torvalds说这是由于他们发现了自己的错误但又不好意思向大家伙儿承认,所以另外创造了一个新的数据类型.

NAME

accept - accept a connection on a socket

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>

int accept(int s, struct sockaddr *addr, socklen_t *addrlen);

DESCRIPTION

The accept function is used with connection-based socket types (SOCK_STREAM, SOCK_SEQPACKET and SOCK_RDM). It extracts the first connection request on the queue of pending connections, creates a new connected socket with mostly the same properties as s, and allocates a new file descriptor for the socket, which is returned. The newly created socket is no longer in the listening state. The original socket s is unaffected by this call. Note that any per file descriptor flags (everything that can be set with the F_SETFL fcntl, like non blocking or async state) are not inherited across an accept.

The argument s is a socket that has been created with socket(2), bound to a local address with bind(2), and is listening for connections after a listen(2).

The argument addr is a pointer to a sockaddr structure. This structure is filled in with the address of the connecting entity, as known to the communications layer. The exact format of the address passed in the addr parameter is determined by the socket's family (see socket(2) and the respective protocol man pages). The addrlen argument is a value-result parameter: it should initially contain the size of the structure pointed to by addr; on return it will contain the actual length (in bytes) of the address returned. When addr is NULL nothing is filled in.

If no pending connections are present on the queue, and the socket is not marked as non-blocking, accept blocks the caller until a connection is present. If the socket is marked non-blocking and no pending connections are present on the queue, accept returns EAGAIN.

In order to be notified of incoming connections on a socket, you can use select(2) or poll(2). A readable event will be delivered when a new connection is attempted and you may then call accept to get a socket for that connection. Alternatively, you can set the socket to deliver SIGIO when activity occurs on a socket; see socket(7) for details.

For certain protocols which require an explicit confirmation, such as DECNet, accept can be thought of as merely dequeuing the next connection request and not implying confirmation. Confirmation can be implied by a normal read or write on the new file descriptor, and rejection can be implied by closing the new socket. Currently only DECNet has these semantics on Linux.

NOTES

There may not always be a connection waiting after a SIGIO is delivered or select(2) or poll(2) return a readability event because the connection might have been removed by an asynchronous network error or another thread before accept is called. If this happens then the call will block waiting for the next connection to arrive. To ensure that accept never blocks, the passed socket s needs to have the O_NONBLOCK flag set (see socket(7)).

RETURN VALUE

The call returns -1 on error. If it succeeds, it returns a non-negative integer that is a descriptor for the accepted socket.

ERROR HANDLING

Linux accept passes already-pending network errors on the new socket as an error code from accept. This behaviour differs from other BSD socket implementations. For reliable operation the application should detect the network errors defined for the protocol after accept and treat them like EAGAIN by retrying. In case of TCP/IP these are ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.

ERRORS

accept shall fail if:

EAGAIN or EWOULDBLOCK: The socket is marked non-blocking and no connections are present to be accepted.
EBADF: The descriptor is invalid.
ENOTSOCK: The descriptor references a file, not a socket.
EOPNOTSUPP: The referenced socket is not of type SOCK_STREAM.
EINTR: The system call was interrupted by a signal that was caught before a valid connection arrived.
ECONNABORTED: A connection has been aborted.
EINVAL: Socket is not listening for connections.
EMFILE: The per-process limit of open file descriptors has been reached.
ENFILE: The system maximum for file descriptors has been reached.

accept may fail if:

EFAULT: The addr parameter is not in a writable part of the user address space.
ENOBUFS, ENOMEM: Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by the system memory.
EPROTO: Protocol error.

Linux accept may fail if:

EPERM: Firewall rules forbid connection.

In addition, network errors for the new socket and as defined for the protocol may be returned. Various Linux kernels can return other errors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT. The value ERESTARTSYS may be seen during a trace.

CONFORMING TO

SVr4, 4.4BSD (the accept function first appeared in BSD 4.2). The BSD man page documents five possible error returns (EBADF, ENOTSOCK, EOPNOTSUPP, EWOULDBLOCK, EFAULT). SUSv3 documents errors EAGAIN, EBADF, ECONNABORTED, EINTR, EINVAL, EMFILE, ENFILE, ENOBUFS, ENOMEM, ENOTSOCK, EOPNOTSUPP, EPROTO, EWOULDBLOCK. In addition, SUSv2 documents EFAULT and ENOSR.

Linux accept does _not_ inherit socket flags like O_NONBLOCK. This behaviour differs from other BSD socket implementations. Portable programs should not rely on this behaviour and always set all required flags on the socket returned from accept.

NOTE

The third argument of accept was originally declared as an `int *' (and is that under libc4 and libc5 and on many other systems like BSD 4.*, SunOS 4, SGI); a POSIX 1003.1g draft standard wanted to change it into a `size_t *', and that is what it is for SunOS 5. Later POSIX drafts have `socklen_t *', and so do the Single Unix Specification and glibc2. Quoting Linus Torvalds: _Any_ sane library _must_ have "socklen_t" be the same size as int. Anything else breaks any BSD socket layer stuff. POSIX initially _did_ make it a size_t, and I (and hopefully others, but obviously not too many) complained to them very loudly indeed. Making it a size_t is completely broken, exactly because size_t very seldom is the same size as "int" on 64-bit architectures, for example. And it _has_ to be the same size as "int" because that's what the BSD socket interface is. Anyway, the POSIX people eventually got a clue, and created "socklen_t". They shouldn't have touched it in the first place, but once they did they felt it had to have a named type for some unfathomable reason (probably somebody didn't like losing face over having done the original stupid thing, so they silently just renamed their blunder).

accept 中文man页面