libev简单使用

libev简单使用

https://zhuanlan.zhihu.com/p/163282654

socket编程最紧迫的需求就是有一个好用的buffer对象可以用，就像libevent的evbuffer类似的东西，现在我们定义一个简单的buffer

buffer.h

#pragma once

#include <stddef.h>

#ifdef __cplusplus
extern "C" {
#endif

struct buffer {
    unsigned char *head, *data, *tail, *end;
};

void buffer_init(struct buffer *buf);
void buffer_free(struct buffer *buf);

size_t buffer_length(const struct buffer *buf);
size_t buffer_size(const struct buffer *buf);
unsigned char *buffer_data(const struct buffer *buf);

void buffer_append_data(struct buffer *buf, const void *data, size_t len);
void buffer_append_null(struct buffer *buf);
void buffer_append_char(struct buffer *buf, char c);
void buffer_append_printf(struct buffer *buf, const char *fmt, ...);

void buffer_drain(struct buffer *buf, size_t len);
void buffer_reset(struct buffer *buf);

void buffer_hexdump(const struct buffer *buf);

#ifdef __cplusplus
}
#endif

我们对buffer的简单需求，就是可以在末尾添加数据(buffer_append_data)，获取一块连续的内存(buffer_data)，消费指定长度的数据(buffer_drain)，这些操作内部都自动完成内存管理，使用者不需要考虑内存问题。

C语言事件循环库除了大名鼎鼎的libevent，还有libev。后者更加轻量。下面我们已libev为例，实现简单的io_copy功能，也即从一个描述符读数据，透传到另一个描述符。

 

先看使用示例：

开一个终端运行

$ nc -v -l 9999
Listening on [0.0.0.0] (family 0, port 9999)

运行

$ cc *.c -lev
$ ./a.out 127.0.0.1 9999 < /var/log/syslog

上面的示例程序把标准输入拷贝到127.0.0.1:9999标示的socket上

下面看main.c文件

// main.c

#include "io.h"
#include "xnet.h"
#include <ev.h>
#include <stdio.h>
#include <stdlib.h>

static void die(const char *s)
{
    perror(s);
    exit(1);
}

static void usage(const char *name)
{
    fprintf(stderr, "usage: %s ip port
", name);
    exit(1);
}

static void completion()
{
    fprintf(stderr, "done
");
}

static void error(const char *errmsg)
{
    fprintf(stderr, "error: %s
", errmsg);
}

int main(int argc, char *argv[])
{
    struct ev_loop *loop;
    int connfd;
    struct io io;

    if (argc != 3)
        usage(argv[0]);

    connfd = dial_tcp(argv[1], atoi(argv[2]));
    if (connfd == -1)
        die("dial_tcp");

    loop = ev_default_loop(0);

    io_init(&io, loop, completion, error);
    io_copy(&io, connfd, 0); // 从描述符0拷贝数据转发给描述符connfd

    ev_run(loop, 0);
}

 

定义struct io

// io.h

#pragma once

#include "buffer.h"
#include <ev.h>

#ifdef __cplusplus
extern "C" {
#endif

typedef void (*completion_cb)();
typedef void (*error_cb)(const char *errmsg);

struct io {
    struct ev_io src_watcher;
    struct ev_io dst_watcher;
    struct buffer buf;
    struct ev_loop *loop;
    completion_cb completion;
    error_cb error;
    int src_done;
};

void io_init(struct io *io, struct ev_loop *loop, completion_cb completion, error_cb error);
void io_copy(struct io *io, int dst, int src);

#ifdef __cplusplus
}
#endif

// io.c

#include "io.h"
#include <errno.h>
#include <unistd.h>

#define HIGH_WATER_MARK (1024 * 1024 * 4)
#define LOW_WATER_MARK  (1024 * 1024 * 1)

static void io_error(struct io *io)
{
    ev_io_stop(io->loop, &io->src_watcher);
    ev_io_stop(io->loop, &io->dst_watcher);

    if (io->error)
        io->error(strerror(errno));
}

static void io_completion(struct io *io)
{
    ev_io_stop(io->loop, &io->src_watcher);
    ev_io_stop(io->loop, &io->dst_watcher);

    if (io->completion)
        io->completion();
}

static void src_read_cb(struct ev_loop *loop, ev_io *w, int revents)
{
    char buf[4096];
    ssize_t nread;
    struct io *io;

    io = (struct io *)w->data;

    nread = read(w->fd, buf, sizeof(buf));
    switch (nread) {
    case -1:
        io_error(io);
        return;
    case 0:
        ev_io_stop(loop, &io->src_watcher);
        ev_io_start(loop, &io->dst_watcher);
        io->src_done = 1; // 标示读完成了
        return;
    default:
        buffer_append_data(&io->buf, buf, nread); // 把读到的数据添加到buffer末尾
        ev_io_start(loop, &io->dst_watcher);

        if (buffer_length(&io->buf) > HIGH_WATER_MARK) // buffer过大时停止读
            ev_io_stop(loop, &io->src_watcher);
        return;
    }
}

static void dst_write_cb(struct ev_loop *loop, ev_io *w, int revents)
{
    ssize_t nwritten;
    struct io *io;

    io = (struct io *)w->data;

    nwritten = write(w->fd, buffer_data(&io->buf), buffer_length(&io->buf));
    if (nwritten == -1) {
        io_error(io);
        return;
    }

    buffer_drain(&io->buf, nwritten); // 消费掉nwritten大小的数据

    if ((buffer_length(&io->buf) < LOW_WATER_MARK) && !io->src_done) // buffer过小时，开始读
        ev_io_start(loop, &io->src_watcher);

    if ((buffer_length(&io->buf) == 0) && io->src_done)
        io_completion(io);
}

void io_init(struct io *io, struct ev_loop *loop, completion_cb completion, error_cb error)
{
    buffer_init(&io->buf);

    io->loop = loop;
    io->completion = completion;
    io->error = error;
    io->src_done = 0;
}

void io_copy(struct io *io, int dst, int src)
{
    ev_io_init(&io->src_watcher, src_read_cb, src, EV_READ);
    ev_io_init(&io->dst_watcher, dst_write_cb, dst, EV_WRITE);

    io->src_watcher.data = io;
    io->dst_watcher.data = io;

    ev_io_start(io->loop, &io->src_watcher);
    ev_io_start(io->loop, &io->dst_watcher);
}

buffer.c实现

#include "buffer.h"
#include <assert.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static inline void *xrealloc(void *ptr, size_t size)
{
    void *p = realloc(ptr, size);
    if (!p) {
        fprintf(stderr, "realloc(%p, %zu): out of memory
", ptr, size);
        exit(1);
    }
    return p;
}

static inline size_t buffer_headroom(const struct buffer *buf)
{
    return buf->data - buf->head;
}

static inline size_t buffer_tailroom(const struct buffer *buf)
{
    return buf->end - buf->tail;
}

static inline void move_data(struct buffer *buf)
{
    size_t data_len;

    data_len = buffer_length(buf);
    memmove(buf->head, buf->data, data_len);
    buf->data = buf->head;
    buf->tail = buf->data + data_len;
}

static void buffer_grow(struct buffer *buf, size_t len)
{
    size_t size, new_size, data_len;

    assert(buf->head == buf->data);

    size = buffer_size(buf) + len;
    new_size = 1024;
    while (new_size < size)
        new_size <<= 1;

    data_len = buffer_length(buf);
    buf->head = xrealloc(buf->head, new_size);
    buf->data = buf->head;
    buf->tail = buf->data + data_len;
    buf->end = buf->head + new_size;
}

static void grow_if_needed(struct buffer *buf, size_t len)
{
    if (buffer_tailroom(buf) >= len)
        return;

    if (buffer_headroom(buf) > 0) {
        move_data(buf);
        if (buffer_tailroom(buf) >= len)
            return;
    }

    buffer_grow(buf, len);
}

void buffer_init(struct buffer *buf)
{
    memset(buf, 0, sizeof(struct buffer));
}

void buffer_free(struct buffer *buf)
{
    free(buf->head);
    memset(buf, 0, sizeof(struct buffer));
}

size_t buffer_length(const struct buffer *buf)
{
    return buf->tail - buf->data;
}

size_t buffer_size(const struct buffer *buf)
{
    return buf->end - buf->head;
}

unsigned char *buffer_data(const struct buffer *buf)
{
    return buf->data;
}

void buffer_append_data(struct buffer *buf, const void *data, size_t len)
{
    grow_if_needed(buf, len);
    memcpy(buf->tail, data, len);
    buf->tail += len;
}

void buffer_append_null(struct buffer *buf)
{
    char null = '';
    buffer_append_data(buf, &null, 1);
}

void buffer_append_char(struct buffer *buf, char c)
{
    buffer_append_data(buf, &c, 1);
}

void buffer_append_printf(struct buffer *buf, const char *fmt, ...)
{
    va_list ap;
    int npr;
    char *str;

    va_start(ap, fmt);
    npr = vsnprintf(NULL, 0, fmt, ap);
    va_end(ap);

    str = alloca(npr);

    va_start(ap, fmt);
    vsprintf(str, fmt, ap);
    va_end(ap);

    buffer_append_data(buf, str, npr);
}

void buffer_drain(struct buffer *buf, size_t len)
{
    buf->data += len;
}

void buffer_reset(struct buffer *buf)
{
    buf->data = buf->tail = buf->head;
}

void buffer_hexdump(const struct buffer *buf)
{
    size_t len, i;

    len = buffer_length(buf);
    for (i = 0; i < len; i++)
        printf("%02x", buf->data[i]);
    printf("
");
}

xnet.h

#pragma once

#ifdef __cplusplus
extern "C" {
#endif

int dial_tcp(const char *ip, int port);

#ifdef __cplusplus
}
#endif

xnet.c

#include "xnet.h"
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

static int dial_net(int socktype, const char *ip, uint16_t port)
{
    int fd, rc;
    struct sockaddr_in addr;

    fd = socket(AF_INET, socktype, 0);
    if (fd == -1)
        return -1;

    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = inet_addr(ip);

    rc = connect(fd, (struct sockaddr *)&addr, sizeof(addr));
    if (rc == -1) {
        close(fd);
        return -1;
    }
    return fd;
}

int dial_tcp(const char *ip, int port)
{
    return dial_net(SOCK_STREAM, ip, port);
}

主要思想就是一边从源描述符读，写到buffer里，一边从buffer里消费数据，写到socket。

==================== End