inet_ntoa的实现

本文主要是介绍inet_ntoa的实现，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

从一段代码开始:

int test_ntoa() {struct sockaddr_in recv = {0};struct sockaddr_in sa = {0};recv.sin_addr.s_addr = -217732928; //192.168.5.243 sa.sin_addr.s_addr = -939415360; //192.168.5.200printf("ip of recv:%s ip of sa:%s\n", inet_ntoa(recv.sin_addr), inet_ntoa(sa.sin_addr));printf("ip of recv:%s\n", inet_ntoa(recv.sin_addr));printf("ip of sa :%s\n", inet_ntoa(sa.sin_addr)); return 0; }

结果有点诧异:

ip of recv:192.168.5.243 ip of sa:192.168.5.243 ip of recv:192.168.5.243 ip of sa :192.168.1.200

C函数出现这类问题,多半是由于其实现使用了静态内存所知.

google其源码:

char * inet_ntoa (struct in_addr in) {__libc_once_define (static, once);char *buffer;unsigned char *bytes;/* If we have not yet initialized the buffer do it now. */__libc_once (once, init);if (static_buf != NULL)buffer = static_buf;else{/* We don't use the static buffer and so we have a key. Use itto get the thread-specific buffer. */buffer = __libc_getspecific (key);if (buffer == NULL){/* No buffer allocated so far. */buffer = malloc (18);if (buffer == NULL)/* No more memory available. We use the static buffer. */buffer = local_buf;else__libc_setspecific (key, buffer);}}bytes = (unsigned char *) ∈__snprintf (buffer, 18, "%d.%d.%d.%d",bytes[0], bytes[1], bytes[2], bytes[3]);return buffer; }
这里使用了静态的缓存buffer,函数把转换后的ip写入缓存buffer,然后返回buffer的首地址值.

看似无奈的选择.glibc里的某些底层函数都使用这类处理方式.

其实也可以选择外部传参的做法解决:

<pre name="code" class="cpp">char * inet_ntoa (struct in_addr in, char* buf, int buflen) {... ... }

这样的处理方式,效率略低于第一种.或许这也是glibc选择第一种方式的原因.

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