网络编程学习笔记(recvfrom很奇怪的一个地方)

recvfrom的最后一个参数不赋值时，返回的是一个很大的数。赋值后就是正常的

服务器端：

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>#define SERV_PORT 9999
#define BUF_LEN 128void dg_echo(int sockfd, struct sockaddr_in *pcliaddr, socklen_t clilen)
{char buf[BUF_LEN]; socklen_t len;int n;for (;;) {len = clilen;if ((n = recvfrom(sockfd, buf, BUF_LEN, 0, (struct sockaddr*)pcliaddr, &len)) < 0) {printf("recvfrom error:%s\n", strerror(errno));continue;} sendto(sockfd, buf, n, 0, (struct sockaddr*)pcliaddr, len);}
}int main(int argc, char **argv)
{struct sockaddr_in servaddr, clientaddr;int sockfd;sockfd = socket(AF_INET, SOCK_DGRAM, 0);   if (sockfd < 0) {printf("socket error:%s\n", strerror(errno));return -1;}memset(&servaddr, 0x00, sizeof(servaddr));servaddr.sin_family = AF_INET;servaddr.sin_addr.s_addr = htonl(INADDR_ANY);servaddr.sin_port = htons(SERV_PORT);if (bind(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0) {printf("bind error:%s\n", strerror(errno));close(sockfd); return -1;}dg_echo(sockfd, &clientaddr, sizeof(clientaddr));return 0;
}

客户端赋值时：

#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <netinet/in.h>
#include <arpa/inet.h>#define SERV_PORT 9999
#define BUF_LEN 128char *sock_ntop(const struct sockaddr* sockaddr, socklen_t len)
{static char str[BUF_LEN];char portstr[7];printf("sa_family1=%d, AF_INET=%d\n", sockaddr->sa_family, AF_INET);switch (sockaddr->sa_family) {case AF_INET:printf("enter...\n");      struct sockaddr_in *sin = (struct sockaddr_in*)sockaddr;if (inet_ntop(AF_INET, &sin->sin_addr, str, sizeof(str)) == (const char *)NULL) {printf("inet_ntop error:%s\n", strerror(errno));return NULL;}  if (ntohs(sin->sin_port) != 0) {snprintf(portstr, sizeof(portstr), ":%d", ntohs(sin->sin_port));strcat(str, portstr);}return str;} return NULL;
}void dg_cli(FILE *fp, int sockfd, const struct sockaddr_in * pservaddr, socklen_t servlen)
{int n;char sendline[BUF_LEN], recvline[BUF_LEN];struct sockaddr reply_addr;socklen_t len;while (fgets(sendline, BUF_LEN, fp) != NULL) {sendto(sockfd, sendline, strlen(sendline), 0, (struct sockaddr*)pservaddr, servlen);   //memset(&reply_addr, 0x00, sizeof(reply_addr));len = servlen;n = recvfrom(sockfd, recvline, BUF_LEN, 0, &reply_addr, &len);printf("sa_family=%d, servlen=%d, len=%d\n", reply_addr.sa_family, servlen, len);    if (len != servlen || memcmp(&reply_addr, pservaddr, len) != 0) {printf("receive (%s) ignored\n", sock_ntop(&reply_addr, len));continue;}recvline[n] = 0;fputs(recvline, stdout);}
}int main(int argc, char **argv)
{struct sockaddr_in servaddr;int sock;memset(&servaddr, 0x00, sizeof(servaddr));    servaddr.sin_family = AF_INET;servaddr.sin_port = htons(SERV_PORT);   if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) < 0) {printf("inet_pton error:%s\n", strerror(errno));return -1;}sock = socket(AF_INET, SOCK_DGRAM, 0);   if (sock < 0) {printf("socket error:%s\n", strerror(errno));return -1;}dg_cli(stdin, sock, &servaddr, sizeof(servaddr));return 0;
}

输出为：

客户端没有赋值时，输出的乱码：

#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <netinet/in.h>
#include <arpa/inet.h>#define SERV_PORT 9999
#define BUF_LEN 128char *sock_ntop(const struct sockaddr* sockaddr, socklen_t len)
{static char str[BUF_LEN];char portstr[7];printf("sa_family1=%d, AF_INET=%d\n", sockaddr->sa_family, AF_INET);switch (sockaddr->sa_family) {case AF_INET:printf("enter...\n");      struct sockaddr_in *sin = (struct sockaddr_in*)sockaddr;if (inet_ntop(AF_INET, &sin->sin_addr, str, sizeof(str)) == (const char *)NULL) {printf("inet_ntop error:%s\n", strerror(errno));return NULL;}  if (ntohs(sin->sin_port) != 0) {snprintf(portstr, sizeof(portstr), ":%d", ntohs(sin->sin_port));strcat(str, portstr);}return str;} return NULL;
}void dg_cli(FILE *fp, int sockfd, const struct sockaddr_in * pservaddr, socklen_t servlen)
{int n;char sendline[BUF_LEN], recvline[BUF_LEN];struct sockaddr reply_addr;socklen_t len;while (fgets(sendline, BUF_LEN, fp) != NULL) {sendto(sockfd, sendline, strlen(sendline), 0, (struct sockaddr*)pservaddr, servlen);   //memset(&reply_addr, 0x00, sizeof(reply_addr));//len = servlen;n = recvfrom(sockfd, recvline, BUF_LEN, 0, &reply_addr, &len);printf("sa_family=%d, servlen=%d, len=%d\n", reply_addr.sa_family, servlen, len);  if (len != servlen || memcmp(&reply_addr, pservaddr, len) != 0) {printf("receive (%s) ignored\n", sock_ntop(&reply_addr, len));continue;}recvline[n] = 0;fputs(recvline, stdout);}
}int main(int argc, char **argv)
{struct sockaddr_in servaddr;int sock;memset(&servaddr, 0x00, sizeof(servaddr));    servaddr.sin_family = AF_INET;servaddr.sin_port = htons(SERV_PORT);   if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) < 0) {printf("inet_pton error:%s\n", strerror(errno));return -1;}sock = socket(AF_INET, SOCK_DGRAM, 0);   if (sock < 0) {printf("socket error:%s\n", strerror(errno));return -1;}dg_cli(stdin, sock, &servaddr, sizeof(servaddr));return 0;
}

输出就不正确，接收的长度及sa_family都是不正确的

哦，原来是这样的

recvfrom()recvfrom() places the received message into the buffer buf.  Thecaller must specify the size of the buffer in len.If src_addr is not NULL, and the underlying protocol provides thesource address of the message, that source address is placed in thebuffer pointed to by src_addr.  In this case, addrlen is a value-result argument.  Before the call, it should be initialized to thesize of the buffer associated with src_addr.  Upon return, addrlen isupdated to contain the actual size of the source address.  Thereturned address is truncated if the buffer provided is too small; inthis case, addrlen will return a value greater than was supplied tothe call.If the caller is not interested in the source address, src_addr andaddrlen should be specified as NULL.

当中有这样一段话：

 Before the call, it should be initialized to thesize of the buffer associated with src_addr.  Upon return, addrlen isupdated to contain the actual size of the source address.

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