Objective-C字符串编码问题(objective-C string encoding problem)

 好吧，我对C套接字相当陌生，但我只需要做一些简单的sendto（）和recvfrom（）调用，使用多播套接字在网络中获取一个字符串。 在环顾四周并阅读了几个指南（包括Beej's）后，我发现下面的代码可以监听通过多播套接字发送的消息（这正是我所需要的）。 程序运行良好时，它在主要，但我的问题出现时，我把它放在一个方法（即方法称为“listenForPackets”）在我的项目中的其他地方，并试图运行它在runloop的另一个线程。 经过调试后，问题归结为变量“mc_addr_str”，该变量在main方法中被赋值为等于argv [1]。 
 
#include <sys/types.h>  // for type definitions
#include <sys/socket.h> // for socket API calls
#include <netinet/in.h> // for address structs
#include <arpa/inet.h>  // for sockaddr_in
#include <stdio.h>      // for printf() and fprintf()
#include <stdlib.h>     // for atoi()
#include <string.h>     // for strlen()
#include <unistd.h>     // for close()

#define MAX_LEN  1024   // maximum receive string size
#define MIN_PORT 1024   // minimum port allowed
#define MAX_PORT 65535  // maximum port allowed

int main(int argc, char *argv[]) {

    int sock;                     // socket descriptor
    int flag_on = 1;              // socket option flag
    struct sockaddr_in mc_addr;   // socket address structure
    char recv_str[MAX_LEN+1];     // buffer to receive string
    int recv_len;                 // length of string received
    struct ip_mreq mc_req;        // multicast request structure
    char* mc_addr_str;            // multicast IP address
    unsigned short mc_port;       // multicast port
    struct sockaddr_in from_addr; // packet source
    unsigned int from_len;        // source addr length

    // validate number of arguments
    if (argc != 3) {
        fprintf(stderr, 
                "Usage: %s <Multicast IP> <Multicast Port>\n", 
                argv[0]);
        exit(1);
    }

    mc_addr_str = argv[1];      // arg 1: multicast ip address
    mc_port = atoi(argv[2]);    // arg 2: multicast port number

    // validate the port range
    if ((mc_port < MIN_PORT) || (mc_port > MAX_PORT)) {
        fprintf(stderr, "Invalid port number argument %d.\n",
                mc_port);
        fprintf(stderr, "Valid range is between %d and %d.\n",
                MIN_PORT, MAX_PORT);
        exit(1);
    }

    // create socket to join multicast group on
    if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
        perror("socket() failed");
        exit(1);
    }

    // set reuse port to on to allow multiple binds per host
    if ((setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &flag_on, sizeof(flag_on))) < 0) {
        perror("setsockopt() failed");
        exit(1);
    }

    // construct a multicast address structure
    memset(&mc_addr, 0, sizeof(mc_addr));
    mc_addr.sin_family      = AF_INET;
    mc_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    mc_addr.sin_port        = htons(mc_port);

    // bind multicast address to socket
    if ((bind(sock, (struct sockaddr *) &mc_addr, sizeof(mc_addr))) < 0) {
        perror("bind() failed");
        exit(1);
    }

    // construct an IGMP join request structure
    mc_req.imr_multiaddr.s_addr = inet_addr(mc_addr_str);
    mc_req.imr_interface.s_addr = htonl(INADDR_ANY);

    // send an ADD MEMBERSHIP message via setsockopt
    if ((setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void*) &mc_req, sizeof(mc_req))) < 0) {
        perror("setsockopt() failed");
        exit(1);
    }

    for (;;) {          // loop forever

        // clear the receive buffers & structs
        memset(recv_str, 0, sizeof(recv_str));
        from_len = sizeof(from_addr);
        memset(&from_addr, 0, from_len);

        // block waiting to receive a packet
        if ((recv_len = recvfrom(sock, recv_str, MAX_LEN, 0, (struct sockaddr*)&from_addr, &from_len)) < 0) {
            perror("recvfrom() failed");
            exit(1);
        }

        // output received string
        printf("Received %d bytes from %s: ", recv_len, inet_ntoa(from_addr.sin_addr));
        printf("%s", recv_str);
    }

    // send a DROP MEMBERSHIP message via setsockopt
    if ((setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, (void*) &mc_req, sizeof(mc_req))) < 0) {
        perror("setsockopt() failed");
        exit(1);
    }

    close(sock);  
}
 
 现在，通过另一个SO成员的帮助，我有一个方法将IP地址作为NSString返回给我（我在我的程序的其他地方也使用它，所以我需要保持它返回NSString）。 
 
-(NSString *)getIPAddress {
    NSString *address = @"error";
    struct ifaddrs *interfaces; // = NULL;
    struct ifaddrs *temp_addr; // = NULL;
    int success = 0;

    // retrieve the current interfaces - returns 0 on success
    success = getifaddrs(&interfaces);
    if (success == 0)  
    {
        // Loop through linked list of interfaces
        temp_addr = interfaces;
        while(temp_addr != NULL)  
        {
            if(temp_addr->ifa_addr->sa_family == AF_INET)
            {
                // Check if interface is en0 which is the wifi connection on the iPhone  
                if([[NSString stringWithUTF8String:temp_addr->ifa_name] isEqualToString:@"en0"])  
                {
                    // Get NSString from C String
                    address = [NSString stringWithUTF8String:inet_ntoa(((struct sockaddr_in *)temp_addr->ifa_addr)->sin_addr)];
                }
            }
            temp_addr = temp_addr->ifa_next;
        }
    }

    // Free memory
    freeifaddrs(interfaces); 
    return address; 
}
 
 我以为我可以做一个简单的小转换 
 
mc_addr_str = (someConversion)getIPAddress;
 
 我到目前为止是： 
 
NSString *ipAddress = [[NSString alloc] initWithString:self.getIPAddress];

mc_addr_str = [ipAddress cStringUsingEncoding:[NSString defaultCStringEncoding]];
 
 当我这样做时，程序会将它传递给setsockopt调用，然后以-1的错误代码失败（我假设这是一个通用错误代码，可让程序知道发生了什么不愉快的事情并需要中止）。 另外，当我在前面的语句中分配mc_addr_str时，我会得到 
 
warning: assignment discards qualifiers from pointer target type
 
 我不确定现在我的问题在哪里。 在分配给mc_addr_str期间，我有铸造错误还是使用了错误的编码？ 任何输入是赞赏！ 

Ok, I'm fairly new to C sockets but I just need to do some simple sendto() and recvfrom() calls to get a string out across a network, using multicast sockets. After looking around and reading several guides (including Beej's), I found the code below which does the job of listening for messages sent over a multicast socket (which is what I need). The program runs fine when it is in main, but my problem arises when I put it in a method (i.e. a method called "listenForPackets") elsewhere in my project and attempt to run it in another thread on a runloop. After debugging though, the problem comes down to the variable "mc_addr_str" which is assigned to equal argv[1] in the main method.
 
#include <sys/types.h>  // for type definitions
#include <sys/socket.h> // for socket API calls
#include <netinet/in.h> // for address structs
#include <arpa/inet.h>  // for sockaddr_in
#include <stdio.h>      // for printf() and fprintf()
#include <stdlib.h>     // for atoi()
#include <string.h>     // for strlen()
#include <unistd.h>     // for close()

#define MAX_LEN  1024   // maximum receive string size
#define MIN_PORT 1024   // minimum port allowed
#define MAX_PORT 65535  // maximum port allowed

int main(int argc, char *argv[]) {

    int sock;                     // socket descriptor
    int flag_on = 1;              // socket option flag
    struct sockaddr_in mc_addr;   // socket address structure
    char recv_str[MAX_LEN+1];     // buffer to receive string
    int recv_len;                 // length of string received
    struct ip_mreq mc_req;        // multicast request structure
    char* mc_addr_str;            // multicast IP address
    unsigned short mc_port;       // multicast port
    struct sockaddr_in from_addr; // packet source
    unsigned int from_len;        // source addr length

    // validate number of arguments
    if (argc != 3) {
        fprintf(stderr, 
                "Usage: %s <Multicast IP> <Multicast Port>\n", 
                argv[0]);
        exit(1);
    }

    mc_addr_str = argv[1];      // arg 1: multicast ip address
    mc_port = atoi(argv[2]);    // arg 2: multicast port number

    // validate the port range
    if ((mc_port < MIN_PORT) || (mc_port > MAX_PORT)) {
        fprintf(stderr, "Invalid port number argument %d.\n",
                mc_port);
        fprintf(stderr, "Valid range is between %d and %d.\n",
                MIN_PORT, MAX_PORT);
        exit(1);
    }

    // create socket to join multicast group on
    if ((sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
        perror("socket() failed");
        exit(1);
    }

    // set reuse port to on to allow multiple binds per host
    if ((setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &flag_on, sizeof(flag_on))) < 0) {
        perror("setsockopt() failed");
        exit(1);
    }

    // construct a multicast address structure
    memset(&mc_addr, 0, sizeof(mc_addr));
    mc_addr.sin_family      = AF_INET;
    mc_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    mc_addr.sin_port        = htons(mc_port);

    // bind multicast address to socket
    if ((bind(sock, (struct sockaddr *) &mc_addr, sizeof(mc_addr))) < 0) {
        perror("bind() failed");
        exit(1);
    }

    // construct an IGMP join request structure
    mc_req.imr_multiaddr.s_addr = inet_addr(mc_addr_str);
    mc_req.imr_interface.s_addr = htonl(INADDR_ANY);

    // send an ADD MEMBERSHIP message via setsockopt
    if ((setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void*) &mc_req, sizeof(mc_req))) < 0) {
        perror("setsockopt() failed");
        exit(1);
    }

    for (;;) {          // loop forever

        // clear the receive buffers & structs
        memset(recv_str, 0, sizeof(recv_str));
        from_len = sizeof(from_addr);
        memset(&from_addr, 0, from_len);

        // block waiting to receive a packet
        if ((recv_len = recvfrom(sock, recv_str, MAX_LEN, 0, (struct sockaddr*)&from_addr, &from_len)) < 0) {
            perror("recvfrom() failed");
            exit(1);
        }

        // output received string
        printf("Received %d bytes from %s: ", recv_len, inet_ntoa(from_addr.sin_addr));
        printf("%s", recv_str);
    }

    // send a DROP MEMBERSHIP message via setsockopt
    if ((setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, (void*) &mc_req, sizeof(mc_req))) < 0) {
        perror("setsockopt() failed");
        exit(1);
    }

    close(sock);  
}
 
Now, via help from another SO member, I have a method that will return the IPaddress to me as a NSString (I'm using it elsewhere in my program also, so I need to keep it returning NSString).
 
-(NSString *)getIPAddress {
    NSString *address = @"error";
    struct ifaddrs *interfaces; // = NULL;
    struct ifaddrs *temp_addr; // = NULL;
    int success = 0;

    // retrieve the current interfaces - returns 0 on success
    success = getifaddrs(&interfaces);
    if (success == 0)  
    {
        // Loop through linked list of interfaces
        temp_addr = interfaces;
        while(temp_addr != NULL)  
        {
            if(temp_addr->ifa_addr->sa_family == AF_INET)
            {
                // Check if interface is en0 which is the wifi connection on the iPhone  
                if([[NSString stringWithUTF8String:temp_addr->ifa_name] isEqualToString:@"en0"])  
                {
                    // Get NSString from C String
                    address = [NSString stringWithUTF8String:inet_ntoa(((struct sockaddr_in *)temp_addr->ifa_addr)->sin_addr)];
                }
            }
            temp_addr = temp_addr->ifa_next;
        }
    }

    // Free memory
    freeifaddrs(interfaces); 
    return address; 
}
 
I thought I could just do a simple little conversion
 
mc_addr_str = (someConversion)getIPAddress;
 
What I have so far is:
 
NSString *ipAddress = [[NSString alloc] initWithString:self.getIPAddress];

mc_addr_str = [ipAddress cStringUsingEncoding:[NSString defaultCStringEncoding]];
 
When I do this, the program makes it to the setsockopt call and then fails with an error code of -1 (I assume that's a general error code that lets the program know something bad happened and needs to abort). Also, when I am assigning mc_addr_str in the previous statement, I get 
 
warning: assignment discards qualifiers from pointer target type
 
I'm not sure where my problem is arising from now. Do I have a casting error during the assignment to mc_addr_str or did I use the wrong encoding? Any input is appreciated!

原文：https://stackoverflow.com/questions/1169043