多个 valgrind 错误：条件跳转或移动取决于未初始化的值

Question

我正在运行 Valgrind，并且收到以下错误（这不是唯一的错误）：

==21743== Conditional jump or move depends on uninitialised value(s)
==21743==    at 0x4A06509: index (mc_replace_strmem.c:164)
==21743==    by 0x33B7CBB3CD: gaih_inet (in /lib64/libc-2.5.so)
==21743==    by 0x33B7CBD629: getaddrinfo (in /lib64/libc-2.5.so)
==21743==    by 0x401A5F: tunnelURL (proxy.c:336)
==21743==    by 0x40142A: client_thread (proxy.c:194)
==21743==    by 0x33B8806616: start_thread (in /lib64/libpthread-2.5.so)
==21743==    by 0x33B7CD3C2C: clone (in /lib64/libc-2.5.so)

我的 tunnelURL() 函数如下所示（C 代码）：

char * tunnelURL(char *url) {
 char * a = strstr(url, "//");
 a += 2;
 char * path = strstr(a, "/");

 char host[256];
 strncpy (host, a, strlen(a)-strlen(path));

 /*
  * The following is courtesy of Beej's Guide
  */
 int status;
 int proxySocketFD;
 struct addrinfo hints;
 struct addrinfo *servinfo; // will point to the results

 memset(&hints, 0, sizeof(hints)); // make sure the struct is empty
 hints.ai_family = AF_INET; // don't care IPv4 or IPv6
 hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
 hints.ai_flags = AI_PASSIVE; // fill in my IP for me

 if ((status = getaddrinfo(host, "80", &hints, &servinfo)) != 0) {
  perror("getaddrinfo() fail");
  exit(1);
 }

 // create socket
 if ((proxySocketFD = socket(servinfo->ai_family, servinfo->ai_socktype, servinfo->ai_protocol)) == -1) {
  perror("proxy socket() fail");
  exit(1);
 }

 // connect
 if (connect(proxySocketFD, servinfo->ai_addr, servinfo->ai_addrlen) != 0) {
  printf("connect() fail");
  exit(1);
 }

 // construct request
 char request[strlen(path) + strlen(host) + 26];
 sprintf(request, "GET %s HTTP/1.1\r\nHost: %s\r\n\r\n", path, host);
 printf("%s", request);

 // send request
 send(proxySocketFD, request, strlen(request), 0);

 // receive response
 int i = 0;
 int amntRecvd = 0;
 char *pageContentBuffer = (char*) malloc(4096 * sizeof(char));
 while ((amntRecvd = recv(proxySocketFD, pageContentBuffer + i, 4096, 0)) > 0) {
  i += amntRecvd;
  realloc(pageContentBuffer, i * 4096 * sizeof(char));
 }

 // close proxy socket
 close(proxySocketFD);

 // deallocate memory
 freeaddrinfo(servinfo);

 return pageContentBuffer;

}

第 336 行对应于 if 语句与 getaddrinfo() 函数调用。我不太确定我还没有初始化什么。我传入“应该”的字符串已经设置。我把它打印出来就好了。我还收到与同一行代码相对应的另一个错误：

==21743== Use of uninitialised value of size 8
==21743==    at 0x33B7D05816: __nscd_cache_search (in /lib64/libc-2.5.so)
==21743==    by 0x33B7D0438B: nscd_gethst_r (in /lib64/libc-2.5.so)
==21743==    by 0x33B7D04B26: __nscd_gethostbyname2_r (in /lib64/libc-2.5.so)
==21743==    by 0x33B7CE9F5E: gethostbyname2_r@@GLIBC_2.2.5 (in /lib64/libc-2.5.so)
==21743==    by 0x33B7CBC522: gaih_inet (in /lib64/libc-2.5.so)
==21743==    by 0x33B7CBD629: getaddrinfo (in /lib64/libc-2.5.so)
==21743==    by 0x401A5F: tunnelURL (proxy.c:336)
==21743==    by 0x40142A: client_thread (proxy.c:194)
==21743==    by 0x33B8806616: start_thread (in /lib64/libpthread-2.5.so)
==21743==    by 0x33B7CD3C2C: clone (in /lib64/libc-2.5.so)

关于可能导致此问题的任何想法？

Answer 1

您没有正确使用realloc()。 realloc() 可能会移动分配的块，并返回该块的新地址 - 因此您需要将该返回值分配给 pageContentBuffer（如果它不是） NULL）。

Answer 2

 // receive response
 int i = 0;
 int amntRecvd = 0;
 char *pageContentBuffer = (char*) malloc(4096 * sizeof(char));
 while ((amntRecvd = recv(proxySocketFD, pageContentBuffer + i, 4096, 0)) > 0) {
  i += amntRecvd;
  realloc(pageContentBuffer, i * 4096 * sizeof(char));
 }

realloc(pageContentBuffer, i * 4096 * sizeof(char)) 看起来不正确。如果您首先收到 4096 字节，则接下来将分配 4096*4096 字节，接下来分配 2*4096*4096 字节，依此类推。
也许你的意思是加法？

Answer 3

您的问题中显示的第一个条件跳转错误是由于您错误地使用了 strncpy 造成的。它不会终止复制的字符串，您必须随后手动执行此操作。那么最好使用 memcpy。

char host[256];
int n = strlen(a)-strlen(path);
memcpy (host, a, n);
host[n] = '\0';