c - 为什么字符串连接可能会在每个连接前加上垃圾前缀？

Question

我对 C 还很陌生，所以如果这是相当标准的知识，我深表歉意。

我有一个像这样的函数，我将一堆 C 风格字符串附加在一起并输出:

char *example(int n, int days, int years){
    char *ret;
    if (n < 5) {
        ret = (char*)malloc(sizeof(char)*256);
        sprintf(ret, "There are %d days in %d years", days, years);
        ret = (char*)malloc(strlen(ret));
        return ret;
    }
    else {
        char *s1;
        char *s2;
        char *s3;
        s1 = example(n/2, days, years);
        s2 = example(n + 5, days, years);
        s3 = example(n--, days, years);

        int length = strlen(s1) + strlen(s2) + strlen(s3);
        ret = (char*)malloc(length);
        strcat(ret, s1);
        strcat(ret, s2);
        strcat(ret, s3);

        return ret;
       }
}

这会在每个新连接前添加一些垃圾字符。我假设我的问题出在内存管理中，但我不确定..这简单吗？我做错了什么？另外，如何才能更干净地完成此操作？

Answer 1

char *example(int n, int days, int years){
   char *ret;
   if (n < 5) {
       ret = (char*)malloc(sizeof(char)*256);
       sprintf(ret, "There are %d days in %d years", days, years);
//     ret = (char*)malloc(strlen(ret));  (DELETE THIS LINE)
       return ret;
   }
   else {
       char *s1;
       char *s2;
       char *s3;
       s1 = example(n/2, days, years);
       s2 = example(n + 5, days, years);  // WILL THIS CAUSE INFINITE RECURSION?
       s3 = example(n--, days, years);

       int length = strlen(s1) + strlen(s2) + strlen(s3) + 1; // ALLOW ROOM FOR TERMINATING '\0'
       ret = (char*)malloc(length);
       strcpy(ret, s1);  // CHANGE TO strcpy()
       strcat(ret, s2);
       strcat(ret, s3);

       return ret;
   }
}