为什么 malloc/calloc 无法为结构数组中的某些成员分配内存？

Question

我想要一些函数 (save()) 来获取指向结构数组 dbEntry 的指针并用数据填充它。这就是内存管理问题的来源。

这是我的代码：

结构本身：

typedef struct dbEntry {
          unsigned int id;
          char* type;
          char* name;
          unsigned int parent_id;
          unsigned char* md5;
} dbEntry;

内存分配和调用save()：

int main(int argc, char const* argv[]){
          if (argc != 5) arg_error();

          if (strcmp(argv[1], "-s") == 0){
              FILE* database = fopen(argv[2], "wb");
              if (database == NULL) file_error();
              dbEntry* entries = malloc(150*sizeof(dbEntry*));
              for (int i=0; i<150; i++){
                  entries[i].id = 0;
                  entries[i].parent_id = 0;
                  entries[i].name = malloc(50*sizeof(char));
                  entries[i].type = malloc(10*sizeof(char));
                  entries[i].md5 = malloc(MD5_DIGEST_LENGTH*sizeof(unsigned char));
              }
              int entry_num = save(argv[4], 0, &entries);

save()

int save(char* dir_path, unsigned int parent, dbEntry** entries) {
          size_t path_len = strlen(dir_path);
          if (dir_path[path_len - 1] != '/') strcat(dir_path, "/");
          char dp_copy[path_len];
          strcpy(dp_copy, dir_path);
          DIR* dir = opendir(dir_path);
          struct dirent* dirent = readdir(dir);
          dirent = readdir(dir);
          dirent = readdir(dir);
          unsigned int id = 0;
          char* fpath;

          if (dirent == NULL) {
                  fprintf(stderr,
                          "Directory %s doesn't exist or can't be "
                          "opened. Check permissions\n",
                          dir_path);
                  exit(-1);
          }

          while (dirent != NULL) {
                  entries[id]->id = id;
                  strcpy(entries[id]->name, dirent->d_name);
                  entries[id]->parent_id = parent;

                  if (dirent->d_type == 8) {
                          strcpy(entries[id]->type, "file");
                          size_t len = strlen(dir_path)+strlen(dirent->d_name);
                          fpath = malloc(len);
                          strcpy(fpath, dir_path);
                          strcat(fpath, dirent->d_name);
                          FILE* file = fopen(fpath, "rb");
                          if (file == NULL){
                                  printf("Could not open file %s for reading\n", fpath);
                                  exit(-1);
                          }
                          md5digest(file, entries[id]->md5);
                          fclose(file);
                          printf("Saved %s\n", strcat(dir_path, dirent->d_name));

GDB 输出：

57              while (dirent != NULL) {
(gdb) n
58                      entries[id]->id = id;
(gdb) n
59                      strcpy(entries[id]->name, dirent->d_name);
(gdb) n
60                      entries[id]->parent_id = parent;
(gdb) n
62                      if (dirent->d_type == 8) {
(gdb) n
63                              strcpy(entries[id]->type, "file");
(gdb) n
64                              size_t len = strlen(dir_path)+strlen(dirent->d_name);
(gdb) n
65                              fpath = malloc(len);
(gdb) n
66                              strcpy(fpath, dir_path);
(gdb) n
67                              strcat(fpath, dirent->d_name);
(gdb) p entries
$1 = (dbEntry **) 0x7ffffffee110
(gdb) p entries[0]
$2 = (dbEntry *) 0x8403490
(gdb) p entries[1]
$3 = (dbEntry *) 0x0
(gdb) p entries[2]
$4 = (dbEntry *) 0x8403260
(gdb) p entries[3]
$5 = (dbEntry *) 0xf625f4cc70b40b00
(gdb) p entries[4]
$6 = (dbEntry *) 0x7ffffffee220
(gdb) p entries[5]
$7 = (dbEntry *) 0x0
(gdb) p entries[6]
$8 = (dbEntry *) 0x8001890 <__libc_csu_init>
(gdb) p entries[7]
$9 = (dbEntry *) 0x7ffffeb51b97 <__libc_start_main+231>
(gdb) p entries[8]
$10 = (dbEntry *) 0x2000000000
(gdb) p entries[0]->name
$11 = 0x8403950 ".bash_history"
(gdb)

由于某种原因，它未能为entries[1]、entries[5] 等分配内存，因此程序在尝试引用entries[1] 时返回Address boundary error。

几乎尝试了所有方法，但问题仍然存在。寻求您的帮助。

Answer 1

您有两个问题：

1. 有

   malloc(150*sizeof(dbEntry*))
   

   您为 dbEntry 结构的 150 个指针分配内存，而不是 dbEntry 结构本身。

   我建议您从 变量本身，如：

   dbEntry *entries = malloc(150 * sizeof *entries);
   

2. 在save 函数中，变量entries 本质上是一个指向dbEntry 数组的指针结构。您将其视为指向结构的指针数组（指向数组的指针与指针数组不同）。您需要取消引用指针以获取原始指针，然后可以像结构数组一样取消引用：

   (*entries)[id].id = id;
   

   或者干脆不把它作为指向指针的指针传递：

   //                                       Note only one asterisk
   //                                                   v
   int save(char* dir_path, unsigned int parent, dbEntry* entries) {
       ...
       entries[id].id = id;  // Entries can be treated like a normal array of structures
       ...
   }