C语言 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的指针dbEntry并用数据填充它。这就是内存管理问题的用武之地。

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 permissionsn",
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 readingn", fpath);
exit(-1);
}
md5digest(file, entries[id]->md5);
fclose(file);
printf("Saved %sn", 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. 跟

   malloc(150*sizeof(dbEntry*))
   

   您为 150 个指向dbEntry结构的指针分配内存，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本质上是指向dbEntry结构数组的指针。您可以将其视为指向结构的指针数组(指向数组的指针与指向指针数组不同(。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该指针：

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

   或者干脆不将其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
   ...
   }