fread 从 c 中的二进制文件读取数据库分段错误

Question

I am writing a function, that helps me read a database of books from a binary file. However when using fread i am having a segmentation fault.

This is the function I am talking about.

Array *readdb (FILE *db, Array *ind) {
    Array *database = NULL;
    Book *book = NULL;
    indexbook *indentry;
    char *buffer, bytesdepillar = 0, titprintby[MAX_ENTRADA];
    int reader = 0;

    if (db == NULL || ind == NULL) 
        return NULL;

    database = malloc(sizeof(Array));

    initArray(database, INIT_ARR_SIZE);

    fseek(db, 0L, SEEK_END);
    bytesdepillar = ftell(db)/sizeof(char);
    fseek(db, 0, SEEK_SET);
    buffer = malloc(bytesdepillar * sizeof(char));
    fread(buffer, sizeof(char), bytesdepillar, db);
    
    for (size_t i = 0; i < ind->used; i++) {
        book = malloc(sizeof(Book));
        indentry = (indexbook*) ind->array[i];

        reader = indentry->offset;

        book->bookID = indentry->key;
        reader+=sizeof(int);
        memcpy(book->ISBN, &buffer[reader], ISBN_LEN);
        reader+=ISBN_LEN;
        memcpy(titprintby, &buffer[reader], indentry->offset + indentry->size - reader);
        strcpy(book->title, strtok(titprintby, "|"));
        strcpy(book->printedBy, strtok(NULL, "|"));
    
        insertArray(database, book);
    }

    return database;
}

ind is the database index with the following structure:

typedef struct {
  int key;
  long int offset;
  size_t size; 
} indexbook;

The file is being opened with

    if ((dbf = fopen(db_name, "rb")) == NULL) {
        fclose(indf);
        printf("%s does not exist", db_name);
        return 1;
    }

Answer 1

As you have stated in the comments section, when the line

fread(buffer, sizeof(char), bytesdepillar, db);

is executed, the variable bytesdepillar has a negative value.

Since you declared the variable to be of type char , depending on your platform, it is probably only able to represent values between -128 and 127 . If the call to ftell returns a value that is higher than 127 , it will probably (it is implementation-defined what exactly happens) be converted to a value between -128 and 127 . That is most likely the reason why it is a negative value.

Therefore, I recommend that you change the declared type of bytesdepillar from char to long , which is the type returned by the function ftell .

If, after changing the declared type to long , the value continues to be negative, then it is probably due to the function ftell returning -1 due to the function failing. In that case, you should verify that db refers to a valid stream (eg verify that the file that has been successfully opened).