读取二进制文件c

Question

So basicaly I have binary file made with such structure

struct data{
char name[30];
char name2[30];
};

I want to read data back into array of structs from file but the problem is I dont know how many records there are in this file. Could somebody explain how could I read whole file not given ammout of records inside?

Answer 1

You can open the file, check it's size:

fseek(fp, 0L, SEEK_END); // Go at the end
sz = ftell(fp);          // Tell me the current position
fseek(fp, 0L, SEEK_SET); // Go back at the beginning

And the number of records inside will be:

N = sz/sizeof(struct data);

Anyway, be careful that if you just write an array of structures to a file, it's possible that it will not be readable others machines, due to different memory alignment. You can use the __attribute__((packed)) option to be sure that the structure will be the same (but it's a GCC specific extension, not part of standard C).

struct __attribute__((packed)) data {
    char name[30];
    char name2[30];
};

Answer 2

Memory mapping your file is your best bet.

int fd = open(filename, O_RDONLY);
struct stat fdstat;
fstat(fd, &fdstat);
struct data * file_contents = mmap(NULL, fdstat.st_size
     , PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);

// assuming the file contains only those structs
size_t num_records = fdstat.st_size / sizeof(*file_contents);

An intelligent OS will then load the data from the file on a first-use basis and will evict pages from memory that have not been accessed recently.