struct中的字符串。 损坏

Question

There is struct like this.

struct Address {
    int id;
    int set;
    char name[MAX_DATA];
    char email[MAX_DATA];
};

And function which set's address.

void Database_set(struct Connection *conn, int id, const char *name, const char *email) {
    struct Address *addr = &conn->db->rows[id];
    if(addr->set) die("Address already set");

    addr->set = 1;
    char *res = strncpy(addr->name, name, MAX_DATA);
    if(!res) die("Name copy failed");

    *res = strncpy(addr->email, email, MAX_DATA);
    if(!res) die("Email copy failed");
}

But first character of addr->name gets corrupted after this line.

*res = strncpy(addr->email, email, MAX_DATA);

Any ideas?

Answer 1

strncpy returns its first argument so after

char *res = strncpy(addr->name, name, MAX_DATA);

the variable res holds addr->name (equivalently, &(addr->name[0]) ) so when

*res = strncpy(addr->email, email, MAX_DATA);

is run it is the equivalent of

addr->name[0] = strncpy(addr->email, email, MAX_DATA);

This assignment is what corrupts the first char of addr->name . As Greg Hewgill says, you don't need to check or even save the return value of strncpy .

Answer 2

I think the strncpy() function is not what you want here. Consider the call:

strncpy(addr->name, name, MAX_DATA);

In the event that name has MAX_DATA or more characters, this will copy the bytes from name into addr->name and will not NUL-terminate the destination. You have two general choices:

1. Manually NUL-terminate the result using code such as

    addr->name[MAX_DATA-1] = '\\0'; 

   however, this is error-prone because you have to remember to do it each time.

2. Use a library function such as strlcpy() (usually available on BSD-derived systems, but not standard) that always NUL-terminates the destination, even if the source won't completely fit.