Bad use of strncpy()

bad_strncpy.c

#include <string.h>
#include <stdio.h>
#include <assert.h>

// Find this with:
// git grep 'strncpy(\([^,]\+\), [^,]\+, sizeof(\1));'

int main(int argc, char *argv[])
{
        // Take a single string argument
        if (argc < 2)
                return 1;

        struct {
                char a[20];
                char b[20];
        } data;

        /**
         * Check that there isn't any padding between the end of 'a' and the
         * beginning of 'b'.
         *
         * Add 'struct __attribute__((packed)) { ...' if this assert fails.
         **/
        assert(sizeof(data) == sizeof(data.a) + sizeof(data.b));

        // Zero the structure, which also will make sure all the strings are
        // full of NUL bytes.
        bzero(&data, sizeof(data));

        // Initialize the second member
        strcpy(data.b, "yellow submarine");

        // *Incorrectly* use strncpy
        strncpy(data.a, argv[1], sizeof(data.a));

        // May echo the first argument, or also "yellow submarine"!
        puts(data.a);
}