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It's certainly not a bad attempt!

void datentry (char *, int);
void datentry(passager *p, int n)

Your function definition and declaration are different, they shouldn't be.
...

Hmm I'm sorry I didn't actually see that you'd put the error messages in as comments :P

We aren't going to spend longer on this than you are.
Tell us the purpose of the program.
Tell us what is going wrong.
Post error messages.