Address booking and writing to file - c

Question

#include <stdio.h>
#define TRUE 1
#define FALSE 0
typedef struct contact {
char firstname [40];
char lastname [40];
char address [100];
char phone[10];
}contact;
int main ()
{ FILE *pFile;
contact entry = {""};
int choice, firstname_flag = TRUE, lastname_flag = TRUE, address_flag = TRUE, phone_flag = TRUE;
pFile = fopen("C:\\contacts.txt", "a+");
if(!pFile){
printf("File could not be open.");
return 1;
}
do{
printf("Choose a selection:\n\n");
printf("1. First name:\n");
printf("2. Last name:\n");
printf("3. Address:\n");
printf("4. Phone number:\n\n");
scanf( "%d", &choice);
}while((choice < 1 || choice > 4));
switch (choice){
case 1:
firstname_flag = FALSE;
printf("Please enter first name: \n");
scanf("%s", &entry.firstname);
break;
case 2:
lastname_flag = FALSE;
printf("Please enter last name: \n");
scanf("%s", &entry.lastname);
break;
case 3:
address_flag = FALSE;
printf("Please enter address: \n");
scanf("%s", &entry.address);
break;
case 4:
phone_flag = FALSE;
printf("Please enter phone number: \n");
scanf("%s", &entry.phone);
break;
default:
break;
}
printf("\nYou will now be asked to enter the other items. \n\n");
if(firstname_flag){
printf("Please enter first name: \n");
scanf("%s", &entry.firstname);
}
if(lastname_flag){
printf("Please enter last name: \n");
scanf("%s", &entry.lastname);
}
if(address_flag){
printf("Please enter address: \n");
scanf("%s", &entry.address);
}
if(phone_flag){
printf("Please enter phone number: \n");
scanf("%s", &entry.phone);
}
fwrite (here)
fclose(pFile);
getchar();
return 0;
}
Here is the code I have so far. First of all is there anything blatant that pokes out as being invalid or wrong practice etc? 2nd I want to write first name,last name,address, and Phone # to a file. I'm unsure if I need to fwrite "entry" or not. Also, I have noticed when selecting Address first that It's almost like the buffer isn't empty and what I put after the space i.e. 123 Park , Park would be used as the first name wrongly and the next entry I would put in would be Last name. Any suggestions on the code usage and anything at all would be greatly appreciated. Thanks again

Answer 1

1
N° 1. In response to: “invalid or wrong practice.” Some of these are more matters of taste…
If you ‘must’ use scanf, don't point it at a string buffer with %s: specify the length of the buffer with something like %32s to reduce (but perhaps not eliminate) someone from just typing too much and crashing your program (or worse…) — But, Read N°3 below for more on this question…
If you're developing something open-source, or for personal use, GNU readline is a very nice option (but it is GPL not LGPL, so it can't be used in "non-libre" works…)
Your structure initializer doesn't cover all the elements. You could use {"", "", "", ""}
Did you mean fwrite(entry)?
If you're checking for missing fields before writing the file, you might want to leave aside the bank of flags, and instead loop on the "invalid condition."
while ('\0' == entry.firstname[0]) {
printf("Please enter first name: \n");
scanf("%40[^\r\n\0\04]", &entry.firstname[0]);
}
No need to open the file so early, and leave it open while waiting for user interaction. Opening the file in a+ mode is reasonably "dangerous" (see man lockf for the skinny), because someone else might try to write to the file while you are; leaving it open in this mode for a long time increases the risk.
You should probably use strerror(errno) to provide the user the details of a failing system call, such as when checking the return codes of fopen, fwrite, and fclose. Due to the way buffered I/O works, fclose could even be reporting a problem that occurred with fwrite under some circumstances.
Print your error messages to stderr using fprintf (stderr,…); instead of the output stream …
If you are going to use flags to indicate that the data is "valid" (or, at least, that it may be), you should probably do so after the user tries to enter it, instead of before.
Functions are your friends … I would probably break up something like this by using a function to collect the user's input (something like prompt_for_field ("first name", &entry.firstname);), a function to check for missing records and prompt for them, and a function to write the record to the file, at least …
It's usually considered good form to exit from main, rather than return, for example by using exit(EXIT_SUCCESS)/exit(EXIT_FAILURE). I believe the idea is to support esoteric and possible extinct systems who might treat some value(s) other than 0 as a successful status code (VMS, perhaps?), but regardless, it's easier to read :-)
2
And N°2, yes, you can fwrite(entry), and as long as you never change the definition of struct contact, you should be able to read it back in all right. Over time, you'll probably want to switch to a more "plain text" type format (#include <json-xml-init-religious-war>), but in this small example, there isn't a pressing need to introduce such complexity.
3
Finally, N°3, you should probably use &entry.address[0] to get the address of the start of a char[] but most significantly: scanf %s does not read a string. It looks like printf %s, but it's not…
s Matches a sequence of non-white-space characters; the next
pointer must be a pointer to character
array that is long enough to hold the input sequence and the terminating null character ('\0'),
which is added automatically. The input string stops at white space or at the maximum field width,
whichever occurs first.
See that "non-white-space?" That's what's got you. The rest of your input (after the first whitespace) is left in the keyboard buffer, waiting for another scanf to read it. A good('ish) way to read from the terminal and accept whitespace is %40[^\r\n\0\04], replacing 40 with the size of your string buffer (char[]). That means, accept (up to) 40 characters, as long as they are none of: carriage return, new line, the null byte, or end-of-file code (^D).
All-in-all, you look to be on the right track, though. Good luck :-)