Why doesn't my C program wait to scan input? - c

Question

I am new to C. I allocated memory with this statement:
patientptr = (char*) calloc (118, sizeof (char));
then I assign data using this (this is a part of the function):
char name[51];
int age;
char agestr[3];
char infectiondate [11];
char address[51];
char *patientptr;
printf("\nEnter the patient name (50 characters at maximum): ");
scanf ("%50s", name);
*patientptr = name;
printf("Enter the patient age: ");
scanf ("%d", &age);
sprintf (agestr, "%2d", age);
*(patientptr + 51) = agestr;
printf("Enter the patient date of infection (in form of dd/mm/year): ");
*(patientptr + 54) = scanf ("%10d", infectiondate);
printf("Enter the patient address (50 characters at maximum): ");
*(patientptr + 65) = scanf ("%50s", address);
*(ptrsptr+patientsnum-1) = patientptr;
printf ("\nPatient added.\n");
Everything goes fine except that after the "enter the patient address: " line, it prints the "patient added" line directly without waiting to scan the address. the output is like this:
Enter the patient name (50 characters at maximum): ahmed
Enter the patient age: 20
Enter the patient date of infection (in form of dd/mm/year): 10/10/2020
Enter the patient address (50 characters at maximum):
Patient added.
is the wrong with my allocated memory?

Answer 1

You may well have used calloc to allocate some memory but examine this snippet:
char *patientptr;
printf("\nEnter the patient name (50 characters at maximum): ");
scanf ("%50s", name);
*patientptr = name;
That first line shadows whatever patientptr was with an uninitialised pointer, hence the final line is undefined behaviour (patientptr now points to some arbitrary address). All bets are off at this point, anything is possible.
Fix that and try again.
In addition, it looks like you believe that:
*(patientptr + 51) = agestr;
is a way to copy a C string from one place to another. In actual fact, this will attempt to place the agestr pointer value into the single character at the memory location &(patientptr[51]), and possibly should have warned you about this.
You need to look into strcpy for this, something along the lines of:
strcpy(patientptr + 51, agestr);
But, if you're looking to do user input, it's often a good idea to work around the limits of scanf. It does, after all, stand for "scan formatted" and there's very little that's less formatted than user input.
I have a favourite function I use for this, which is shown below, along with the modifications to your own code to use it (using both that function and with quite a bit of other validation specific to your case):
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
// Bullet-proof line input function.
#define OK 0
#define NO_INPUT 1
#define TOO_LONG 2
static int getLine (char *prmpt, char *buff, size_t sz) {
int ch, extra;
// Get line with buffer overrun protection.
if (prmpt != NULL) {
printf ("%s", prmpt);
fflush (stdout);
}
if (fgets (buff, sz, stdin) == NULL)
return NO_INPUT;
// If it was too long, there'll be no newline. In that case, we flush
// to end of line so that excess doesn't affect the next call.
if (buff[strlen(buff)-1] != '\n') {
extra = 0;
while (((ch = getchar()) != '\n') && (ch != EOF))
extra = 1;
return (extra == 1) ? TOO_LONG : OK;
}
// Otherwise remove newline and give string back to caller.
buff[strlen(buff)-1] = '\0';
return OK;
}
// Ensure a character array is non-empty and all digits.
int checkDigits(char *arr, size_t sz) {
if (sz == 0) {
return 0;
}
while (sz-- > 0) {
if (! isdigit(*arr++)) {
return 0;
}
}
return 1;
}
// Get customer data line, return NULL if okay, error if not.
// Output record must be long enough for format string below
// and a one-character end-string marker.
static char *getPatientData(char *patientData) {
// Keep this format string in sync with field sizes below.
static char *fmtString = "%-50.50s" "%3d" "%-10.10s" "%-50.50s";
char name[51];
char ageStr[4];
char infectionDate[11];
char address[51];
if (getLine("Patient name: ", name, sizeof(name)) != OK) {
return "Error getting name.";
}
if (getLine("Patient age: ", ageStr, sizeof(ageStr)) != OK) {
return "Error getting age.";
}
if (! checkDigits(ageStr, strlen(ageStr))) {
return "Error, age contains non-digit data.";
}
int age = atoi(ageStr);
// Further age sanity checking, if desired. Example: ensure <= 150.
if (getLine("Infection date (dd/mm/yyyy): ", infectionDate, sizeof(infectionDate)) != OK) {
return "Error getting infection date.";
}
if (
strlen(infectionDate) != 10
|| infectionDate[2] != '/'
|| infectionDate[5] != '/'
|| ! checkDigits(infectionDate, 2)
|| ! checkDigits(infectionDate + 3, 2)
|| ! checkDigits(infectionDate + 6, 4)
) {
return "Error, incorrect format.";
}
// Further checking if desired. Example: valid year/month/day combo.
if (getLine("Patient address: ", address, sizeof(address)) != OK) {
return "Error getting address.";
}
sprintf(patientData, fmtString, name, age, infectionDate, address);
return NULL;
}
int main(void) {
char *patientPtr = malloc (50 + 3 + 10 + 50 + 1);
char *result = getPatientData(patientPtr);
if (result != NULL) {
printf("*** %s\n", result);
return 1;
}
printf("Got '%s'\n", patientPtr);
return 0;
}
A sample run follows:
Patient name: Pax Diablo
Patient age: 55
Infection date (dd/mm/yyyy): 25/05/2020
Patient address: No fixed abode
Got 'Pax Diablo 5525/05/2020No fixed abode '