File processing in C; won't take input

Question

I'm starting to learn file processing in C. The point of this specific program is to make a file called "clients.dat" where I store the account number, name, and balance of clients at a bank, lets say. I've worked and refined the code so that its a perfect replica of what the textbook provides as an example, yet for some reason mine loops endlessly after the first "scanf" and reprints question marks unto oblivion, without ever making it to the scanf statement inside the while loop. Would anyone have any suggestions as to why this is happening? My compiler is Netbeans and I'm running it on Linux-Ubuntu.

#include <stdio.h>
#include <stdlib.h>

/*
 * 
 */
int main() {

    unsigned int actNumber;
    char actName[30];
    long double actBalance;

    FILE *fPtr;

    if((fPtr = fopen("clients.dat", "w")) == NULL) {
        printf("File could not be found.\n");
    }

    else {
        printf("Enter the Account Number, Name, and Balance.\n Hit the EoF to exit.\n");
        printf("%s","?");
        scanf("%d%29s%lf", &actNumber, actName, &actBalance);

        while (!feof(stdin)) {            
            fprintf(fPtr, "%d, %29s, %.2lf\n", actNumber, actName, actBalance);
            printf("%s", "?");
            scanf("%d%29s%lf", &actNumber, actName, &actBalance);                        
        }

        fclose(fPtr);
    }
    return;
}

[`while(!feof(file))` is always wrong](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — The Paramagnetic Croissant, Jun 29 '15 at 21:52
`while (!feof(stdin)) {` means you need to hit Ctrl-D on the keyboard to terminate stdin stream — fukanchik, Jun 29 '15 at 21:53
For the sake of clarification Netbeans is not your compiler but your IDE. There is some underlying C compiler used by Netbeans to build your code. — SeriousBusiness, Jun 29 '15 at 21:58
@fukanchik euh, I know? It's still wrong. Read the linked answer. — The Paramagnetic Croissant, Jun 30 '15 at 06:24

Iharob Al Asimi · Accepted Answer · 2015-06-29T21:58:19.397

The end of file marker is set on stdin only when you press a special key combination on the console.

You can make your loop work correctly by using the return value of scanf(), like this

while (scanf("%d%29s%lf", &actNumber, actName, &actBalance) == 3) 
{            
    fprintf(fPtr, "%d, %29s, %.2lf\n", actNumber, actName, actBalance);
    printf("%s", "?");
}

After the first scanf() a '\n' character is left in the input stream, when you call scanf() again inside the loop, the character is consumed then ignored, and scanf() fails returning a value that is less than 3, the process is repeated over and over causing the infinite loop.

The following solution, however, is better. Using fgets() allows a better handiling of the '\n' character left unread in the stdin,

char line[100];
while (fgets(line, sizeof(line), stdin) != NULL)
 {
    if (sscanf(line, "%d%29s%lf", &actNumber, actName, &actBalance) != 3)
        continue;
    fprintf(fPtr, "%d;%29s;%.2lf\n", actNumber, actName, actBalance);
    fprintf(stdout, "?");
 }

note that I remove the white spaces in the printf() format, and replaced the , with ; because in some locales the , is the decimal separator, it's just instintictive not to use it, you can use it if you ensure that . is the decimal separator.

all right yeah touche, working this through standard loop conditions feels much more intuitive. Thanks @iharob — David Tamrazov, Jun 29 '15 at 22:03

File processing in C; won't take input

1 Answers1