Traverse FILE line by line using fscanf

Question

Ok so i have a text file database.txt.

Each line is a user with the format below

"John Smith"| 4| 80.00| "123 Lollipop Lane"| "New Jersey"| "08080"  

When I try do the following:

database = fopen(fileName,"r"); 
    while(fscanf(database,"%[^\n]", buffer) != EOF){ 
        printf("ATTEMPT TO: Insert user %s\n\n", buffer); 
        if (insert_Customer(clients, create_Customer(buffer)) < 0){ 
            printf("\nERROR: Failed to insert and create customer %s\n", buffer); 
        } 
        else printf("\nSUCCESS: Inserted Customer %s\n\n", buffer); 
    } 

It runs fine for the first line, sending in the ENTIRE line to the create_Customer(char * buffer) function. For some reason I do not understand, after this first line it continually tries to send "John Smith" instead of moving on to the next line. I believe my format is incorrect but I can't figure out how to accomplish what I am trying to accomplish which is read this file line by line and pass each line into the create_Customer function.

Answer 1

I think you need a closing \n in your pattern. I saw the a bunch of scrolling text with the same first line with %[^\n], but with %[^\n]\n I saw the expected output.

However, if the line in the file exceeds that of the buffer, you may run into a problem where your buffer doesn't have enough space. The results

Test v1:

Code

#include <stdio.h>


int main(int argc, char * argv[])
{
    FILE * database;
    char buffer[30];

    database = fopen("test.txt", "r");

    if (NULL == database)
    {
         perror("opening database");
         return (-1);
    }

    while (EOF != fscanf(database, "%[^\n]\n", buffer))
    {
         printf("> %s\n", buffer);
    }

    fclose(database);
    return (0);
}

Results

> 12343456567856789
> zbasdflkjasdfkjklsdafjklas
> zxcvjkleryjkldhfg
> 1234567890123456789012341234
> 12345678901234567890123123asdfjklzxcv;jkl;eqwrtjklkzlcxvjkleqrt41234
*** stack smashing detected ***: ./fscanf terminated                                                                                                                   
Segmentation fault (core dumped)

Test v2:

Let's fix the code so that we don't overrun our buffer. We do this by changing %[^\n]\n to %30[^\n]\n. This tells fscanf that it can only use upto the size of the buffer -- 30.

Code

#include <stdio.h>


int main(int argc, char * argv[])
{
    FILE * database;
    char buffer[30];

    database = fopen("test.txt", "r");

    if (NULL == database)
    {
         perror("opening database");
         return (-1);
    }

    while (EOF != fscanf(database, "%30[^\n]\n", buffer))
    {
         printf("> %s\n", buffer);
    }

    fclose(database);
    return (0);
}

Results

It looks like it worked!

> 12343456567856789
> zbasdflkjasdfkjklsdafjklas
> zxcvjkleryjkldhfg
> 1234567890123456789012341234
> 12345678901234567890123123asdf
> jklzxcv;jkl;eqwrtjklkzlcxvjkle
> qrt41234

test.txt

This is what the test file looks like that I was using.

12343456567856789
zbasdflkjasdfkjklsdafjklas
zxcvjkleryjkldhfg
1234567890123456789012341234
12345678901234567890123123asdfjklzxcv;jkl;eqwrtjklkzlcxvjkleqrt41234

---

Any reason you are using fscanf and not fgets?

One thing to note if you use fgets, you may want to strip the \n character because it will be included as part of the string.

From cplusplus.com fgets:

Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or the end-of-file is reached, whichever happens first.

Example:

/* fgets example */
#include <stdio.h>

int main()
{
    FILE * database;
    int res;
    char buffer [100];

    database = fopen(fileName,"r");

    if (NULL == database) {
        perror("opening database file");
        return (-1);
    }

    /* while not end-of-file */
    while (!feof(database)) { 

        /* we expect buffer pointer back if all is well, get out if we don't get it */
        if (buffer != fgets(buffer, 100, database))
            break;

        /* strip /n */
        int len = strlen(buffer);
        if (buffer[len - 1] == '\n')
            buffer[len - 1] = 0;

        printf("ATTEMPT TO: Insert user %s\n\n", buffer); 

        if (insert_Customer(clients, create_Customer(buffer)) < 0)
            printf("\nERROR: Failed to insert and create customer %s\n", buffer); 
        else
            printf("\nSUCCESS: Inserted Customer %s\n\n", buffer);
    }

    /* a little clean-up */
    fclose(database);

    return (0);
}

Answer 2

The problem is that your fscanf will never read the newline at the end of the first line. So when it is called the second time, it will fail (returning 0, not EOF) and read nothing, leaving buffer unchanged.

You could add a call to fscanf("%*[\n]"); at the end of your while loop to skip the newline (and any blank lines that might occur). Or you could just use fgets instead, which also makes it easier to avoid the potential buffer overflow problem.

Answer 3

You have to use fgets, instead of fscanf!The fscanf reads until it meats \n (new line) character ,whereas you can use fgets to read all lines with the exact same code you used! Hope that helped :)

Answer 4

You should put a blank after specifier "%[^\n] " or use "%[^\n]\n". Both will allow you to skip the newline character.

fscanf(database, "%[\n] ", buffer);

or

fscanf(database, "%[\n]\n", buffer);