How to scan a formatted string using fscanf()?

Question

I have this file with multiple lines. Each line follows a format of

`userID:$An Integer$$String:`

For example:

pyc1:$1$$Tnq7a6/C1wwyKyt0V/.BP/:
pyc2:$6$$Tnq7a6/C1wwyKyt0V/.BP/:

How should I use fscanf() to retrieve the userID, the integer, and the string between $$ and :`?

This is what I have now, but it is not working.

int main(){
    FILE * file = fopen("shadow.txt", "r");
    char line[256] = {0};
    char userID[30] = {0};
    int hashType;
    char hash[256] = {0};
    int result = fscanf(file, "%s :$ %d $$ %s :", userID, &hashType, hash);
    printf("%s\n", userID );
    printf("%d\n", hashType );
    printf("%s\n", hash );

}

I don't know much about the format I should specify for fscanf() to achieve this. Please help.

@MadPhysicist The userID, Integer and hash is not scanning into the variables. It appears as the whole line from the file is scanned into userID — Milk_QD
– Milk_QD, Commented Dec 7, 2017 at 13:44

gsamaras · Accepted Answer · 2017-12-08 08:43:49Z

3

Try a regex, like this for example:

fscanf (file, "%[^:]:$%d$$%[^:]:", userID, &hashType, hash);

I created a regex101 link that explains the regex expression used above.

PS: Don't forget to check for the return value of fscanf(), in order to ensure that everything was read parsed correctly. You want to parse three elements, thus the return value should be 3.

edited Dec 8, 2017 at 8:43

answered Dec 7, 2017 at 13:48

gsamaras

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5 Comments

Milk_QD Over a year ago

Thank you so much. It works. So my understand for %[^:] is it will scan in any character except for : Is that correct?

Milk_QD Over a year ago

Does all the regex rules can be applied inside %[]? Such as * for any number of characters

gsamaras Over a year ago

There are many things you can do with a regex expression @Anlinyang. Play around with it in the link I posted, if you like!

user3629249 Over a year ago

the '%[^:]' will input any characters, but will stop (and not input) the ':' when a ':' is encountered

user3629249 Over a year ago

this fails to check the returned value from fscanf(), so errors can occur and the code would not catch those errors. Strongly suggest checking the returned value against 3. Any other returned value means an error occurred

Jay Joshi · Accepted Answer · 2017-12-07 13:46:20Z

0

Try this:

while(ret = fscanf(file," %[^:] : $ %d $$ %[^:] :",userID,&hashtype,hash))
{

    if(ret == EOF)
    {
        break;
    }

    printf("value of ret: %d\n",ret);
    printf("%s , %d , %s",userid,id,str);

    // . . .

}

here, int ret is:

EOF, if the pointer is reached end of file.
0, if no input matched with the variable
>0, number of matched variables with the file input

edited Dec 7, 2017 at 13:46

answered Dec 7, 2017 at 13:30

Jay Joshi

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1 Comment

user3629249 Over a year ago

the correct value to be checking as returned from fscanf() is (in the current scenario) 3. There are other error events possible besides EOF

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