I Can't figure out how to change a String and add more characters to the end

Question

I'm trying to make a file pointer and I want the user to enter in the title and then have the program add .txt after the the title and here's one of the ways I tried to do it

int main()
{
    File * fpointer;
    char name;

    printf("Type name of program");
    scanf("%s.txt", &name);
    puts(&name);
}

you are scanning more than 1 charcter, but char only holds one. Look at an array of char https://stackoverflow.com/questions/5406935/reading-a-string-with-scanf — kenny, Oct 24 '17 at 18:04
Note that `scanf("%s.txt", &name);` does not add `".txt"` to the input: it demands a match. — Weather Vane, Oct 24 '17 at 18:05

score 3 · Accepted Answer · answered Oct 24 '17 at 18:06

scanf's approach will validate an input sequence to have .txt suffix. In addition, you do not have enough space for a string, because name can hold a single character.

If you want to force a suffix on an existing input, use strcat function:

#include <stdio.h>
#include <string.h>

#define MAX_NAME 40
int main() {
    File * fpointer;
    char name[MAX_NAME+5];
    printf("Type name of program: ");
    scanf("%40s", name);
    strcat(name, ".txt");
    puts(name);
    return 0;
}

Demo.

score 1 · Answer 2 · answered Oct 24 '17 at 18:45

A char object can only hold a single character value. To store a string, you need to use an array of char¹:

#define MAX_NAME_LENGTH 20 // or whatever
...
char name[MAX_NAME_LENGTH+1]; // +1 for string terminator

Remember that in C, a string is a sequence of character values including a 0-valued terminator - for example, the string "hello" is represented as the character sequence {'h', 'e', 'l', 'l', 'o', 0 }. So your array needs to be large enough to handle the longest string you expect plus an extra element for the 0 terminator. If you expect your largest string to be 20 characters long, then the array needs to be able to hold at least 21 characters.

For various historical reasons not worth going into right now, C I/O routines won't protect you if you try to write more characters to the array than it's sized to hold. If name is sized to hold 20 characters plus the 0 terminator, and you read it as

scanf( "%s", name );

if someone enters 200 characters, then 180 characters will be written to the memory immediately following the name array, potentially leading to all kinds of mayhem. Buffer overflows are a popular malware exploit, so it's a security issue as well.

To protect against that, you need to specify the max number of characters to read. Unfortunately, scanf requires you to hardcode it as part of the conversion specifier, rather than providing it as a separate argument like printf. You can't just use the macro like

scanf( "%MAX_NAME_LENs", name );

you either have to specify it as a numeric literal:

scanf( "%20s", name );

or use some preprocessor trickery to expand the macro and concatenate it with the format string:

#define EXPAND(x) #x
#define STR(x) EXPAND(x)
...
scanf( "%" STR(MAX_NAME_LEN) "s", name );

Ater preprocessing, that expands to

scanf( "%" "20" "s", name );

which is evaluated as

scanf( "%20s", name );

Or, you could go the easy route and use fgets:

fgets( name, sizeof name, stdin );

which is what I recommend.

^{Or wchar_t for "wide" strings, which you don't need to worry about right now.}

I Can't figure out how to change a String and add more characters to the end

2 Answers2