How does scanf() work for strings in C99?

Question

So I'm working on a documentary of learning the C Language and I want to make examples within pointers and input. Basically, I made a Hello, (name). program like this using C99:

#include <stdio.h>

int main() {
    char *name;
    printf("What's your name? ");
    scanf("%s", name);
    printf("Hello, %s.\n", name);
}

When I tried compiling it, the error it gave me was this:

C3.c:5:48: error: variable 'name' is uninitialized when used here [-Werror,-Wuninitialized]
    printf("What is your name? "); scanf("%s", name);
                                               ^~~~
C3.c:4:17: note: initialize the variable 'name' to silence this warning
    char *name;
               ^
                = NULL
1 error generated.
<builtin>: recipe for target 'C3' failed
make: *** [C3] Error 1

So I modified line 2 into char *result = "";, which actually worked this time, but the output was this:

What's your name? Bro
Segmentation fault

This is a little confusing for me, since I'm not actually used to the scanf() function.

So my problem is, what would be a better way to get strings by input using scanf() in C99 without any weird output right after?

I'm pretty sure `int main(void)` was preferred in C99. `int main()` is pretty old. — William Pursell, Sep 20 '20 at 14:30

Mureinik · Accepted Answer · 2020-09-20T14:32:33.140

2

scanf doesn't allocate any memory. You need to allocate the string's memory yourself. You should also specify the length scanf is allowed to read to avoid buffer overflows.

E.g.:

char name[21]; // 20 chars plus the null-terminator
printf("What's your name? ");
scanf("%20s", name);

edited Sep 20 '20 at 14:32

answered Sep 20 '20 at 14:26

Mureinik

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`scanf("%19s", name);` would be better. – William Pursell Sep 20 '20 at 14:28
1

@WilliamPursell good point, thanks! Edited in an explanation about it. – Mureinik Sep 20 '20 at 14:33
Nice explanation about the `char *` problems. I will try it now. – Sep 24 '20 at 10:44

How does scanf() work for strings in C99?

1 Answers1