Variable string limit "%*s" with sscanf() ==> "data argument not used by format string"

Question

int y = strlen(target);
sscanf(p,"%*s",y,buffer);

Why does the above code result in warning: data argument not used by format string ??

Compiled with Apple clang version 11.0.0 (clang-1100.0.33.17).

Aim : To get the same number of characters as in a string target into the string buffer, where p is a char * pointing to some element of a string.

`*` in a `sscanf` format specifier is different than `*` in a `printf` format specifier. Be sure to consult the documentation for `sscanf`. — user3386109, Aug 23 '20 at 04:56
You don't. `sscanf` does not provide a variable string limit. — user3386109, Aug 23 '20 at 04:58
Wow, is there no way to get a variable number of characters from a string in C ? — user13863346, Aug 23 '20 at 05:01
You may want to [edit] the question to explain what it is that you're trying to accomplish. `sscanf` is probably not the right tool for the job. — user3386109, Aug 23 '20 at 05:01
Re "*Wow, is there no way to get a variable number of characters from a string in C*", `%s` and `%5s` both read a variable number of characters. (Unbound in the first case, and up to five in the latter.) — ikegami, Aug 23 '20 at 05:05
@ikegami no, `%5s` wouldn't be variable, from what I mean, it would be a number known at compile-time. — user13863346, Aug 23 '20 at 05:07
Copying anywhere from 1 to 5 character as `%5s` is indeed copying a variable number of characters. You actually want the opposite. You want to copy of specific number of characters. You can use `memcpy` for that. — ikegami, Aug 23 '20 at 05:12
@M. Nejat Aydin , My apologies. I thought you used `sscanf`. Comment deleted. — ikegami, Aug 23 '20 at 05:17

score 4 · Answer 1 · answered Aug 23 '20 at 05:19

Other answers explain that sscanf does not provide for taking field widths from arguments. But with respect to this:

Aim : To get the same number of characters as in a string target into the string buffer, where p is a char * pointing to some element of a string.

I/O functions such as sscanf() are comparatively heavyweight. If all you want to do is copy (part of) one string to another, then memcpy() or strncpy() would be more suited to the task. Or strncat(), whose properties make it superior to the other two for this particular task, as with it you do not have the risk of overrunning your source string that memcpy() would bring, or the need to manually ensure termination of the result that comes with both memcpy() and strncpy(). Example:

*buffer = '\0';         // start with an empty string
strncat(buffer, p, y);  // concatenate the wanted region of the source string

score 3 · Answer 2 · answered Aug 23 '20 at 04:58

3

With sscanf(), the * in "%*s" is an argument suppression, not a size.

sscanf(p,"%*s",y,buffer); simply scans p for a non-white-space string without saving anything. y, buffer are unused.

Recommend to research fgets().

answered Aug 23 '20 at 04:58

chux - Reinstate Monica

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ikegami · Accepted Answer · 2020-08-23T05:15:06.670

2

I believe you want

#include <string.h>

memcpy(buffer, p, strlen(target));

Note that this will not add a terminating NUL. This can be achieved as follows:

#include <string.h>

size_t len = strlen(target);
memcpy(buffer, p, len);
buffer[len] = 0;

edited Aug 23 '20 at 05:15

answered Aug 23 '20 at 05:08

ikegami

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Variable string limit "%*s" with sscanf() ==> "data argument not used by format string"

3 Answers3