In a C-scanf format, how do I specify, that I want a character ^?
"%[^]" does not work with GNU scanf, because ^ at start has the negation meaning.
scanf is it possible to specify a string consisting of a number of
^characters?
Doing a simple %[^] is impossible.
The %[^] is actually invalid - the initial [ is not closed. A %[^]] is interpreted as all characters except ].
Assuming %[^] would be valid, then it would present an ambiguity: %[^]] could be interpreted as a string consisting only from ^ followed by a ]. Or imagine like $[^]abc]. I believe the ability to scan strings only consisting of ^ was sacrificed to give ^ its functionality, which makes a reasonable sacrifice.
To solve the problem in practice, do not use scanf and write it yourself. Or you could do something like "%[\01^] - scan also something else that will not be in the input, like 0x01 byte.
From C99 7.19.6.2 (and this pdf) (emphasis mine):
[[...]
The conversion specifier includes all subsequent characters in the format string, up to and including the matching right bracket (
]). The characters between the brackets (the scan list) compose the scanset, unless the character after the left bracket is a circumflex (^), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket. If the conversion specifier begins with[]or[^], the right bracket character is in the scanlist and the next following right bracket character is the matching right bracket that ends the specification; otherwise the first following right bracket character is the one that ends the specification. [...]
So if the conversion is %[] or %[^] then the ] is in the scanlist, and the next ] will end the scanlist.
As a workaround, you can specify in the scanlist ^ negation of all characters except ^, effectively scanning only for ^ - %[^^].