[SOLVED] Negating brackets in a C scanf() scanset

Negating brackets in a C scanf() scanset

I'm trying to use scanf() to read lines of the form mov r0, r1, r2 or ldr r0, [r1, 0x4] in a text buffer, negating any possible square brackets, commas and white spaces. The following code is not working:

void ReadBuffer(const char* buffer)
{
 char command[2048];
 char var[3][2048];
 sscanf(buffer,"%[^ ,[]\n\t] %[^ ,[]\n\t], %[^ ,[]\n\t], %[^ ,[]\n\t]", command, va[0], var[1], var[2]);
}

How can I add the square brackets to the negated scanset?

Solution

C 2018 describes the scanf conversion specifiers in 7.21.6.2 12, and for the [ conversion it says:

… 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…

Therefore, put the ] immediately after the ^. The [ may go anywhere in the scanlist (the characters between the [ and the closing ]).

Example:

#include <stdio.h>


int main(void)
{
    char a[100], b, c[100], d, e[100], f;
    int result = sscanf("foo[bar, baz]", " %[^][, ] %c %[^][, ] %c %[^][, ] %c", a, &b, c, &d, e, &f);
    printf("scanf returned %d.\n", result);
    printf("a = %s.\n", a);
    printf("b = %c.\n", b);
    printf("c = %s.\n", c);
    printf("d = %c.\n", d);
    printf("e = %s.\n", e);
    printf("f = %c.\n", f);
}

Output:

scanf returned 6.
a = foo.
b = [.
c = bar.
d = ,.
e = baz.
f = ].

Note this is a painstaking way to parse assembly code. It is usually better to write your own C code to analyze the characters, especially after studying some parsing theory, and even better to use an off-the-shelf lexical analyzer generator and a grammar parser generator. Doing it with scanf requires careful attention to return values, buffer sizes, and more.