Specifying the maximum string length to scanf dynamically in C (like "%*s" in printf)

Basic answer

There isn't an analog to the printf() format specifier * in scanf().

In The Practice of Programming, Kernighan and Pike recommend using snprintf() to create the format string:

size_t sz = 64;
char format[32];
snprintf(format, sizeof(format), "%%%zus", sz);
if (scanf(format, buffer) != 1) { …oops… }

Extra information

Upgrading the example to a complete function:

int read_name(FILE *fp, char *buffer, size_t bufsiz)
{
    char format[16];
    snprintf(format, sizeof(format), "%%%zus", bufsiz - 1);
    return fscanf(fp, format, buffer);
}

This emphasizes that the size in the format specification is one less than the size of the buffer (it is the number of non-null characters that can be stored without counting the terminating null). Note that this is in contrast to fgets() where the size (an int, incidentally; not a size_t) is the size of the buffer, not one less. There are multiple ways of improving the function, but it shows the point. (You can replace the s in the format with [^\n] if that's what you want.)

Also, as Tim Čas noted in the comments, if you want (the rest of) a line of input, you're usually better off using fgets() to read the line, but remember that it includes the newline in its output (whereas %63[^\n] leaves the newline to be read by the next I/O operation). For more general scanning (for example, 2 or 3 strings), this technique may be better — especially if used with fgets() or getline() and then sscanf() to parse the input.

Also, the TR 24731-1 'safe' functions, implemented by Microsoft (more or less) and standardized in Annex K of ISO/IEC 9899-2011 (the C11 standard), require a length explicitly:

if (scanf_s("%[^\n]", buffer, sizeof(buffer)) != 1)
    ...oops...

This avoids buffer overflows, but probably generates an error if the input is too long. The size could/should be specified in the format string as before:

if (scanf_s("%63[^\n]", buffer, sizeof(buffer)) != 1)
    ...oops...

if (scanf_s(format, buffer, sizeof(buffer)) != 1)
    ...oops...

Note that the warning (from some compilers under some sets of flags) about 'non-constant format string' has to be ignored or suppressed for code using the generated format string.

There is indeed no variable width specifier in the scanf family of functions. Alternatives include creating the format string dynamically (though this seems a bit silly if the width is a compile-time constant) or simply accepting the magic number. One possibility is to use preprocessor macros for specifying both the buffer and format string width:

#define STR_VALUE(x) STR(x)
#define STR(x) #x

#define MAX_LEN 63

char buffer[MAX_LEN + 1];
fscanf(file, "%" STR_VALUE(MAX_LEN) "[^\n]", buffer);