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

Question

I can specify the maximum amount of characters for scanf to read to a buffer using this technique:

char buffer[64];

/* Read one line of text to buffer. */
scanf("%63[^\n]", buffer);

But what if we do not know the buffer length when we write the code? What if it is the parameter of a function?

void function(FILE *file, size_t n, char buffer[n])
{
    /* ... */
    fscanf(file, "%[^\n]", buffer); /* WHAT NOW? */
}

This code is vulnerable to buffer overflows as fscanf does not know how big the buffer is.

I remember seeing this before and started to think that it was the solution to the problem:

fscanf(file, "%*[^\n]", n, buffer);

My first thought was that the * in "%*[*^\n]" meant that the maximum string size is passed an argument (in this case n). This is the meaning of the * in printf.

When I checked the documentation for scanf I found out that it means that scanf should discard the result of [^\n].

This left me somewhat disappointed as I think that it would be a very useful feature to be able to pass the buffer size dynamically for scanf.

Is there any way I can pass the buffer size to scanf dynamically?

Answer 1

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.

Answer 2

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);