Understanding the Problem: Track Buffer Offsets with vsscanf

When creating custom stream abstractions in C that handle both file streams (FILE*) and in-memory byte buffers, unified pointer management becomes a key challenge. While vfscanf automatically advances the file indicator for a FILE*, vsscanf operates on a static string buffer without updating any internal offset position.

To advance the buffer pointer manually, you need to know exactly how many characters vsscanf consumed. The standard C library approach is to append the %n specifier to the format string and supply an additional int* argument. However, when working inside a wrapper function that receives a va_list, appending an argument dynamically is not possible in standard C, as vsscanf expects a va_list parameter directly rather than variadic arguments.

Why the Naive Approach Fails

Consider the following attempt to perform a second pass with %n:

int consumed = 0;
vsscanf(buf + pos, new_fmt, ap2, &consumed); // Compilation error!

This fails to compile or causes undefined behavior because vsscanf takes exactly three parameters:

int vsscanf(const char *s, const char *format, va_list arg);

You cannot pass extra variadic arguments after ap2. Furthermore, standard C provides no portable mechanism to append values onto an existing va_list.

Solution 1: Unify Streams Using POSIX fmemopen (Recommended)

The cleanest, standard-conforming approach in POSIX environments (POSIX.1-2008) is to eliminate vsscanf entirely for buffer reading. Instead, open the memory buffer as a FILE* stream using fmemopen() and use vfscanf() for both memory and file inputs.

Because vfscanf() advances the file pointer automatically, you can retrieve the exact number of characters consumed using ftell() or fgetpos().

Example Implementation

#include <stdio.h>
#include <stdarg.h>

int read_from_buffer(const char *buf, size_t buf_len, size_t *pos, const char *fmt, va_list ap) {
    // Create a memory-backed file stream for the buffer region
    FILE *mem_stream = fmemopen((void *)(buf + *pos), buf_len - *pos, "r");
    if (!mem_stream) {
        return -1; // Memory stream creation failed
    }

    // Use vfscanf directly on the memory stream
    int items_read = vfscanf(mem_stream, fmt, ap);

    // Determine bytes consumed via ftell
    long consumed = ftell(mem_stream);
    if (consumed > 0) {
        *pos += (size_t)consumed;
    }

    fclose(mem_stream);
    return items_read;
}

Advantages

  • No Format String Modification: Works transparently with caller-supplied format strings.
  • Single-Pass Execution: Avoids performance overhead associated with scanning data twice.
  • Unified Code Paths: Eliminates distinct logic branches between memory buffers and actual file pointers.

Solution 2: Cross-Platform Compatibility (Windows)

If you are targeting non-POSIX systems like Windows where fmemopen() is not available natively, you have two options:

  1. Use a Lightweight fmemopen Polyfill: Include an open-source implementation of fmemopen built on Windows memory mapping or custom C runtime streams (e.g., using funopen on BSD/macOS or custom buffer routines).
  2. Format String Parsing: If custom stream abstractions must remain header-only or self-contained without platform dependencies, manually inspect the format string to count specifiers, extract arguments, or use higher-level string parsing (e.g., strtod, strtol, sscanf) that natively report end pointers.

Conclusion

Attempting to append %n to a format string when forwarding a va_list is unsupported by the C standard library layout. By converting memory buffers into stream handles via fmemopen(), you leverage vfscanf() to handle offset tracking automatically, keeping code clean, safe, and maintainable.