How to skip unwanted characters before a specific specific character (comma) when reading a file in C

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Summary

This incident stemmed from a malformed fscanf format string that attempted to “skip unwanted characters” before a comma using invalid scanset syntax. The result was undefined behavior, buffer corruption, and inconsistent parsing across lines with long fields.

Root Cause

The failure was caused by incorrect use of scansets in fscanf, specifically patterns like:

  • *[^,] — invalid and undefined
  • %49[^,]*[^,] — mixes a scanset with a literal *, which does not mean “skip characters”
  • Overly complex format strings attempting to sanitize input inline

Key issues:

  • Scanset syntax does not support repetition operators (*, +, etc.)
  • fscanf stops reading when the scanset fails, leaving unread characters in the stream
  • Long fields overflow the intended buffer, causing the parser to desynchronize

Why This Happens in Real Systems

Real-world file parsers often fail because:

  • Developers overestimate what scanf-family functions can safely do
  • Input formats evolve, but parsing code remains rigid
  • Malformed or oversized fields break assumptions
  • Skipping characters inline leads to unread garbage, which cascades into later fields

Real-World Impact

These bugs commonly cause:

  • Silent data corruption
  • Partial reads, where only the first few records load
  • Infinite loops when the parser never consumes the failing character
  • Crashes due to buffer overflows or mismatched types

Example or Code (if necessary and relevant)

A safe and correct approach uses bounded scansets without invalid operators:

fscanf(f, " %49[^,],%d,%d,%d,%49[^,],%19[^,],%d,%d,%d",
       osobe[i].imeprezime,
       &osobe[i].datumrodjenja.dan,
       &osobe[i].datumrodjenja.mjesec,
       &osobe[i].datumrodjenja.godina,
       osobe[i].vozilo.model,
       osobe[i].vozilo.vrsta_goriva,
       &osobe[i].vozilo.registracija.dan,
       &osobe[i].vozilo.registracija.mjesec,
       &osobe[i].vozilo.registracija.godina);

This pattern:

  • Reads up to the comma
  • Enforces maximum field length
  • Avoids undefined scanset behavior

How Senior Engineers Fix It

Experienced engineers avoid fragile inline parsing and instead:

  • Use simple, bounded scansets (%49[^,])
  • Consume delimiters explicitly
  • Validate return values rigorously
  • Switch to fgets + strtok or manual parsing for reliability
  • Reject malformed lines early, rather than trying to salvage them

They prioritize predictable behavior over clever one-liners.

Why Juniors Miss It

Less experienced developers often:

  • Assume scanf works like regex engines
  • Believe * means “skip characters” inside scansets
  • Don’t realize that undefined behavior can appear to work on some inputs
  • Try to solve parsing in a single format string instead of breaking it down
  • Lack familiarity with robust input-handling patterns

A small misunderstanding of scanset rules leads to disproportionately large failures.

If you want, I can also show a fully robust fgets-based parser that never risks undefined behavior.

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