sprintf()处理-INF时返回正INF，此现象是Bug还是预期行为？

Great question! The short answer is: this is almost certainly a bug, not intended design behavior—assuming you’re working with a C implementation that follows the C99 (or later) standard.

Let me walk through why:

• What the C Standard Requires: Starting with C99, the specification explicitly mandates that when formatting a negative infinity value with sprintf() (using format specifiers like %f, %g, or their uppercase equivalents), the output must include a leading - sign. For example, -inf (or -INF if you use %F) is the correct output for negative infinity. A compliant implementation should never drop that negative sign.
• Exceptions for Non-Standard Environments: If you’re using a very old pre-C99 compiler, or a lightweight embedded library that skips some C99 features, this might be an oversight. But even in those cases, it’s not "expected design"—it’s a missing feature or bug in that specific library, not intentional behavior.
• Quick Test to Confirm: You can verify this behavior with a simple code snippet:

  #include <stdio.h>
  #include <math.h>
  
  int main() {
      char buffer[20];
      sprintf(buffer, "%f", -INFINITY);
      printf("Output: %s\n", buffer); // Should print "-inf" (or "-INF" with %F)
      return 0;
  }
  

  If this outputs just "inf" or "INF" instead of the signed version, your sprintf() implementation isn’t following the standard.

In most modern, standard-compliant C environments (like glibc on Linux, MSVC on Windows, or Clang), this would be treated as a bug and fixed if reported.

内容的提问来源于stack exchange，提问作者mvorisek