isblank(3)

isblank3

Space or tab character test

Usage

#include <ctype.h>

int

isblank(int c);

int

isblank_l(int c, locale_t loc);

Library

Standard C Library (libc, -lc)

Overview

The isblank() and isblank_l() functions test for a space or tab character. For any locale, this includes the following standard characters:

“\t”“ ”

In the "C" locale, a successful isblank() or isblank_l() test is limited to these characters only. The value of the argument must be representable as an unsigned char or the value of EOF.

The isblank_l() function takes an explicit locale argument, whereas the isblank() function uses the current global or per-thread locale.

Return values

The isblank() and isblank_l() functions return zero if the character tests false and return non-zero if the character tests true.

Compatibility

The 4.4BSD extension of accepting arguments outside of the range of the unsigned char type in locales with large character sets is considered obsolete and may not be supported in future releases. The iswblank() or iswblank_l() function should be used instead.

Releases

87 releases since FreeBSD 2.0.5-RELEASE; the text changed 44 times.

  1. 2.0.5–2.2.2
  2. 2.2.5–3.4
  3. 3.5–3.5.1
  4. 4.0
  5. 4.1
  6. 4.1.1–4.4
  7. 4.5–4.11
  8. 5.0
  9. 5.1–5.2.1
  10. 5.3–5.5
  11. 6.0–6.3
  12. 6.4
  13. 7.0
  14. 7.1
  15. 7.2
  16. 7.3
  17. 7.4
  18. 8.0
  19. 8.1
  20. 8.2
  21. 8.3
  22. 8.4
  23. 9.0
  24. 9.1
  25. 9.2
  26. 9.3
  27. 10.0
  28. 10.1
  29. 10.2
  30. 10.3
  31. 10.4
  32. 11.0
  33. 11.1
  34. 11.2
  35. 11.3
  36. 11.4
  37. 12.0
  38. 12.1
  39. 12.2
  40. 12.3
  41. 12.4
  42. 13.0–13.1
  43. 13.2
  44. 13.3–14.5
  45. 15.0–16.0

Standards

The isblank() function conforms to ISO/IEC 9899:1999 (“ISO C99”). The isblank_l() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

From base 16.0-CURRENT-20261027-61c70dc16a79 in FreeBSD 16.0-CURRENT (2026-10-27). Parsed by carta from the page's mdoc source.