tolower(3)

tolower3

Upper case to lower case letter conversion

Usage

#include <ctype.h>

int

tolower(int c);

int

tolower_l(int c, locale_t loc);

Library

Standard C Library (libc, -lc)

Overview

The tolower() and tolower_l() functions convert an upper-case letter to the corresponding lower-case letter. The argument must be representable as an unsigned char or the value of EOF.

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

Return values

If the argument is an upper-case letter, the tolower() and tolower_l() functions return the corresponding lower-case letter if there is one; otherwise, the argument is returned unchanged.

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 towlower() or towlower_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. 2.1.5–2.1.7.1
  3. 2.2.2–3.4
  4. 3.5–3.5.1
  5. 4.0
  6. 4.1
  7. 4.1.1–4.2
  8. 4.3–4.4
  9. 4.5–4.11
  10. 5.0–5.5
  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 tolower() function conforms to ISO/IEC 9899:1990 (“ISO C90”).

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