Add STRNEQ(), and use it instead of its pattern

[alejandro-colomar]

These APIs are like streq() and strprefix(), but for nonstrings (thus, the usual strn*() name).

The first parameter is a nonstring, and the second is a string.

---

Revisions:

v2

• Drop STRNPFX() from this patch set. I'll keep that for later.

$ git range-diff master gh/strneq strneq 
1:  cc5fcce1 = 1:  cc5fcce1 lib/, src/: Use strncmp(3) instead of explicit byte comparisons
2:  c6f4546b = 2:  c6f4546b lib/string/strcmp/: strneq(), STRNEQ(): Add APIs
3:  c7bc9f1d = 3:  c7bc9f1d lib/, src/: Use STRNEQ() instead of their pattern
4:  b1f734cf < -:  -------- lib/string/strcmp/: strnpfx(), STRNPFX(): Add APIs
5:  fbdade07 < -:  -------- src/logoutd.c: Use STRNPFX() instead of its pattern

v2b

• Rebase

$ git rd
1:  cc5fcce1 = 1:  f41a6069 lib/, src/: Use strncmp(3) instead of explicit byte comparisons
2:  c6f4546b = 2:  23b111ae lib/string/strcmp/: strneq(), STRNEQ(): Add APIs
3:  c7bc9f1d = 3:  fc42cc1b lib/, src/: Use STRNEQ() instead of their pattern

v3c

• Update lib/string/README.

$ git range-diff shadow/master gh/strneq strneq 
1:  f41a6069 = 1:  f41a6069 lib/, src/: Use strncmp(3) instead of explicit byte comparisons
2:  23b111ae ! 2:  30c4e70d lib/string/strcmp/: strneq(), STRNEQ(): Add APIs
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
        string/strcmp/strprefix.h \
        string/strcpy/stpecpy.c \
     
    + ## lib/string/README ##
    +@@ lib/string/README: Don't use some libc functions without Really Good Reasons:
    +   The return value of strcmp(3) is confusing.
    +   strcmp(3) would be legitimate for sorting strings.
    + 
    ++    strncmp(3)
    ++  Use STRNEQ(), or strpfx(), or else, depending on what you want.
    ++  The return value of strncmp(3) is confusing,
    ++  and it is unclear the purpose of its use when you read it.
    ++
    +     strcasecmp(3)
    +   Use strcaseeq() instead.
    + 
    +@@ lib/string/README: strcmp/ - String comparison
    +   Like strsfx(), but ignore upper-/lower-case.
    + 
    +   n/
    +-    strneq()  // Unimplemented
    ++    strneq()
    +   Return true if a [[gnu::nonstring]] is equal to a string.
    +-    STRNEQ()  // Unimplemented
    ++    STRNEQ()
    +   Like strneq(), but takes an array.
    + 
    +     strnpfx()  // Unimplemented
    +
      ## lib/string/strcmp/strneq.c (new) ##
     @@
     +// SPDX-FileCopyrightText: 2025, Alejandro Colomar <[email protected]>
3:  fc42cc1b = 3:  49a51b7e lib/, src/: Use STRNEQ() instead of their pattern

v4

• Use [[gnu::nonstring]].

$ git range-diff shadow/master gh/strneq strneq 
1:  f41a6069 = 1:  f41a6069 lib/, src/: Use strncmp(3) instead of explicit byte comparisons
-:  -------- > 2:  cb760ff7 lib/attr.h: __has_c_attribute(): Define fallback
-:  -------- > 3:  44ff1464 lib/attr.h: ATTR_NONSTRING: Add attribute [[gnu::nonstring]]
2:  30c4e70d ! 4:  a302e99e lib/string/strcmp/: strneq(), STRNEQ(): Add APIs
    @@ lib/string/strcmp/strneq.h (new)
     +
     +
     +ATTR_STRING(2)
    -+inline bool strneq(const char *strn, const char *s, size_t size);
    ++inline bool strneq(ATTR_NONSTRING const char *strn, const char *s, size_t size);
     +
     +
     +// nonstring equal
3:  49a51b7e = 5:  39c64afc lib/, src/: Use STRNEQ() instead of their pattern

v4b

• Rebase

$ git rd 
1:  f41a6069 = 1:  a9f960b9 lib/, src/: Use strncmp(3) instead of explicit byte comparisons
2:  cb760ff7 = 2:  419505c3 lib/attr.h: __has_c_attribute(): Define fallback
3:  44ff1464 = 3:  40602dd4 lib/attr.h: ATTR_NONSTRING: Add attribute [[gnu::nonstring]]
4:  a302e99e = 4:  db0d2614 lib/string/strcmp/: strneq(), STRNEQ(): Add APIs
5:  39c64afc = 5:  cb9791cc lib/, src/: Use STRNEQ() instead of their pattern