Add http_strncasecmp(3)

POSIX.1-2008 does not any locale-specific version of strncasecmp(3), so
conversions to lowercase depend on the system locale.

Since HTTP header fields must be checked without case sensitivity and
not depend on the system locale, a specialised function that forces the
"POSIX" locale is required.

2 files changed, 53 insertions, 0 deletions

diff --git a/doc/man3/Makefile b/doc/man3/Makefile
index cdfff3d..ffdbf16 100644
--- a/doc/man3/Makefile
+++ b/doc/man3/Makefile
@@ -28,6 +28,7 @@ OBJECTS = \
 	$(DESTDIR)$(man3dir)/http_encode_url.3 \
 	$(DESTDIR)$(man3dir)/http_free.3 \
 	$(DESTDIR)$(man3dir)/http_response_add_header.3 \
+	$(DESTDIR)$(man3dir)/http_strncasecmp.3 \
 	$(DESTDIR)$(man3dir)/http_update.3
 
 all:
diff --git a/doc/man3/http_strncasecmp.3 b/doc/man3/http_strncasecmp.3
new file mode 100644
index 0000000..0d5449f
--- /dev/null
+++ b/doc/man3/http_strncasecmp.3
@@ -0,0 +1,52 @@
+.TH HTTP_STRNCASECMP 3 2026-02-27 0.6.0 "libweb Library Reference"
+
+.SH NAME
+http_strncasecmp \- compare two strings ignoring case as "POSIX" locale
+
+.SH SYNOPSIS
+.LP
+.nf
+#include <libweb/http.h>
+.P
+char *http_strncasecmp(const char *\fIs1\fP, const char *\fIs2\fP, size_t \fIn\fP);
+.fi
+
+.SH DESCRIPTION
+The
+.IR http_strncasecmp ()
+function compares two strings as done by
+.IR strncasecmp (3),
+but forces the use of the "POSIX" locale, so that characters are converted
+to lowercase with
+.IR tolower_l (3),
+rather than relying on the locale configured by the system.
+
+This makes
+.IR http_strncasecmp ()
+ideal to compare HTTP headers, which must be checked without case sensitivity
+according to RFC 9110.
+
+.SH RETURN VALUE
+If the comparison is successful, zero is returned. If the two strings
+are not equal (case ignored) or any of their characters could not be converted
+by
+.IR tolower_l(3) ,
+a positive integer is returned.
+Otherwise, a negative integer is returned.
+
+.SH ERRORS
+No errors are defined.
+
+.SH SEE ALSO
+.BR libweb_http (7),
+.BR http_strcasecmp (3).
+
+.B https://www.rfc-editor.org/rfc/rfc9110#name-field-names
+
+.SH COPYRIGHT
+Copyright (C) 2023-2026 libweb contributors
+.P
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.