[PATCH] stpncpy.3, string_copying.7: Clarify that st[rp]ncpy() do NOT produce a string

[Alejandro Colomar <alx@kernel.org>]

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These copy *from* a string.  But the destination is a simple character
sequence within an array; not a string.

Suggested-by: DJ Delorie <dj@redhat.com>
Cc: Jonny Grant <jg@jguk.org>
Cc: Matthew House <mattlloydhouse@gmail.com>
Cc: Oskari Pirhonen <xxc3ncoredxx@gmail.com>
Cc: Thorsten Kukuk <kukuk@suse.com>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Zack Weinberg <zack@owlfolio.org>
Cc: "G. Branden Robinson" <g.branden.robinson@gmail.com>
Cc: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---

Resending, including the mailing lists, which I forgot.

 man3/stpncpy.3        | 17 +++++++++++++----
 man7/string_copying.7 | 20 ++++++++++----------
 2 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/man3/stpncpy.3 b/man3/stpncpy.3
index b6bbfd0a3..f86ff8c29 100644
--- a/man3/stpncpy.3
+++ b/man3/stpncpy.3
@@ -6,9 +6,8 @@
 .TH stpncpy 3 (date) "Linux man-pages (unreleased)"
 .SH NAME
 stpncpy, strncpy
-\- zero a fixed-width buffer and
-copy a string into a character sequence with truncation
-and zero the rest of it
+\-
+fill a fixed-width null-padded buffer with bytes from a string
 .SH LIBRARY
 Standard C library
 .RI ( libc ", " \-lc )
@@ -37,7 +36,7 @@ .SH SYNOPSIS
         _GNU_SOURCE
 .fi
 .SH DESCRIPTION
-These functions copy the string pointed to by
+These functions copy bytes from the string pointed to by
 .I src
 into a null-padded character sequence at the fixed-width buffer pointed to by
 .IR dst .
@@ -110,6 +109,16 @@ .SH CAVEATS
 These functions produce a null-padded character sequence,
 not a string (see
 .BR string_copying (7)).
+For example:
+.P
+.in +4n
+.EX
+strncpy(buf, "1", 5);       // { \[aq]1\[aq],   0,   0,   0,   0 }
+strncpy(buf, "1234", 5);    // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq],   0 }
+strncpy(buf, "12345", 5);   // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
+strncpy(buf, "123456", 5);  // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
+.EE
+.in
 .P
 It's impossible to distinguish truncation by the result of the call,
 from a character sequence that just fits the destination buffer;
diff --git a/man7/string_copying.7 b/man7/string_copying.7
index cadf1c539..0e179ba34 100644
--- a/man7/string_copying.7
+++ b/man7/string_copying.7
@@ -41,15 +41,11 @@ .SS Strings
 .\" ----- SYNOPSIS :: Null-padded character sequences --------/
 .SS Null-padded character sequences
 .nf
-// Zero a fixed-width buffer, and
-// copy a string into a character sequence with truncation.
-.BI "char *stpncpy(char " dst "[restrict ." sz "], \
+// Fill a fixed-width null-padded buffer with bytes from a string.
+.BI "char *strncpy(char " dst "[restrict ." sz "], \
 const char *restrict " src ,
 .BI "               size_t " sz );
-.P
-// Zero a fixed-width buffer, and
-// copy a string into a character sequence with truncation.
-.BI "char *strncpy(char " dst "[restrict ." sz "], \
+.BI "char *stpncpy(char " dst "[restrict ." sz "], \
 const char *restrict " src ,
 .BI "               size_t " sz );
 .P
@@ -240,14 +236,18 @@ .SS Truncate or not?
 .\" ----- DESCRIPTION :: Null-padded character sequences --------------/
 .SS Null-padded character sequences
 For historic reasons,
-some standard APIs,
+some standard APIs and file formats,
 such as
-.BR utmpx (5),
+.BR utmpx (5)
+and
+.BR tar (1),
 use null-padded character sequences in fixed-width buffers.
 To interface with them,
 specialized functions need to be used.
 .P
-To copy strings into them, use
+To copy bytes from strings into these buffers, use
+.BR strncpy (3)
+or
 .BR stpncpy (3).
 .P
 To copy from an unterminated string within a fixed-width buffer into a string,
-- 
2.42.0

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