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Add STRNCPYTAIL(), and use it instead of its pattern #1304

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Add STRNCPYTAIL(), and use it instead of its pattern #1304

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4638da0
lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
alejandro-colomar Jul 18, 2025
cd8af17
lib/utmp.c: Use STRNCPYTAIL() instead of its pattern
alejandro-colomar Jul 18, 2025
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2 changes: 2 additions & 0 deletions lib/Makefile.am
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Original file line number Diff line number Diff line change
Expand Up @@ -239,6 +239,8 @@ libshadow_la_SOURCES = \
string/strcpy/strncat.h \
string/strcpy/strncpy.c \
string/strcpy/strncpy.h \
string/strcpy/strncpytail.c \
string/strcpy/strncpytail.h \
string/strcpy/strtcpy.c \
string/strcpy/strtcpy.h \
string/strdup/strdup.c \
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13 changes: 13 additions & 0 deletions lib/string/strcpy/strncpytail.c
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Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
// SPDX-FileCopyrightText: 2025, Alejandro Colomar <[email protected]>
// SPDX-License-Identifier: BSD-3-Clause


#include "config.h"

#include "string/strcpy/strncpytail.h"

#include <stddef.h>


extern inline char *strncpytail(char *restrict dst, const char *restrict src,
size_t dsize);
36 changes: 36 additions & 0 deletions lib/string/strcpy/strncpytail.h
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@@ -0,0 +1,36 @@
// SPDX-FileCopyrightText: 2025, Alejandro Colomar <[email protected]>
// SPDX-License-Identifier: BSD-3-Clause


#ifndef SHADOW_INCLUDE_LIB_STRING_STRCPY_STRNCPYTAIL_H_
#define SHADOW_INCLUDE_LIB_STRING_STRCPY_STRNCPYTAIL_H_


#include "config.h"

#include <stddef.h>
#include <string.h>
#include <sys/param.h>

#include "attr.h"
#include "sizeof.h"
#include "string/strchr/strnul.h"


#define STRNCPYTAIL(dst, src) strncpytail(dst, src, countof(dst))


ATTR_STRING(2)
inline char *strncpytail(char *restrict dst, const char *restrict src,
size_t dsize);


// nonstring copy tail-of-string
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@hallyn

I'm standing by this term. For the mnemonic comment explaining the name, nonstring is the most appropriate thing to use. You can think of strn as "string-non".

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@alejandro-colomar alejandro-colomar Aug 12, 2025 •
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Also need to mention that GCC means by this term "do not use strlen() and other string-based function because the data MAY be not NUL-terminated or could be binary data", but here, in this project, it means different: "it is a text data in fixed size buffer/array, which terminates at the end of the array without NUL-termination or terminated early by first NUL".

Hm, do we need to define a new term?

We already have "fixed-width null-padded character array", which is what utmp(5) members are.

nonstring is a more generic thing, which englobes this and other creatures. But for this mnemonic use case, nonstring is easier to remember.

@Karlson2k Please do not respond here. I already know your opinion, and am not going to agree with it. Anyone interested in it, can read the lengthy thread above.

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@hallyn Please merge when you're happy with the name. Feel free to close all conversations, if you need that to be able to merge.

inline char *
strncpytail(char *restrict dst, const char *restrict src, size_t dsize)
{
return strncpy(dst, strnul(src) - MIN(strlen(src), dsize), dsize);
}


#endif // include guard
3 changes: 2 additions & 1 deletion lib/utmp.c
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Expand Up @@ -35,6 +35,7 @@
#include "string/strcmp/strneq.h"
#include "string/strcmp/strprefix.h"
#include "string/strcpy/strncpy.h"
#include "string/strcpy/strncpytail.h"
#include "string/strcpy/strtcpy.h"
#include "string/strdup/strdup.h"
#include "string/strdup/strndup.h"
Expand Down Expand Up @@ -298,7 +299,7 @@ prepare_utmp(const char *name, const char *line, const char *host,
&& ('\0' != ut->ut_id[0])) {
STRNCPY(utent->ut_id, ut->ut_id);
} else {
STRNCPY(utent->ut_id, strnul(line) - MIN(strlen(line), countof(utent->ut_id)));
STRNCPYTAIL(utent->ut_id, line);
}
#if defined(HAVE_STRUCT_UTMPX_UT_NAME)
STRNCPY(utent->ut_name, name);
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