Add strncpytail_a(), and use it instead of its pattern by alejandro-colomar · Pull Request #1304 · shadow-maint/shadow

alejandro-colomar · 2025-07-18T13:28:14Z

Revisions:

v1b

Rebase

$ git range-diff 17a3bb51df92^..gh/strncpytail shadow/master..strncpytail 
1:  17a3bb51 = 1:  52ed2fb5 lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
2:  0747fecc = 2:  25ed13db lib/utmp.c: Use STRNCPYTAIL() instead of its pattern

v1c

Rebase

$ git rd 
1:  52ed2fb5 = 1:  a9207ca3 lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
2:  25ed13db ! 2:  22ce0a47 lib/utmp.c: Use STRNCPYTAIL() instead of its pattern
    @@ lib/utmp.c
      #include "string/strdup/xstrdup.h"
      #include "string/strdup/xstrndup.h"
     @@ lib/utmp.c: prepare_utmp(const char *name, const char *line, const char *host,
    -   if (NULL != ut) {
    +       && ('\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)));

v1d

Rebase

$ git range-diff a9207ca3^..22ce0a47 5f7630f8^..6b0a0ac0
1:  a9207ca3 = 1:  5f7630f8 lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
2:  22ce0a47 = 2:  6b0a0ac0 lib/utmp.c: Use STRNCPYTAIL() instead of its pattern

v1e

Rebase

$ git rd 
1:  5f7630f8 = 1:  4fc3f696 lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
2:  6b0a0ac0 = 2:  df8b604d lib/utmp.c: Use STRNCPYTAIL() instead of its pattern

v1f

Rebase

$ git rd 
1:  4fc3f696 = 1:  163d8535 lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
2:  df8b604d ! 2:  95ab3cb6 lib/utmp.c: Use STRNCPYTAIL() instead of its pattern
    @@ Commit message
     
      ## lib/utmp.c ##
     @@
    - #include "string/strcmp/streq.h"
    + #include "string/strcmp/strneq.h"
      #include "string/strcmp/strprefix.h"
      #include "string/strcpy/strncpy.h"
     +#include "string/strcpy/strncpytail.h"

v1g

Rebase

$ git rd 
1:  163d8535 ! 1:  4638da01 lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
     +  string/strcpy/strncpytail.h \
        string/strcpy/strtcpy.c \
        string/strcpy/strtcpy.h \
    -   string/strdup/strndupa.c \
    +   string/strdup/strdup.c \
     
      ## lib/string/strcpy/strncpytail.c (new) ##
     @@
2:  95ab3cb6 ! 2:  cd8af171 lib/utmp.c: Use STRNCPYTAIL() instead of its pattern
    @@ lib/utmp.c
      #include "string/strcpy/strncpy.h"
     +#include "string/strcpy/strncpytail.h"
      #include "string/strcpy/strtcpy.h"
    - #include "string/strdup/xstrdup.h"
    - #include "string/strdup/xstrndup.h"
    + #include "string/strdup/strdup.h"
    + #include "string/strdup/strndup.h"
     @@ lib/utmp.c: prepare_utmp(const char *name, const char *line, const char *host,
            && ('\0' != ut->ut_id[0])) {
                STRNCPY(utent->ut_id, ut->ut_id);

v2

Rename s/STRNCPYTAIL/strncpytail_a/.
Improve comments.

$ git rd 
1:  4638da014 ! 1:  f498a3873 lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
    @@ Metadata
     Author: Alejandro Colomar <alx@kernel.org>
     
      ## Commit message ##
    -    lib/string/strcpy/: strncpytail(), STRNCPYTAIL(): Add APIs
    +    lib/string/strcpy/: strncpytail[_a](): Add APIs
     
         This works similar to strncpy(3), except that it truncates from the
         start of the string if the string doesn't fit.  It is useful for utmp(5)
    @@ lib/string/strcpy/strncpytail.h (new)
     +#include "string/strchr/strnul.h"
     +
     +
    -+#define STRNCPYTAIL(dst, src)  strncpytail(dst, src, countof(dst))
    ++// strncpytail_a - nonstring copy tail-of-string array
    ++#define strncpytail_a(dst, src)  strncpytail(dst, src, countof(dst))
     +
     +
     +ATTR_STRING(2)
    @@ lib/string/strcpy/strncpytail.h (new)
     +    size_t dsize);
     +
     +
    -+// nonstring copy tail-of-string
    ++// strncpytail - nonstring copy tail-of-string
     +inline char *
     +strncpytail(char *restrict dst, const char *restrict src, size_t dsize)
     +{
2:  cd8af171e ! 2:  36c4c50ef lib/utmp.c: Use STRNCPYTAIL() instead of its pattern
    @@ Metadata
     Author: Alejandro Colomar <alx@kernel.org>
     
      ## Commit message ##
    -    lib/utmp.c: Use STRNCPYTAIL() instead of its pattern
    +    lib/utmp.c: Use strncpytail_a() instead of its pattern
     
         Signed-off-by: Alejandro Colomar <alx@kernel.org>
     
    @@ lib/utmp.c: prepare_utmp(const char *name, const char *line, const char *host,
                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);
    ++          strncpytail_a(utent->ut_id, line);
        }
      #if defined(HAVE_STRUCT_UTMPX_UT_NAME)
        STRNCPY(utent->ut_name, name);

v2b

Update lib/string/README.

$ git rd 
1:  f498a3873 ! 1:  593027522 lib/string/strcpy/: strncpytail[_a](): Add APIs
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
        string/strcpy/strtcpy.h \
        string/strdup/strdup.c \
     
    + ## lib/string/README ##
    +@@ lib/string/README: strcpy/ - String copying
    +     strncpytail()
    +   Like strncpy(), but when truncating, the tail of the string is
    +   kept instead of the beginning.  This is useful for ut_id.
    +-    STRNCPYTAIL()
    ++    strncpytail_a()
    +   Like strncpytail, but takes an array.
    + 
    +     STRNCAT()  // To be removed
    +
      ## lib/string/strcpy/strncpytail.c (new) ##
     @@
     +// SPDX-FileCopyrightText: 2025, Alejandro Colomar <alx@kernel.org>
2:  36c4c50ef = 2:  982f944a5 lib/utmp.c: Use strncpytail_a() instead of its pattern

v2c

Rebase

$ git rd 
1:  593027522 = 1:  4404b232e lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  982f944a5 ! 2:  7d6800182 lib/utmp.c: Use strncpytail_a() instead of its pattern
    @@ lib/utmp.c
      #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"
    + 
     @@ lib/utmp.c: prepare_utmp(const char *name, const char *line, const char *host,
            && ('\0' != ut->ut_id[0])) {
                STRNCPY(utent->ut_id, ut->ut_id);

v2d

Rebase

$ git rd 
1:  4404b232e = 1:  e2c0cd3d7 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  7d6800182 = 2:  e78027182 lib/utmp.c: Use strncpytail_a() instead of its pattern

v2e

Rebase

$ git rd 
1:  e2c0cd3d7 ! 1:  485973f1a lib/string/strcpy/: strncpytail[_a](): Add APIs
    @@ lib/string/README: strcpy/ - String copying
     +    strncpytail_a()
        Like strncpytail, but takes an array.
      
    -     STRNCAT()  // To be removed
    +     strncat_a()  // To be removed
     
      ## lib/string/strcpy/strncpytail.c (new) ##
     @@
2:  e78027182 ! 2:  4ffb5bc27 lib/utmp.c: Use strncpytail_a() instead of its pattern
    @@ lib/utmp.c
      
     @@ lib/utmp.c: prepare_utmp(const char *name, const char *line, const char *host,
            && ('\0' != ut->ut_id[0])) {
    -           STRNCPY(utent->ut_id, ut->ut_id);
    +           strncpy_a(utent->ut_id, ut->ut_id);
        } else {
    --          STRNCPY(utent->ut_id, strnul(line) - MIN(strlen(line), countof(utent->ut_id)));
    +-          strncpy_a(utent->ut_id, strnul(line) - MIN(strlen(line), countof(utent->ut_id)));
     +          strncpytail_a(utent->ut_id, line);
        }
      #if defined(HAVE_STRUCT_UTMPX_UT_NAME)
    -   STRNCPY(utent->ut_name, name);
    +   strncpy_a(utent->ut_name, name);

v2f

Rebase

$ git rd 
1:  485973f1a = 1:  0d001eb2b lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  4ffb5bc27 = 2:  5bccdef18 lib/utmp.c: Use strncpytail_a() instead of its pattern

v2g

Rebase

$ git rd 
1:  0d001eb2b = 1:  559ac55c6 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  5bccdef18 = 2:  cf1424804 lib/utmp.c: Use strncpytail_a() instead of its pattern

v2h

Rebase

$ git rd 
1:  559ac55c6 = 1:  5c06da618 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  cf1424804 = 2:  2f4793faa lib/utmp.c: Use strncpytail_a() instead of its pattern

v2i

Rebase

$ git rd 
1:  5c06da618 = 1:  63d4e4ac0 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  2f4793faa = 2:  22b11999a lib/utmp.c: Use strncpytail_a() instead of its pattern

v2j

Rebase

$ git rd 
1:  63d4e4ac0 = 1:  136b63d95 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  22b11999a = 2:  c33c3d5c0 lib/utmp.c: Use strncpytail_a() instead of its pattern

v3

Use strneq_a() instead of its pattern.

$ git range-diff shadow/master c33c3d5c0044 HEAD
1:  136b63d95 = 1:  136b63d95 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  c33c3d5c0 = 2:  c33c3d5c0 lib/utmp.c: Use strncpytail_a() instead of its pattern
-:  --------- > 3:  e950aa7b1 lib/utmp.c: Use strneq_a() instead of its pattern

v3b

Rebase

$ git rd 
1:  136b63d95 = 1:  f102f5586 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  c33c3d5c0 = 2:  dc7d35898 lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  e950aa7b1 = 3:  6a97a418b lib/utmp.c: Use strneq_a() instead of its pattern

v3c

Rebase

$ git rd 
1:  f102f5586 = 1:  7163e0365 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  dc7d35898 = 2:  9d24cf8a3 lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  6a97a418b = 3:  760f6b1db lib/utmp.c: Use strneq_a() instead of its pattern

v3d

Rebase

$ git rd 
1:  7163e0365637 ! 1:  5832c049d908 lib/string/strcpy/: strncpytail[_a](): Add APIs
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
     +  string/strcpy/strncpytail.h \
        string/strcpy/strtcpy.c \
        string/strcpy/strtcpy.h \
    -   string/strdup/strdup.c \
    +   string/strdup/memdup.c \
     
      ## lib/string/README ##
     @@ lib/string/README: strcpy/ - String copying
2:  9d24cf8a3169 ! 2:  2473dffe0b85 lib/utmp.c: Use strncpytail_a() instead of its pattern
    @@ lib/utmp.c
      #include "string/strcmp/strprefix.h"
      #include "string/strcpy/strncpy.h"
     +#include "string/strcpy/strncpytail.h"
    + #include "string/strdup/memdup.h"
      #include "string/strdup/strdup.h"
      #include "string/strdup/strndup.h"
    - 
     @@ lib/utmp.c: prepare_utmp(const char *name, const char *line, const char *host,
            && ('\0' != ut->ut_id[0])) {
                strncpy_a(utent->ut_id, ut->ut_id);
3:  760f6b1db1b1 = 3:  500ff245cdd5 lib/utmp.c: Use strneq_a() instead of its pattern

v3e

Rebase

$ git rd 
1:  5832c049 = 1:  d4eb7969 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  2473dffe = 2:  1532dbcd lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  500ff245 = 3:  0c7decf8 lib/utmp.c: Use strneq_a() instead of its pattern

v3f

Rebase

$ git rd
1:  d4eb7969 = 1:  4eab81f2 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  1532dbcd = 2:  d41c2740 lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  0c7decf8 = 3:  2251bbd8 lib/utmp.c: Use strneq_a() instead of its pattern

v3g

Rebase

$ git rd 
1:  4eab81f25668 = 1:  7ebd8a18affd lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  d41c27406926 = 2:  b7bfe85054ad lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  2251bbd8cc49 = 3:  9eaf0ae3955c lib/utmp.c: Use strneq_a() instead of its pattern

v3h

Rebase

$ git rd 
 1:  3af81f598574 =  1:  1781d0d7fa72 lib/logind.c: Fix style
 2:  8d5bb01d3e0c =  2:  d8fa47da16b5 lib/logind.c: Remove dead initializations
 3:  1aaa11a73d84 !  3:  d0552ddd4958 lib/: Move utmp.c and logind.c prototypes to new "session_management.h" header
    @@ lib/utmp.c
     
      ## src/login.c ##
     @@
    - #include "getdef.h"
    + #include "io/fprintf.h"
      #include "prototypes.h"
      #include "pwauth.h"
     +#include "session_management.h"
 4:  8507b3d84fd5 =  4:  2aef494855a1 lib/logind.c: get_session_host(): free() earlier, to avoid a goto
 5:  cdfb40d1847b =  5:  dbaff831cc43 lib/logind.c: Wrap libsystemd function in our wrapper get_session()
 6:  b6e7134e97b8 =  6:  41dc548502b5 lib/logind.c: Wrap libsystemd function in our wrapper session_get_remote_host()
 7:  4a0633c9869a =  7:  2c698463cbec configure.ac, lib/utmp.c: Assume utmpx(5) has ut_host
 8:  481b6acecd22 =  8:  aff3bcd25b67 lib/, src/: get_session_host(): Return the host
 9:  7868607ef562 =  9:  def7dafe282b lib/utmp.c: get_session_host(): Don't exit from library code

v3i

Rebase

$ git rd 
1:  7ebd8a18affd = 1:  04a0d5f4b814 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  b7bfe85054ad = 2:  82bed05771c2 lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  9eaf0ae3955c = 3:  f329bd7a9288 lib/utmp.c: Use strneq_a() instead of its pattern

v3j

Rebase

$ git rd 
1:  04a0d5f4b814 = 1:  d8876fa89605 lib/string/strcpy/: strncpytail[_a](): Add APIs
2:  82bed05771c2 = 2:  140f3b837e60 lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  f329bd7a9288 = 3:  11dc1b2f8b1c lib/utmp.c: Use strneq_a() instead of its pattern

v4

Include <memory.h> for strncpy(3).

$ git rd 
1:  d8876fa89605 ! 1:  a4820c9a2095 lib/string/strcpy/: strncpytail[_a](): Add APIs
    @@ lib/string/strcpy/strncpytail.h (new)
     +
     +#include "config.h"
     +
    ++#include <memory.h>
     +#include <stddef.h>
     +#include <string.h>
     +#include <sys/param.h>
2:  140f3b837e60 = 2:  55e24c70b935 lib/utmp.c: Use strncpytail_a() instead of its pattern
3:  11dc1b2f8b1c = 3:  7ed78c5da107 lib/utmp.c: Use strneq_a() instead of its pattern

v5

Move API to a new lib/memory/. [@ikerexxe ]

$ git rd --creation-factor=99
1:  a4820c9a2095 ! 1:  32b835a4bcdc lib/string/strcpy/: strncpytail[_a](): Add APIs
    @@ Metadata
     Author: Alejandro Colomar <alx@kernel.org>
     
      ## Commit message ##
    -    lib/string/strcpy/: strncpytail[_a](): Add APIs
    +    lib/memory/memcpy/: strncpytail[_a](): Add APIs
     
         This works similar to strncpy(3), except that it truncates from the
         start of the string if the string doesn't fit.  It is useful for utmp(5)
    @@ Commit message
     
      ## lib/Makefile.am ##
     @@ lib/Makefile.am: 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/memdup.c \
    +   lockpw.c \
    +   loginprompt.c \
    +   mail.c \
    ++  memory/memcpy/strncpytail.c \
    ++  memory/memcpy/strncpytail.h \
    +   motd.c \
    +   myname.c \
    +   nss.c \
     
    - ## lib/string/README ##
    -@@ lib/string/README: strcpy/ - String copying
    -     strncpytail()
    -   Like strncpy(), but when truncating, the tail of the string is
    -   kept instead of the beginning.  This is useful for ut_id.
    --    STRNCPYTAIL()
    -+    strncpytail_a()
    -   Like strncpytail, but takes an array.
    - 
    -     strncat_a()  // To be removed
    -
    - ## lib/string/strcpy/strncpytail.c (new) ##
    + ## lib/memory/memcpy/strncpytail.c (new) ##
     @@
    -+// SPDX-FileCopyrightText: 2025, Alejandro Colomar <alx@kernel.org>
    ++// SPDX-FileCopyrightText: 2025-2026, Alejandro Colomar <alx@kernel.org>
     +// SPDX-License-Identifier: BSD-3-Clause
     +
     +
     +#include "config.h"
     +
    -+#include "string/strcpy/strncpytail.h"
    ++#include "memory/memcpy/strncpytail.h"
     +
     +#include <stddef.h>
     +
    @@ lib/string/strcpy/strncpytail.c (new)
     +extern inline char *strncpytail(char *restrict dst, const char *restrict src,
     +    size_t dsize);
     
    - ## lib/string/strcpy/strncpytail.h (new) ##
    + ## lib/memory/memcpy/strncpytail.h (new) ##
     @@
    -+// SPDX-FileCopyrightText: 2025, Alejandro Colomar <alx@kernel.org>
    ++// SPDX-FileCopyrightText: 2025-2026, Alejandro Colomar <alx@kernel.org>
     +// SPDX-License-Identifier: BSD-3-Clause
     +
     +
    -+#ifndef SHADOW_INCLUDE_LIB_STRING_STRCPY_STRNCPYTAIL_H_
    -+#define SHADOW_INCLUDE_LIB_STRING_STRCPY_STRNCPYTAIL_H_
    ++#ifndef SHADOW_INCLUDE_LIB_MEMORY_MEMCPY_STRNCPYTAIL_H_
    ++#define SHADOW_INCLUDE_LIB_MEMORY_MEMCPY_STRNCPYTAIL_H_
     +
     +
     +#include "config.h"
    @@ lib/string/strcpy/strncpytail.h (new)
     +
     +
     +#endif  // include guard
    +
    + ## lib/string/README ##
    +@@ lib/string/README: strcpy/ - String copying
    +     strncpytail()
    +   Like strncpy(), but when truncating, the tail of the string is
    +   kept instead of the beginning.  This is useful for ut_id.
    +-    STRNCPYTAIL()
    ++    strncpytail_a()
    +   Like strncpytail, but takes an array.
    + 
    +     strncat_a()  // To be removed
2:  55e24c70b935 ! 2:  75f21c72ec08 lib/utmp.c: Use strncpytail_a() instead of its pattern
    @@ Commit message
     
      ## lib/utmp.c ##
     @@
    - #include "string/strcmp/strneq.h"
    - #include "string/strcmp/strprefix.h"
    - #include "string/strcpy/strncpy.h"
    -+#include "string/strcpy/strncpytail.h"
    - #include "string/strdup/memdup.h"
    - #include "string/strdup/strdup.h"
    - #include "string/strdup/strndup.h"
    + #include "alloc/malloc.h"
    + #include "attr.h"
    + #include "io/syslog.h"
    ++#include "memory/memcpy/strncpytail.h"
    + #include "sizeof.h"
    + #include "string/strchr/strnul.h"
    + #include "string/strcmp/streq.h"
     @@ lib/utmp.c: prepare_utmp(const char *name, const char *line, const char *host,
            && ('\0' != ut->ut_id[0])) {
                strncpy_a(utent->ut_id, ut->ut_id);
3:  7ed78c5da107 = 3:  b95da0678ae4 lib/utmp.c: Use strneq_a() instead of its pattern

Karlson2k · 2025-07-18T14:49:58Z

Nice.
Improves code readability.

Karlson2k · 2025-07-18T15:12:57Z

For the records.
The next code could be faster:

inline char *
strncpytail(char *restrict dst, const char *restrict src, size_t dsize)
{
  size_t src_len;
  src_len = strlen(src);

  if (dsize > src_len)
    return memcpy (dst, src, src_len + 1);

  return memcpy (dst, src + (src_len - dsize), dsize);
}

Large strings / buffers can be processed much faster.

But if performance is not an issue, the code in the PR will work fine.

Note: the binary is still a few bytes larger.

alejandro-colomar · 2025-07-18T17:26:59Z

For the records. The next code could be faster:
inline char *
strncpytail(char *restrict dst, const char *restrict src, size_t dsize)
{
  size_t src_len;
  src_len = strlen(src);

  if (dsize > src_len)
    return memcpy (dst, src, src_len + 1);

  return memcpy (dst, src + (src_len - dsize), dsize);
}
Large strings / buffers can be processed much faster.

But if performance is not an issue, the code in the PR will work fine.

Note: the binary is still a few bytes larger.

Indeed, since this API is designed for short strings (ut_id), I'll optimize for readability for now.

alejandro-colomar · 2025-08-11T16:00:47Z

@hallyn The term nonstring comes from https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute. (To avoid you having to read the lengthy discussion in the other thread.)

hallyn · 2025-08-11T16:04:25Z

@hallyn The term nonstring comes from https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute. (To avoid you having to read the lengthy discussion in the other thread.)

Thanks

hallyn · 2025-08-12T16:20:20Z

@hallyn The term nonstring comes from https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute. (To avoid you having to read the lengthy discussion in the other thread.)

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?

Karlson2k · 2025-08-12T16:24:43Z

@hallyn The term nonstring comes from https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute. (To avoid you having to read the lengthy discussion in the other thread.)

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?

Probably.
@alejandro-colomar insisting that it is perfectly described by "fixed size array with zero-padded data". It also assumed that zero-padding must not be checked and data reading must be stopped at the first zero. Also assumed that the data cannot have zeros because zero is used for padding.
I'm trying to convince Alejandro that what is described is not zero-padding, but is just optional early zero-termination.

I any case, I think both terms "non-string" and "zero-padded array" are confusing and not fully describe this kind of entities.

alejandro-colomar · 2025-08-12T20:32:43Z

Please, do not continue the discussion unthreaded. That makes it impossible to follow the page. If you want to have a lengthy discussion, open a comment on any line of code, which can be hidden easily.

Karlson2k · 2025-11-28T17:17:06Z

Besides used terms, the code is OK.

alejandro-colomar · 2025-11-28T20:40:59Z

Besides used terms, the code is OK.

Thanks!

alejandro-colomar · 2026-01-08T02:35:00Z

Cc: @kees

This works similar to strncpy(3), except that it truncates from the start of the string if the string doesn't fit. It is useful for utmp(5) ut_id, where the tail of the string is more useful and distinctive. Signed-off-by: Alejandro Colomar <alx@kernel.org>

Signed-off-by: Alejandro Colomar <alx@kernel.org>

alejandro-colomar requested review from hallyn and ikerexxe July 18, 2025 13:28

alejandro-colomar force-pushed the strncpytail branch 2 times, most recently from 9acb3e0 to 0747fec Compare July 18, 2025 13:31

alejandro-colomar force-pushed the strncpytail branch from 0747fec to 25ed13d Compare July 18, 2025 20:09

Karlson2k approved these changes Jul 18, 2025

View reviewed changes

alejandro-colomar added the Simpler A good issue for a new beginner label Jul 20, 2025

hallyn reviewed Aug 9, 2025

View reviewed changes

Comment thread lib/string/strcpy/strncpytail.h Outdated

alejandro-colomar force-pushed the strncpytail branch from 25ed13d to 22ce0a4 Compare August 10, 2025 08:15

This comment was marked as off-topic.

Sign in to view

alejandro-colomar commented Aug 12, 2025

View reviewed changes

Comment thread lib/string/strcpy/strncpytail.h Outdated

alejandro-colomar commented Aug 12, 2025

View reviewed changes

Comment thread lib/memory/memcpy/strncpytail.h

alejandro-colomar force-pushed the strncpytail branch from 22ce0a4 to 6b0a0ac Compare August 16, 2025 12:34

alejandro-colomar mentioned this pull request Sep 29, 2025

4.19 Release plan (Herve) - 2025-12 #1354

Closed

alejandro-colomar force-pushed the strncpytail branch from 6b0a0ac to df8b604 Compare September 30, 2025 09:48

alejandro-colomar force-pushed the strncpytail branch from df8b604 to 95ab3cb Compare October 19, 2025 09:27

alejandro-colomar force-pushed the strncpytail branch 2 times, most recently from cd8af17 to 36c4c50 Compare October 31, 2025 09:23

alejandro-colomar changed the title ~~Add STRNCPYTAIL(), and use it instead of its pattern~~ Add strncpytail_a(), and use it instead of its pattern Oct 31, 2025

alejandro-colomar force-pushed the strncpytail branch 2 times, most recently from 982f944 to 7d68001 Compare November 4, 2025 12:05

alejandro-colomar force-pushed the strncpytail branch from 7d68001 to e780271 Compare November 6, 2025 13:02

alejandro-colomar force-pushed the strncpytail branch from e780271 to 4ffb5bc Compare November 28, 2025 15:55

alejandro-colomar force-pushed the strncpytail branch 4 times, most recently from 2f4793f to 22b1199 Compare December 7, 2025 13:03

alejandro-colomar self-assigned this Dec 11, 2025

alejandro-colomar force-pushed the strncpytail branch from 22b1199 to c33c3d5 Compare December 12, 2025 01:31

alejandro-colomar force-pushed the strncpytail branch from e950aa7 to 6a97a41 Compare December 23, 2025 22:14

alejandro-colomar force-pushed the strncpytail branch from 6a97a41 to 760f6b1 Compare December 31, 2025 02:31

alejandro-colomar requested a review from hallyn December 31, 2025 02:32

alejandro-colomar added the New API label Jan 1, 2026

alejandro-colomar force-pushed the strncpytail branch from 760f6b1 to 500ff24 Compare February 23, 2026 13:42

alejandro-colomar force-pushed the strncpytail branch from 500ff24 to 0c7decf Compare March 4, 2026 16:11

alejandro-colomar force-pushed the strncpytail branch from 0c7decf to 2251bbd Compare April 12, 2026 07:19

alejandro-colomar force-pushed the strncpytail branch from 2251bbd to 9eaf0ae Compare July 20, 2026 20:02

alejandro-colomar force-pushed the strncpytail branch from 9eaf0ae to f329bd7 Compare August 19, 2026 09:14

alejandro-colomar mentioned this pull request Aug 23, 2026

5.0 Release plan ~ 2028 #1722

Open

alejandro-colomar force-pushed the strncpytail branch 2 times, most recently from 11dc1b2 to 7ed78c5 Compare August 25, 2026 15:46

alejandro-colomar added 3 commits August 26, 2026 14:39

lib/utmp.c: Use strncpytail_a() instead of its pattern

75f21c7

Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/utmp.c: Use strneq_a() instead of its pattern

b95da06

Signed-off-by: Alejandro Colomar <alx@kernel.org>

alejandro-colomar force-pushed the strncpytail branch from 7ed78c5 to b95da06 Compare August 26, 2026 12:41

Uh oh!

Conversation

alejandro-colomar commented Jul 18, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Karlson2k commented Jul 18, 2025

Uh oh!

Karlson2k commented Jul 18, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

alejandro-colomar commented Jul 18, 2025

Uh oh!

Uh oh!

alejandro-colomar commented Aug 11, 2025

Uh oh!

hallyn commented Aug 11, 2025

Uh oh!

This comment was marked as off-topic.

hallyn commented Aug 12, 2025

Uh oh!

Karlson2k commented Aug 12, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment was marked as off-topic.

alejandro-colomar commented Aug 12, 2025

Uh oh!

Uh oh!

Uh oh!

Karlson2k commented Nov 28, 2025

Uh oh!

alejandro-colomar commented Nov 28, 2025

Uh oh!

alejandro-colomar commented Jan 8, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

alejandro-colomar commented Jul 18, 2025 •

edited

Loading

Karlson2k commented Jul 18, 2025 •

edited

Loading

Karlson2k commented Aug 12, 2025 •

edited

Loading