Re: [PATCH v5 4/5] stpncpy.3, strncpy.3: Document in a single page

[Alejandro Colomar <alx.manpages@gmail.com>]

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Formatted stpncpy(3):

stpncpy(3)                 Library Functions Manual                 stpncpy(3)

NAME
        stpncpy,  strncpy  - zero a fixed‐width buffer and copy a string into a
        character sequence with truncation and zero the rest of it

LIBRARY
        Standard C library (libc, -lc)

SYNOPSIS
        #include <string.h>

        char *stpncpy(char dst[restrict .sz], const char *restrict src,
                       size_t sz);
        char *strncpy(char dst[restrict .sz], const char *restrict src,
                       size_t sz);

    Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

        stpncpy():
            Since glibc 2.10:
                _POSIX_C_SOURCE >= 200809L
            Before glibc 2.10:
                _GNU_SOURCE

DESCRIPTION
        These functions copy the string pointed to by src  into  a  null‐padded
        character sequence at the fixed‐width buffer pointer to by dst.  If the
        destination buffer, limited by its size, isn’t large enough to hold the
        copy,  the resulting character sequence is truncated.  They only differ
        in the return value.

        An implementation of these functions might be:

            char *
            stpncpy(char *restrict dst, const char *restrict src, size_t sz)
            {
                bzero(dst, sz);
                return mempcpy(dst, src, strnlen(src, sz));
            }

            char *
            strncpy(char *restrict dst, const char *restrict src, size_t sz)
            {
                stpncpy(dst, src, sz);
                return dst;
            }

RETURN VALUE
        stpncpy()
               returns a pointer to one after the last character in the  desti‐
               nation character sequence.

        strncpy()
               returns dst.

ATTRIBUTES
        For  an  explanation  of  the  terms  used in this section, see attrib‐
        utes(7).
        ┌────────────────────────────────────────────┬───────────────┬─────────┐
        │Interface                                   │ Attribute     │ Value   │
        ├────────────────────────────────────────────┼───────────────┼─────────┤
        │stpncpy(), strncpy()                        │ Thread safety │ MT‐Safe │
        └────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS
        stpncpy()
               POSIX.1‐2008.

        strncpy()
               POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD.

CAVEATS
        The name of these functions is confusing.  These  functions  produce  a
        null‐padded character sequence, not a string (see string_copy(7)).

        Truncation  should  be  determined by comparing the length of the input
        string with the size of the destination buffer.

        If you’re going to use this function in chained calls, it would be use‐
        ful to develop a similar function that accepts a pointer  to  one  past
        the end of the destination buffer instead of its size.

EXAMPLES
        #include <err.h>
        #include <stdio.h>
        #include <stdlib.h>
        #include <string.h>

        int
        main(void)
        {
            char    *end;
            char    buf1[20];
            char    buf2[20];
            size_t  len;

            if (sizeof(buf1) < strlen("Hello world!"))
                warnx("stpncpy: truncating character sequence");
            end = stpncpy(buf1, "Hello world!", sizeof(buf1));
            len = end - buf1;

            printf("[len = %zu]: ", len);
            printf("%.*s\n", (int) len, buf1);  // "Hello world!"

            if (sizeof(buf2) < strlen("Hello world!"))
                warnx("strncpy: truncating character sequence");
            strncpy(buf2, "Hello world!", sizeof(buf));
            len = strnlen(buf2, sizeof(buf2));

            printf("[len = %zu]: ", len);
            printf("%.*s\n", (int) len, buf2);  // "Hello world!"

            exit(EXIT_SUCCESS);
        }

SEE ALSO
        wcpncpy(3), string_copy(7)

Linux man‐pages (unreleased)        (date)                          stpncpy(3)

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<http://www.alejandro-colomar.es/>

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