4 * Implementation of POSIX.1-2008 conforming strnlen(), and a wrapper
5 * extending it to conform with ISO-C11 TR-24731-1 strnlen_s().
9 * Written by Keith Marshall <keithmarshall@users.sourceforge.net>
10 * Copyright (C) 2016, 2017, MinGW.org Project
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33 * This file implements a POSIX.1-2008 conforming strnlen() function, under
34 * the alternative public name of __mingw_strnlen(), (to avoid any possible
35 * conflict with Microsoft's DLL implementations, or a replacement function
36 * implemented by the user).
38 * Note that POSIX does not prescribe any error returns for its strnlen()
39 * function; in particular, it does not specify handling of a NULL pointer,
40 * passed as the string argument to count, and thus, a segmentation fault
41 * is a plausible outcome. This is consistent with the behaviour exhibited
42 * by Microsoft's implementation, and this implementation acts likewise.
44 * OTOH, ISO-C11 TR-24731-1 prescribes that strnlen_s() shall return zero,
45 * when passed a NULL pointer as the string argument; the alternative entry
46 * point, __mingw_strnlen_s(), checks for this anomaly, and acts accordingly,
47 * before continuing as for __mingw_strnlen(), when passed a valid pointer
48 * to a string argument; (this is consistent with the documented behaviour
49 * of Microsoft's strnlen_s() implementation).
51 * Note that I've chosen to implement this in assembly language to enable
52 * sharing of common code between two distinct entry points, with subtly
53 * differing behaviours, while avoiding the overhead of an extra function
54 * call to invoke such shared code; this also allows the code to exploit
55 * the CPU's string scanning instructions, (which GCC does not), and so
56 * achieves a more compact, (and likely more efficient), implementation.
59 #if defined UNICODE || defined _UNICODE
60 /* The algorithm defined herein is effectively the same as that required
61 * as a substitute for wcsnlen()/wcsnlen_s(); map entry point names to the
62 * appropriate alternatives for scanning wchar_t *, (rather than char *),
63 * strings, (i.e. Microsoft's so-called UNICODE strings).
65 # define ___mingw_strnlen ___mingw_wcsnlen
66 # define ___mingw_strnlen_s ___mingw_wcsnlen_s
68 /* The fundamental change required to scan whcar_t * strings, rather than
69 * char * strings, is that we must use the scasw instruction in place of
70 * the scasb instruction; map it accordingly.
77 .globl ___mingw_strnlen
78 .def ___mingw_strnlen; .scl 2; .type 32; .endef
81 /* Implements: size_t __mingw_strnlen (const char *string, size_t maxlen );
83 * Scans at most maxlen chars, returning the lesser of strlen (string) and
84 * maxlen; does NOT check for string == NULL, which may thus induce failure
85 * with a segmentation fault. Note that initialization of return count to
86 * zero, in EAX, also serves as the NUL char reference for SCASB, in AL.
88 pushl %edi /* must preserve this */
89 xorl %eax, %eax /* initialize return count to zero */
90 movl 8(%esp), %edx /* load address of string argument */
92 movl 12(%esp), %ecx /* load maxlen ... */
93 jecxz .L4 /* and jump to end, if it's zero */
95 cld /* scan string from low-->high address */
96 movl %edx, %edi /* using this as the scan pointer ... */
97 repne scasb /* as required by this CPU scan */
98 mov %edi, %eax /* note where we stopped ... */
99 jnz .L3 /* no NUL found; count is complete ... */
100 decl %eax /* NUL found and counted; discount it */
102 sub %edx, %eax /* compute effective count to return */
104 #if defined UNICODE || defined _UNICODE
105 /* By the time we get to here, the EAX register contains the effective
106 * length of the scanned string in bytes; this is the correct value to be
107 * returned, in the case of char * strings, but it is twice the correct
108 * length for wchar_t * strings, (possibly with one odd residual byte,
109 * left over from a partially discounted NUL wchar_t); we may adjust this,
110 * for the wchar_t * case, by a simple logical right shift, effectively
111 * dividing the even extent of the (unsigned) count by two, extracting
112 * the full wchar_t count, leaving any odd byte residual as remainder
113 * in the carry flag, where we may simply (and safely) ignore it.
115 shrl %eax /* convert byte count to wchar_t count */
118 .L4: popl %edi /* restore saved register ... */
119 ret /* and we're done */
122 .globl ___mingw_strnlen_s
123 .def ___mingw_strnlen_s; .scl 2; .type 32; .endef
126 /* Implements: size_t __mingw_strnlen_s (const char *string, size_t maxlen );
128 * Exhibits identical behaviour to __mingw_strnlen(), EXCEPT that it DOES
129 * check for string == NULL, returning zero when found.
131 pushl %edi /* must preserve this */
132 xorl %eax, %eax /* initialize return count to zero */
133 movl 8(%esp), %edx /* load address of string argument ... */
134 testl %edx, %edx /* checking for NULL pointer, and ... */
135 jnz .L1 /* proceeding as strnlen(), if not ... */
136 popl %edi /* otherwise restore saved register ... */
137 ret /* and return zero count value */
139 /* $RCSfile$: end of file */