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/* copy no more than N bytes from SRC to DEST, returning the address of
the terminating '\0' in DEST.
For Intel 80x86, x>=3.
Copyright (C) 1994-2025 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <sysdep.h>
#include "asm-syntax.h"
#define PARMS 4+4 /* space for 1 saved reg */
#define RTN PARMS
#define DEST RTN
#define SRC DEST+4
#define LEN SRC+4
.text
ENTRY (__stpncpy)
pushl %esi
cfi_adjust_cfa_offset (4)
movl DEST(%esp), %eax
movl SRC(%esp), %esi
cfi_rel_offset (esi, 0)
movl LEN(%esp), %ecx
subl %eax, %esi /* magic: reduce number of loop variants
to one using addressing mode */
jmp L(1) /* jump to loop "head" */
ALIGN(4)
/* Four times unfolded loop with two loop counters. We get the
third value (the source address) by using the index+base
addressing mode. */
L(2): movb (%eax,%esi), %dl /* load current char */
movb %dl, (%eax) /* and store it */
testb %dl, %dl /* was it NUL? */
jz L(7) /* yes, then exit */
movb 1(%eax,%esi), %dl /* load current char */
movb %dl, 1(%eax) /* and store it */
testb %dl, %dl /* was it NUL? */
jz L(6) /* yes, then exit */
movb 2(%eax,%esi), %dl /* load current char */
movb %dl, 2(%eax) /* and store it */
testb %dl, %dl /* was it NUL? */
jz L(5) /* yes, then exit */
movb 3(%eax,%esi), %dl /* load current char */
movb %dl, 3(%eax) /* and store it */
testb %dl, %dl /* was it NUL? */
jz L(4) /* yes, then exit */
addl $4, %eax /* increment loop counter for full round */
L(1): subl $4, %ecx /* still more than 4 bytes allowed? */
jae L(2) /* yes, then go to start of loop */
/* The maximal remaining 15 bytes are not processed in a loop. */
addl $4, %ecx /* correct above subtraction */
jz L(9) /* maximal allowed char reached => go to end */
movb (%eax,%esi), %dl /* load current char */
movb %dl, (%eax) /* and store it */
testb %dl, %dl /* was it NUL? */
jz L(3) /* yes, then exit */
incl %eax /* increment pointer */
decl %ecx /* decrement length counter */
jz L(9) /* no more allowed => exit */
movb (%eax,%esi), %dl /* load current char */
movb %dl, (%eax) /* and store it */
testb %dl, %dl /* was it NUL? */
jz L(3) /* yes, then exit */
incl %eax /* increment pointer */
decl %ecx /* decrement length counter */
jz L(9) /* no more allowed => exit */
movb (%eax,%esi), %dl /* load current char */
movb %dl, (%eax) /* and store it */
testb %dl, %dl /* was it NUL? */
jz L(3) /* yes, then exit */
incl %eax /* increment pointer */
jmp L(9) /* we don't have to test for counter underflow
because we know we had a most 3 bytes
remaining => exit */
/* When coming from the main loop we have to adjust the pointer. */
L(4): decl %ecx /* decrement counter */
incl %eax /* increment pointer */
L(5): decl %ecx /* increment pointer */
incl %eax /* increment pointer */
L(6): decl %ecx /* increment pointer */
incl %eax /* increment pointer */
L(7):
addl $3, %ecx /* correct pre-decrementation of counter
at the beginning of the loop; but why 3
and not 4? Very simple, we have to count
the NUL char we already wrote. */
jz L(9) /* counter is also 0 => exit */
/* We now have to fill the rest of the buffer with NUL. This
is done in a tricky way. Please note that the addressing mode
used below is not the same we used above. Here we use the
%ecx register. */
L(8):
movb $0, (%ecx,%eax) /* store NUL char */
L(3): decl %ecx /* all bytes written? */
jnz L(8) /* no, then again */
L(9): popl %esi /* restore saved register content */
cfi_adjust_cfa_offset (-4)
cfi_restore (esi)
ret
END (__stpncpy)
libc_hidden_def (__stpncpy)
#ifndef __stpncpy
weak_alias (__stpncpy, stpncpy)
#endif
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