"alloca“代表什么？

Question

alloca()函数在调用方的堆栈帧中分配内存。alloca最初代表什么？关于这个名字的词源有什么来源吗？

Answer 1

我在历史上做过一些调查，似乎"alloca“中的"a”代表“自动”，就像自动释放一样。

GNU手册页说：

There is evidence that the alloca() function appeared in 32V,
PWB, PWB.2, 3BSD, and 4BSD.  There is a man page for it in
4.3BSD.  Linux uses the GNU version.

从此提交消息链接，我找到了UNIX/32V和PWB/UNIX源代码的档案，每个源都包含一个alloca.s程序集文件：

32v/usr/src/libc/sys/alloca.s

# like alloc, but automatic
# automatic free in return

.globl  _alloca
_alloca:
    .word   0x0000
    subl2   4(ap),sp    # crude allocation
    movl    16(fp),r1   # pc
    movq    8(fp),ap    # new (old) ap and fp
    bicl2   $3,sp       # 4-byte align
    addl2   $7*4,sp     # reuse space of mscp
    movl    sp,r0       # return value
    jmp     (r1)        # funny return

spencer_pwb/sys/source/s4/util/alloca.s

/   wdptr = alloca(nbytes);
/ like alloc, but automatic
/ automatic free upon return from calling function

.globl _alloca
_alloca:
    mov (sp)+,r1    / return
    mov (sp),r0     /count
    inc r0
    bic $1,r0       / round up
    sub r0,sp       / take core
    mov sp,r0
    tst (r0)+       / returned value; will generate
                / a memory fault if there isn't enough core
    jmp (r1)
.data
    <@(#)alloca 1.1\0>

我相信引用的alloc函数将是基于PWB中的函数定义的现代malloc的前身：

spencer_pwb/sys/source/s4/alloc.c

/*      alloc - old-style memory allocator
 *      returns -1 on fail rather than 0
*/
alloc(n)
{
        register p;
        p = malloc(n);
        return(p?p:-1);
}