将-1作为文件描述符传递给mmap [英] passing -1 as file descriptor to mmap

问题描述

我在FC17 Linux中对"ls"命令进行了跟踪.

I did an strace on the "ls" command in FC17 Linux.

以下是输出.

execve("/usr/bin/ls", ["ls"], [/* 48 vars */]) = 0
brk(0)                                  = 0x27c1000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc765fa6000
...

我没有得到将-1作为文件描述符传递给mmap调用的目的和结果，有人可以对此有所了解吗?

I am not getting the purpose and result of passing -1 as the file descriptor to the mmap call, can some one shed some light on this ?

推荐答案

有两种映射(将虚拟内存区域映射到进程):文件支持的映射和匿名(非文件支持的)映射.有两种方法可以请求匿名映射:

There are two kinds of mappings (areas of virtual memory mapped to a process): file-backed mappings, and anonymous (non file-backed) mappings. There are two ways to request an anonymous mapping:

• (BSD)将MAP_ANONYMOUS(以前称为MAP_ANON)传递到mmap().没有关联的文件，因此您应该将-1作为file参数传递.一些操作系统会忽略file参数，其他操作系统则将其设为-1(BSD IIRC).
• (Sys V)映射/dev/zero.在这种情况下，file显然是有意义的.

• (BSD) Pass MAP_ANONYMOUS (formerly MAP_ANON) to mmap(). There is no associated file, so you should pass -1 as file parameter. Some OSes ignore the file parameter, others require it to be -1 (BSD IIRC).
• (Sys V) Map /dev/zero. In this case, file is obviously meaningful.

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