c - mmap: map_anonymous 为什么它给 SIGSEGV？

Question

为什么这段代码会报segmentation fault？

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <string.h>

int main()
{
    void *ptr;

    ptr=mmap(NULL, 10, PROT_READ|PROT_WRITE, MAP_ANONYMOUS, -1, 0);
    strcpy(ptr, "Hello");

}

或者更好的是，我希望:char *ptr=malloc(10); 然后将此参数传递给 mmap。两者都给出 SIGSEGV。

Answer 1

检查系统调用的返回值!

mmap 的flags 参数必须恰好具有以下两个选项之一:

MAP_SHARED
  Share  this mapping.  Updates to the mapping are visible to other processes
  that map this file, and are carried through to the underlying file. The file
  may not actually  be updated until msync(2) or munmap() is called.

MAP_PRIVATE
  Create  a private copy-on-write mapping.  Updates to the mapping are not
  visible to other processes mapping the same file, and are not carried through
  to the underlying file.   It is  unspecified whether changes made to the file
  after the mmap() call are visible in the mapped region.

你没有提供它，所以 mmap 很可能失败(返回 (void*)-1)，errno 设置为EINVAL.