mallinfo 不显示 mmap 分配的信息

Question

In mallinfo structure there are two fields hblks and hblkhd . The man documentation says that they are responsible for the number of blocks allocated by mmap and the total number of bytes. But when I run next code

void * ptr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
*(int *) ptr = 10;

Fields hblks and hblkhd are also zero. While the total number of free bytes in the blocks decreases. Could you please explain why this behavior is observed?

I also tried to allocate all free space and use mmap after it. But in this situation fields also equal to zero

Compiler: gcc 9.4.0 OS: Ubuntu 20.04.1

Answer 1

I did some experiments and they led me to the conclusion that this field is filled only when mmap occurred when calling malloc . A normal mmap call doesn't show up in this statistic, which is logical, because this is a system call, and the statistic is collected in user-space