C++ Malloc 不调用 mmap 或 brk？

Question

在 ubuntu 18.04 我写了下面的 c 程序运行它的输入是 100

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    int x;
    scanf("%d", &x);
    usleep(1);
    void *tmp = malloc(x);
    usleep(1);
    free(tmp);
    return 0;
}

终端中的输出是：（使用 strace）

strace ./a.out
execve("./a.out", ["./a.out"], 0x7ffd69236c10 /* 50 vars */) = 0
brk(NULL)                               = 0x55a719717000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=96020, ...}) = 0
mmap(NULL, 96020, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f7acb879000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "7ELF[=11=][=11=][=11=][=11=][=11=][=11=][=11=][=11=][=11=]>[=11=][=11=][=11=][=11=][=11=][=11=][=11=][=11=][=11=]"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2030928, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f7acb877000
mmap(NULL, 4131552, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f7acb277000
mprotect(0x7f7acb45e000, 2097152, PROT_NONE) = 0
mmap(0x7f7acb65e000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1e7000) = 0x7f7acb65e000
mmap(0x7f7acb664000, 15072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f7acb664000
close(3)                                = 0
arch_prctl(ARCH_SET_FS, 0x7f7acb8784c0) = 0
mprotect(0x7f7acb65e000, 16384, PROT_READ) = 0
mprotect(0x55a718d34000, 4096, PROT_READ) = 0
mprotect(0x7f7acb891000, 4096, PROT_READ) = 0
munmap(0x7f7acb879000, 96020)           = 0
fstat(0, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
brk(NULL)                               = 0x55a719717000
brk(0x55a719738000)                     = 0x55a719738000
read(0, 100
"100\n", 1024)                  = 4
nanosleep({tv_sec=0, tv_nsec=1000}, NULL) = 0
nanosleep({tv_sec=0, tv_nsec=1000}, NULL) = 0
lseek(0, -1, SEEK_CUR)                  = -1 ESPIPE (Illegal seek)
exit_group(0)                           = ?
+++ exited with 0 +++

这不是st运行ge吗？为什么 malloc 没有使用 brk 或 mmap 来分配内存（在两次调用 nanosleep() 之间）。这种 st运行ge 行为的原因是什么？

如果我将输入更改为 200000，我会在中间看到对 mmap 的调用。

Answer 1

Isn't this strange?

没有

why malloc didn't use brk or mmap for the allocation of memory (between the 2 calls for nanosleep()).

场景 1：您启用了优化，并且优化器发现忽略对 malloc 的调用是不可观察的，因此它从未被调用。

场景 2：malloc 被调用，但它大概不需要在此时调用 brk 或 mmap。 brk 和 mmap 之前都被调用过，可能是为了 malloc.

C++ Malloc 不调用 mmap 或 brk？

C++ Malloc Doesn't call mmap or brk?

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sbrk