58 多次基于 malloc 分配内存的一些现象分析-CFANZ编程社区

问题来自于文章内存分配的小例子请教

内存分配的小例子请教 - 内核源码-Chinaunix

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malloc 虚拟内存分配的调试(1)

malloc 虚拟内存分配的调试(2)

printf 的调试

测试用例

主题是这一系列 malloc

#include<stdio.h>

int main(int argc, char **argv) {

    void *p, *p1, *p2, *p3, *p4, *p5, *p6, *p7;

    printf(" sbrk(0) is 0x%02x \n", sbrk(0));

    p = malloc(1024);
    printf(" sbrk(0) is 0x%02x after malloc  \n", sbrk(0));

    p1 = malloc(2 * 1024);
    p2 = malloc(4 * 1024);
    p3 = malloc(8 * 1024);
    p4 = malloc(16 * 1024);
    p5 = malloc(32 * 1024);
    p6 = malloc(64 * 1024);
    p7 = malloc(128 * 1024);

    printf(" p is 0x%02x  \n", p);
    printf(" p1 is 0x%02x, p2 is 0x%02x, p3 is 0x%02x  \n", p1, p2, p3);
    printf(" p4 is 0x%02x, p5 is 0x%02x, p6 is 0x%02x  \n", p4, p5, p6);
    printf(" p7 is 0x%02x  \n", p7);

    printf(" sbrk(0) is 0x%02x at final  \n", sbrk(0));

    return 0;

}

为什么 p1 和 p0 差 1040, 而不是 1024
为什么 p7 malloc 128K 后地址是在远端 0xf7fcd010
为什么一系列 malloc 之后 brk 依然在 0x623000
反应伙伴系统的分配策略?

执行结果如下

root@ubuntu:~/linux/tmp# ./Test15MallocManyTimes
 sbrk(0) is 0x602000
 sbrk(0) is 0x623000 after malloc
 p is 0x602420
 p1 is 0x602830, p2 is 0x603040, p3 is 0x604050
 p4 is 0x606060, p5 is 0x60a070, p6 is 0x612080
 p7 is 0xf7fcd010
 sbrk(0) is 0x623000 at final

strace 执行观察系统调用日志如下

root@ubuntu:~/linux/tmp# strace ./Test15MallocManyTimes
execve("./Test15MallocManyTimes", ["./Test15MallocManyTimes"], [/* 23 vars */]) = 0
brk(NULL)                               = 0x602000
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)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=25854, ...}) = 0
mmap(NULL, 25854, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7ffff7ff1000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0`\t\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7ff0000
mmap(NULL, 3971488, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7ffff7a0d000
mprotect(0x7ffff7bcd000, 2097152, PROT_NONE) = 0
mmap(0x7ffff7dcd000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7ffff7dcd000
mmap(0x7ffff7dd3000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ffff7dd3000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7fef000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7fee000
arch_prctl(ARCH_SET_FS, 0x7ffff7fef700) = 0
mprotect(0x7ffff7dcd000, 16384, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ)     = 0
mprotect(0x7ffff7ffc000, 4096, PROT_READ) = 0
munmap(0x7ffff7ff1000, 25854)           = 0
brk(NULL)                               = 0x602000
fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 0), ...}) = 0
brk(0x623000)                           = 0x623000
write(1, " sbrk(0) is 0x602000 \n", 22 sbrk(0) is 0x602000
) = 22
write(1, " sbrk(0) is 0x623000 after mallo"..., 36 sbrk(0) is 0x623000 after malloc
) = 36
mmap(NULL, 135168, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ffff7fcd000
write(1, " p is 0x602420  \n", 17 p is 0x602420
)      = 17
write(1, " p1 is 0x602830, p2 is 0x603040,"..., 50 p1 is 0x602830, p2 is 0x603040, p3 is 0x604050
) = 50
write(1, " p4 is 0x606060, p5 is 0x60a070,"..., 50 p4 is 0x606060, p5 is 0x60a070, p6 is 0x612080
) = 50
write(1, " p7 is 0xf7fcd010  \n", 20 p7 is 0xf7fcd010
)   = 20
write(1, " sbrk(0) is 0x623000 at final  \n", 32 sbrk(0) is 0x623000 at final
) = 32
exit_group(0)                           = ?
+++ exited with 0 +++