multiple malloc calls internally calling mmap only once

Question

When I try the below code I am not clearly able to analyze malloc api internal calls.What I am not clear is about the system call mmap is called only once for 2 or more malloc calls.If I am assigning more then 4069 bytes also it is calling only one mmap internally(trace is identified by using strace -p processid ).

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

main()
{
int *p,*q;
sleep(20);
p=malloc(5096);
printf("p=%p\n",p);
q=malloc(4096);
printf("q=%p\n",q);
sleep(2);
return 0;
}

strace OUTPUT:

root@TEST:/home/harish# strace  -p 6109
Process 6109 attached
restart_syscall(<... resuming interrupted call ...>
) = 0
brk(0)                                  = 0xeca000
brk(0xeec000)                           = 0xeec000
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 14), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f10b7bc7000
write(1, "p=0xeca010\n", 11)            = 11
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({20, 0},
0x7ffc34a51790)      = 0
write(1, "q=0xecb020\n", 11)            = 11
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({2, 0}, 0x7ffc34a51790)       = 0
exit_group(0)                           = ?
+++ exited with 0 +++

What I am looking is ,if malloc is used more then once will it call more then one mmap since memory is exceeding in two malloc's beyond 4096

Answer 1

malloc() does not result into mmap() call. Generally it would result into brk(). However, not each call will result into brk(). It depends a lot on currently allocated pages, asked memory and other things.

Answer 2

Your process' internal heap (accessed via malloc, free and realloc) manages memory as it sees fit - this includes:

• growing the heap by large or fixed increments to amortize the cost of expensive brk/sbrk syscalls over multiple (de)allocations
• dealing with smaller (de)allocations inside that heap area itself
• managing (de)fragmentation of allocated records

It's also common to use different mechanisms for large and small allocations, for example small objects are allocated from that contiguous area managed by brk/sbrk, but individual large objects may be allocated directly with mmap.