why I dont see malloc and free systemcall when run strace on stress command

The --vm flag of stress command says that, > -m, --vm N spawn N workers spinning on malloc()/free() I wanted to see what it is actually doing so I ran this command, strace stress --vm 1 I get the following output, execve("/bin/stress", ["stress", "--vm", "1"], [/* 27 vars */]) = 0 brk(NULL) = 0x55f05bef2000 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fce9f15a000 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=40388, ...}) = 0 mmap(NULL, 40388, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fce9f150000 close(3) = 0 open("/lib64/libm.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\0pS\0\0\0\0\0\0"..., 832) = 832 fstat(3, {st_mode=S_IFREG|0755, st_size=1139680, ...}) = 0 mmap(NULL, 3150136, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fce9ec38000 mprotect(0x7fce9ed39000, 2093056, PROT_NONE) = 0 mmap(0x7fce9ef38000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x100000) = 0x7fce9ef38000 close(3) = 0 open("/lib64/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\0P%\2\0\0\0\0\0"..., 832) = 832 fstat(3, {st_mode=S_IFREG|0755, st_size=2173512, ...}) = 0 mmap(NULL, 3981792, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fce9e86b000 mprotect(0x7fce9ea2e000, 2093056, PROT_NONE) = 0 mmap(0x7fce9ec2d000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c2000) = 0x7fce9ec2d000 mmap(0x7fce9ec33000, 16864, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fce9ec33000 close(3) = 0 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fce9f14f000 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fce9f14d000 arch_prctl(ARCH_SET_FS, 0x7fce9f14d740) = 0 mprotect(0x7fce9ec2d000, 16384, PROT_READ) = 0 mprotect(0x7fce9ef38000, 4096, PROT_READ) = 0 mprotect(0x55f05abdb000, 4096, PROT_READ) = 0 mprotect(0x7fce9f15b000, 4096, PROT_READ) = 0 munmap(0x7fce9f150000, 40388) = 0 getpid() = 5305 fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0 mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fce9f159000 write(1, "stress: info: dispatching"..., 64stress: info: dispatching hogs: 0 cpu, 0 io, 1 vm, 0 hdd ) = 64 clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fce9f14da10) = 5306 wait4(-1 The output just stuck here and I don't see any malloc() calls either. How can I see it making malloc() and free() systemcalls?