Why is brk(0) called?

Question

On a first shell I run the command tail with no argument.

On a second shell I kill the first tail using strace to monitor the system calls.

qdii@nomada ~ $ strace kill 1713
execve("/bin/kill", ["kill", "1713"], [/* 82 vars */]) = 0
brk(0)                                  = 0x2533000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f9dacfe8000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3

I didn’t know the brk command so I looked it up and found that it changes the end of the data segment of the process. Why would kill do that? especially setting the data segment to 0 seems to make little sense.

Answer 1

The run-time library calls sbrk to read the current end of the data segment. sbrk is implemented on top of the brk syscall; as you can see, calling brk with an invalid address just returns the current value.