|BRK(2)||System Calls Manual||BRK(2)|
LIBRARY¶Standard C Library (libc, -lc)
sbrk() functions are legacy interfaces from before the advent of modern virtual memory management. They are deprecated and not present on the arm64 or riscv architectures. The mmap(2) interface should be used to allocate pages instead.
sbrk() functions are used to change the amount of
memory allocated in a process's data segment. They do this by moving the
location of the “break”. The break is the first address after
the end of the process's uninitialized data segment (also known as the
brk() function sets the break to
sbrk() function raises the break by
incr bytes, thus allocating at least
incr bytes of new memory in the data segment. If
incr is negative, the break is lowered by
NOTES¶While the actual process data segment size maintained by the kernel will only grow or shrink in page sizes, these functions allow setting the break to unaligned values (i.e., it may point to any address inside the last page of the data segment).
The current value of the program break may be determined by
sbrk(0). See also
The getrlimit(2) system call may be used to
determine the maximum permissible size of the data segment. It will not be
possible to set the break beyond “etext
+ rlim.rlim_max” where
the rlim.rlim_max value is returned from a call to
&rlim). (See end(3) for the
definition of etext).
brk() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
sbrk() function returns the prior
break value if successful; otherwise the value (void
*)-1 is returned and the global variable errno
is set to indicate the error.
sbrk() functions will fail if:
SEE ALSO¶execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3)
brk() function appeared in Version 7 AT&T UNIX. FreeBSD 11.0 introduced the arm64 and riscv architectures which do not support
sbrk() with malloc(3), free(3), or similar functions will result in non-portable program behavior.
Setting the break may fail due to a temporary lack of swap space. It is not possible to distinguish this from a failure caused by exceeding the maximum size of the data segment without consulting getrlimit(2).
sbrk() is sometimes used to monitor heap
use by calling with an argument of 0. The result is unlikely to reflect
actual utilization in combination with an mmap(2) based
are not thread-safe.
|June 2, 2018||Linux 4.19.0-10-amd64|