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Man Pages
BRK(2) FreeBSD System Calls Manual BRK(2)

brk, sbrk
change data segment size

Standard C Library (libc, -lc)

#include <unistd.h>
int
brk(const void *addr);
void *
sbrk(intptr_t incr);

The brk() and 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.
The brk() and 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 “BSS”).
The brk() function sets the break to addr.
The 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 incr bytes.

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 calling sbrk(0). See also end(3).
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 getrlimit(RLIMIT_DATA, &rlim). (See end(3) for the definition of etext).

The 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.
The 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.

The brk() and sbrk() functions will fail if:
[]
The requested break value was beyond the beginning of the data segment.
[]
The data segment size limit, as set by setrlimit(2), was exceeded.
[]
Insufficient space existed in the swap area to support the expansion of the data segment.

execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3)

The brk() function appeared in Version 7 AT&T UNIX. FreeBSD 11.0 introduced the arm64 and riscv architectures which do not support brk() or sbrk().

Mixing brk() or 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 malloc.
brk() and sbrk() are not thread-safe.
June 2, 2018 FreeBSD 12.0-RELEASE

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