NAME
brk, sbrk - change data segment size
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
int
brk(const void *addr);
void *
sbrk(intptr_t incr);
DESCRIPTION
The brk() and sbrk() functions are legacy interfaces from before the
advent of modern virtual memory management.
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.
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 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).
RETURN VALUES
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.
ERRORS
The brk() and sbrk() functions will fail if:
[EINVAL] The requested break value was beyond the beginning of
the data segment.
[ENOMEM] The data segment size limit, as set by setrlimit(2),
was exceeded.
[ENOMEM] Insufficient space existed in the swap area to support
the expansion of the data segment.
SEE ALSO
execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3)
HISTORY
The brk() function appeared in Version 7 AT&T UNIX.
BUGS
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).