brk, sbrk - Change space allocation
void *addr ); void *sbrk(
intptr_t incr );
The following function prototypes do not conform to current
standards and are supported only for backward compatibility:
char *addr ); void *sbrk(
ssize_t incr ); void *sbrk(
long incr );
Standard C Library (libc)
Interfaces documented on this reference page conform to
industry standards as follows:
brk(), sbrk(): XSH4.2
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Points to the effective address of the maximum available
data. Specifies the number of bytes to be added to the
current break. The value of incr may be positive or negative.
The brk() function sets the lowest data segment location
not used by the program (called the break) to addr.
In the alternate function sbrk(), incr more bytes are
added to the program's data space, and a pointer to the
start of the new area is returned.
When a program begins execution with the execve() function,
the break is set at the highest location defined by
the program and data storage areas. Therefore, only programs
with growing data areas should need to use sbrk().
The current value of the program break is reliably
returned by the call sbrk(0). The getrlimit() function may
be used to determine the maximum permissible size of the
data segment. It is not possible to set the break beyond
the value returned from a call to the getrlimit() function.
If the data segment was locked at the time of the brk()
function, additional memory allocated to the data segment
by brk() will also be locked.
Programmers should be aware that the concept of a current
break is a historical remnant of earlier UNIX systems.
Many existing UNIX programs were designed using this memory
model, and these programs typically use the brk() or
sbrk() functions to increase or decrease their available
The use of the mmap() function is now preferred because it
can be used portably with all other memory allocation
functions and with any function that uses other allocation
Upon successful completion, the brk() function returns a
value of 0 (zero), and the sbrk() function returns the
prior break value. If either call fails, a value of -1 is
returned and errno is set to indicate the error.
If the brk() or sbrk() function fails, no additional memory
is allocated and errno is set to one of the following
values: [Tru64 UNIX] Indicates an attempt to set a break
value to less than the initial value at program startup.
The requested change would allocate more space than
allowed by the limit as returned by the getrlimit() function.
Functions: exec(2), getrlimit(2), mmap(2), plock(2), malloc(3)
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