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brk, sbrk - Change space allocation
void *addr );
intptr_t incr );
The following function prototypes do not conform to current standards and
are supported only for backward compatibility:
char *addr );
ssize_t incr );
long incr );
Standard C Library (libc)
Interfaces documented on this reference page conform to industry standards
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
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
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 memory.
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 functions.
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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