brk

, sbrk

- change the amount of space allocated for the calling process's data segment

Synopsis

#include <unistd.h>

int brk(void *endds);

void *sbrk(intptr_t incr);

Description

The brk() and sbrk() functions are used to change dynamically the amount of space allocated for the calling process's data segment (see exec(2)). The change is made by resetting the process's break value and allocating the appropriate amount of space. The break value is the address of the first location beyond the end of the data segment. The amount of allocated space increases as the break value increases. Newly allocated space is set to zero. If, however, the same memory space is reallocated to the same process its contents are undefined.

When a program begins execution using execve() the break is set at the highest location defined by the program and data storage areas.

The getrlimit(2) function may be used to determine the maximum permissible size of the data segment; it is not possible to set the break beyond the rlim_max value returned from a call to getrlimit(), that is to say, “end + rlim.rlim_max.” See end(3C).

The brk() function sets the break value to endds and changes the allocated space accordingly.

The sbrk() function adds incr function bytes to the break value and changes the allocated space accordingly. The incr function can be negative, in which case the amount of allocated space is decreased.

Return Values

Upon successful completion, brk() returns 0. Otherwise, it returns -1 and sets errno to indicate the error.

Upon successful completion, sbrk() returns the prior break value. Otherwise, it returns (void *)-1 and sets errno to indicate the error.

Errors

The brk() and sbrk() functions will fail and no additional memory will be allocated if:

ENOMEM

The data segment size limit as set by setrlimit() (see getrlimit(2)) would be exceeded; the maximum possible size of a data segment (compiled into the system) would be exceeded; insufficient space exists in the swap area to support the expansion; or the new break value would extend into an area of the address space defined by some previously established mapping (see mmap(2)).

EAGAIN

Total amount of system memory available for private pages is temporarily insufficient. This may occur even though the space requested was less than the maximum data segment size (see ulimit(2)).

Usage

The behavior of brk() and sbrk() is unspecified if an application also uses any other memory functions (such as malloc(3C), mmap(2), free(3C)). The brk() and sbrk() functions have been used in specialized cases where no other memory allocation function provided the same capability. The use of mmap(2) is now preferred because it can be used portably with all other memory allocation functions and with any function that uses other allocation functions.

It is unspecified whether the pointer returned by sbrk() is aligned suitably for any purpose.

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

exec(2), getrlimit(2), mmap(2), shmop(2), ulimit(2), end(3C), free(3C), malloc(3C)

Notes

The value of incr may be adjusted by the system before setting the new break value. Upon successful completion, the implementation guarantees a minimum of incr bytes will be added to the data segment if incr is a positive value. If incr is a negative value, a maximum of incr bytes will be removed from the data segment. This adjustment may not be necessary for all machine architectures.

The value of the arguments to both brk() and sbrk() are rounded up for alignment with eight-byte boundaries.

Bugs

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().