NAME

brk, sbrk - change data segment size

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

#include <unistd.h>

int brk(void *addr);
void *sbrk(intptr_t increment);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

brk(), sbrk():

    Since glibc 2.19:
        _DEFAULT_SOURCE
            || ((_XOPEN_SOURCE >= 500) &&
                ! (_POSIX_C_SOURCE >= 200112L))
    From glibc 2.12 to glibc 2.19:
        _BSD_SOURCE || _SVID_SOURCE
            || ((_XOPEN_SOURCE >= 500) &&
                ! (_POSIX_C_SOURCE >= 200112L))
    Before glibc 2.12:
        _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION

brk() and sbrk() change the location of the program break, which defines the end of the process's data segment (i.e., the program break is the first location after the end of the uninitialized data segment). Increasing the program break has the effect of allocating memory to the process; decreasing the break deallocates memory.

brk() sets the end of the data segment to the value specified by addr, when that value is reasonable, the system has enough memory, and the process does not exceed its maximum data size (see setrlimit(2)).

sbrk() increments the program's data space by increment bytes. Calling sbrk() with an increment of 0 can be used to find the current location of the program break.

RETURN VALUE

On success, brk() returns zero. On error, -1 is returned, and errno is set to ENOMEM.

On success, sbrk() returns the previous program break. (If the break was increased, then this value is a pointer to the start of the newly allocated memory). On error, (void *) -1 is returned, and errno is set to ENOMEM.

STANDARDS

None.

HISTORY

4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001.

NOTES

Avoid using brk() and sbrk(): the malloc(3) memory allocation package is the portable and comfortable way of allocating memory.

Various systems use various types for the argument of sbrk(). Common are int, ssize_t, ptrdiff_t, intptr_t.

C library/kernel differences

The return value described above for brk() is the behavior provided by the glibc wrapper function for the Linux brk() system call. (On most other implementations, the return value from brk() is the same; this return value was also specified in SUSv2.) However, the actual Linux system call returns the new program break on success. On failure, the system call returns the current break. The glibc wrapper function does some work (i.e., checks whether the new break is less than addr) to provide the 0 and -1 return values described above.

On Linux, sbrk() is implemented as a library function that uses the brk() system call, and does some internal bookkeeping so that it can return the old break value.