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brk(2)                           System Calls                           brk(2)



NAME
       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  permis-
       sible  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 allo-
       cated space accordingly.

       The sbrk() function adds  incr function bytes to the  break  value  and
       changes the allocated space accordingly. The incr function can be nega-
       tive, 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.  Oth-
       erwise, 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 capa-
       bility.  The use of mmap(2) is now preferred because  it  can  be  used
       portably  with all other memory allocation functions and with any func-
       tion 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:


       tab()     allbox;     cw(2.750000i)|    cw(2.750000i)    lw(2.750000i)|
       lw(2.750000i).  ATTRIBUTE TYPEATTRIBUTE VALUE MT-LevelMT-Safe


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 nec-
       essary 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().




SunOS 5.10                        14 Jan 1997                           brk(2)