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sbrk(2)


NAME

   brk, sbrk - change data segment size

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 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.

CONFORMING TO

   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.

SEE ALSO

   execve(2), getrlimit(2), end(3), malloc(3)

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.



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