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.

SEE ALSO

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

Linux man-pages 6.7                                2023-10-31                                             brk(2)