brk(2) — Linux manual page


brk(2)                     System Calls Manual                    brk(2)

NAME         top

       brk, sbrk - change data segment size

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <unistd.h>

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

   Feature Test Macro Requirements for glibc (see

       brk(), sbrk():
           Since glibc 2.19:
                   || ((_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         top

       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         top

       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         top


HISTORY         top

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

NOTES         top

       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         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                         brk(2)

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