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SBRK(2)                  DragonFly System Calls Manual                 SBRK(2)


NAME

     sbrk -- change data segment size (obsolete)


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <sys/types.h>
     #include <unistd.h>

     void *
     sbrk(intptr_t incr);


DESCRIPTION

     The sbrk() function is a legacy interface from before the advent of
     modern virtual memory management.  sbrk() has only limited functionality
     due to having to play nice with modern system calls such as mmap(2).

     The sbrk() function is used to change the amount of memory allocated in a
     process's data segment.  It does this by moving the location of the
     ``break''.  The break is the first address after the end of the process's
     uninitialized data segment (also known as the ``BSS'').

     The break range is limited by the RLIMIT_DATA resource limit applied to
     the process.

     The sbrk() function raises the break by incr bytes, returning a pointer
     to the base of the new memory.  Any attempt to lower the break point will
     return (void *)-1 and set errno to EOPNOTSUPP.


NOTES

     While the actual process data segment size maintained by the kernel will
     only grow or shrink in page sizes, this function allows setting the break
     to unaligned values (i.e., it may point to any address inside the last
     page of the data segment).

     The current value of the program break may be determined by calling
     sbrk(0).

     The sbrk() function is thread-safe.  See also end(3).

     The getrlimit(2) system call may be used to determine the maximum
     permissible size of the data segment.  It will not be possible to set the
     break beyond ``etext + rlim.rlim_max'' where the rlim.rlim_max value is
     returned from a call to getrlimit(RLIMIT_DATA, &rlim).  (See end(3) for
     the definition of etext).


RETURN VALUES

     The sbrk() function returns the prior break pointer if successful;
     otherwise the value (void *)-1 is returned and the global variable errno
     is set to indicate the error.


ERRORS

     sbrk() will fail if:

     [EINVAL]           The requested break value was beyond the beginning of
                        the data segment.

     [ENOMEM]           The data segment size limit, as set by setrlimit(2),
                        was exceeded.

     [ENOMEM]           Insufficient space existed in the swap area to support
                        the expansion of the data segment.

     [EOPNOTSUPP]       An attempt has been made to perform an action that is
                        no longer supported by this function.


SEE ALSO

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


BUGS

     Mixing sbrk() with malloc(3), free(3), or similar functions will result
     in non-portable program behavior.

     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(2).

DragonFly 5.5                    March 1, 2019                   DragonFly 5.5

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