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sbrk(2) [freebsd man page]

BRK(2)							      BSD System Calls Manual							    BRK(2)

NAME
brk, sbrk -- change data segment size LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #include <unistd.h> int brk(const void *addr); void * sbrk(intptr_t incr); DESCRIPTION
The brk() and sbrk() functions are legacy interfaces from before the advent of modern virtual memory management. The brk() and sbrk() functions are used to change the amount of memory allocated in a process's data segment. They do 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 brk() function sets the break to addr. The sbrk() function raises the break by incr bytes, thus allocating at least incr bytes of new memory in the data segment. If incr is nega- tive, the break is lowered by incr bytes. NOTES
While the actual process data segment size maintained by the kernel will only grow or shrink in page sizes, these functions allow 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). 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 brk() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. The sbrk() function returns the prior break value if successful; otherwise the value (void *)-1 is returned and the global variable errno is set to indicate the error. ERRORS
The brk() and sbrk() functions 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. SEE ALSO
execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3) HISTORY
The brk() function appeared in Version 7 AT&T UNIX. BUGS
Mixing brk() or 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). BSD
July 12, 1999 BSD

Check Out this Related Man Page

BRK(2)							      BSD System Calls Manual							    BRK(2)

NAME
brk, sbrk -- change data segment size SYNOPSIS
#include <unistd.h> void * brk(const void *addr); void * sbrk(int incr); DESCRIPTION
The brk and sbrk functions are historical curiosities left over from earlier days before the advent of virtual memory management. The brk() function sets the break or lowest address of a process's data segment (uninitialized data) to addr (immediately above bss). Data addressing is restricted between addr and the lowest stack pointer to the stack segment. Memory is allocated by brk in page size pieces; if addr is not evenly divisible by the system page size, it is increased to the next page boundary. The current value of the program break is reliably returned by ``sbrk(0)'' (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 the rlim_max value returned from a call to getrlimit, e.g. ``qetext + rlp->rlim_max.'' (see end(3) for the definition of etext). RETURN VALUES
Brk returns a pointer to the new end of memory if successful; otherwise -1 with errno set to indicate why the allocation failed. The sbrk function returns a pointer to the base of the new storage if successful; otherwise -1 with errno set to indicate why the allocation failed. ERRORS
Sbrk will fail and no additional memory will be allocated if one of the following are true: [ENOMEM] The limit, as set by setrlimit(2), was exceeded. [ENOMEM] The maximum possible size of a data segment (compiled into the system) was exceeded. [ENOMEM] Insufficient space existed in the swap area to support the expansion. SEE ALSO
execve(2), getrlimit(2), malloc(3), mmap(2), end(3) 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. HISTORY
A brk() function call appeared in Version 7 AT&T UNIX. 4th Berkeley Distribution December 11, 1993 4th Berkeley Distribution

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