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BRK(2)			 DragonFly System Calls Manual			BRK(2)

NAME

brk, sbrk -- change data segment size (obsolete)

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. brk() is no longer imple- mented by DragonFly and sbrk() has only limited functionality due to hav- ing to play nice with modern system calls such as mmap(2). 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 loca- tion 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 brk() function sets the break to addr. DragonFly no longer imple- ments this function. The sbrk() function raises the break by incr bytes, returning a pointer to the base of the new memory. In the traditional call, a negative incr lowers the break address by the specified number of bytes. However, DragonFly no longer supports using this legacy function to lower the break address. The reason is because the resource limit can be adjusted upward and downward at run-time and indirectly allow normal memory-mappings via mmap() to infiltrate the tra- ditional data area. In addition, lowering the break address in this man- ner is not thread safe. 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, 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). The sbrk() function is thread-safe. See also end(3). The getrlimit(2) system call may be used to determine the maximum permis- sible 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 pointer if successful; other- wise the value (void *)-1 is returned and the global variable errno is set to indicate the error.

ERRORS

brk() or 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)

HISTORY

A brk() function call 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). DragonFly 5.5 July 12, 1999 DragonFly 5.5

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