brk, sbrk - change data segment size


#include <unistd.h>

int brk(void *end_data_segment);

void *sbrk(intptr_t increment);


brk() sets the end of the data segment to the value specified by end_data_segment, when that value is reasonable, the system does have enough memory and the process does not exceed its max data size (see setrlimit(2)).

sbrk() increments the program's data space by increment bytes. sbrk() isn't a system call, it is just a C library wrapper. Calling sbrk() with an increment of 0 can be used to find the current location of the program break.


On success, brk() returns zero. On error, -1 is returned, and errno is set to ENOMEM. (But see LINUX NOTES below.)

On success, sbrk() returns a pointer to the start of the new area. On error, -1 is returned, and errno is set to ENOMEM.


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

brk() and sbrk() are not defined in the C Standard and are deliberately excluded from the POSIX.1 standard (see paragraphs B. and B.8.3.3).


Various systems use various types for the parameter of sbrk(). Common are int, ssize_t, ptrdiff_t, intptr_t.


The return value described above for brk() is the behaviour provided by the glibc wrapper function for the Linux brk() system call. (On most other implementations, the return value from brk() is the same.) However, the actual Linux system call returns the new program break on success. On failure, the system call returns the current break (thus for example, the call brk(0) can be used to obtain the current break). The glibc wrapper function does some work 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.


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