sbrk(), brk() 함수

malloc(), new()
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시스템 콜
-> brk() - sbrk()

데이터 세그먼트의 영역을 넓혀준다. (세그먼트 끝을 연장)

 

 

- man 페이지 -BRK(2) Linux Programmer's Manual BRK(2) NAME brk, sbrk - change data segment size SYNOPSIS #include <unistd.h> int brk(void *addr); void *sbrk(intptr_t increment); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): brk(), sbrk(): Since glibc 2.12: _BSD_SOURCE || _SVID_SOURCE || (_XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) && !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) Before glibc 2.12: _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED DESCRIPTION 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; decreas‐ ing the break deallocates memory. brk() sets the end of the data segment to the value specified by addr, when that value is reason‐ able, the system has enough memory, and the process does not exceed its maximum data size (see setr‐ limit(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 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 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. CONFORMING TO 4.3BSD; SUSv1, marked LEGACY in SUSv2, removed in POSIX.1-2001. NOTES Avoid using brk() and sbrk(): the malloc(3) memory allocation package is the portable and comfort‐ able way of allocating memory. Various systems use various types for the argument of sbrk(). Common are int, ssize_t, ptrdiff_t, intptr_t. Linux Notes 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 execve(2), getrlimit(2), end(3), malloc(3) COLOPHON This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

 

 

 

brk(), sbrk()의 반환형의 차이를 보면

 

 ->

 

man 페이지에도 나왔듯이 brk()는 성공시 0, 실패시 -1을 반환하고 sbrk()는 성공시 세그먼트의 break주소(마지막 경계선?)을 반환하며 실패시 -1을 반환한다.

 

 

인자값을 살펴보면

- brk()의 경우에는 brk(0x8efe000)처럼 사용해서 0x8efe000까지 할당하는 것이고

(만약 세그먼트의 break주소가 이미 0x8efe000을 넘은 경우에는 아무일도 일어나지 않는다)

 

- sbrk()이 경우에는 sbrk(0x1000)처럼 사용해서 이전 세그먼트에서 + 0x1000만큼 해주는 것이다.

 

 

데이터 세그먼트의 최하위 주소를 얻을때는 sbrk(0)을 호출해서 얻는다 .

 

 

malloc(1000)을 해도 brk() 시스템 콜이 사용됩니다. ()

// sbrk(1000)은 경우에는 정확히 1000을 할당할텐데 malloc(1000)으로 하면 좀더 많은 메모리를 반환해서 나중에 다시 brk호출 되는 경우는 방지해서 시스템 성능을 향상 -