3.2.3 System Calls: brk() and sbrk()
The four routines we've covered (malloc(), calloc(), realloc(), and free()) are the standard, portable functions to use for dynamic memory management.
On Unix systems, the standard functions are implemented on top of two additional, very primitive routines, which directly change the size of a process's address space. We present them here to help you understand how GNU/Linux and Unix work ("under the hood" again); it is highly unlikely that you will ever need to use these functions in a regular program. They are declared as follows:
#include Common
#include /* Necessary for GLIBC 2 systems */
int brk(void *end_data_segment);
void *sbrk(ptrdiff_t increment);
The brk() system call actually changes the process's address space. The address is a pointer representing the end of the data segment (really the heap area, as shown earlier in Figure ). Its argument is an absolute logical address representing the new end of the address space. It returns 0 on success or -1 on failure.
The sbrk() function is easier to use; its argument is the increment in bytes by which to change the address space. By calling it with an increment of 0, you can determine where the address space currently ends. Thus, to increase your address space by 32 bytes, use code like this:
char *p = (char *) sbrk(0); /* get current end of address space */
if (brk(p + 32) < 0) {
/* handle error */
}
/* else, change worked */
Practically speaking, you would not use brk() directly. Instead, you would use sbrk() exclusively to grow (or even shrink) the address space. (We show how to do this shortly, in Section 3.2.5, "Address Space Examination"," page 78.)
Even more practically, you should never use these routines. A program using them can't then use malloc() also, and this is a big problem, since many parts of the standard library rely on being able to use malloc(). Using brk() or sbrk() is...