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brk(2) System Calls Manual brk(2) NAME brk, sbrk - Change space allocation LIBRARY Standard C Library (libc.so, libc.a) SYNOPSIS #include <unistd.h> int brk( void *addr ); void *sbrk( int incr ); The following function definitions do not conform to current standards and are supported only for backward compatibility: int brk( char *addr ); void *sbrk( ssize_t incr ); STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: brk(), sbrk(): XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry standards and associated tags. PARAMETERS Points to the effective address of the maximum available data. Speci- fies the number of bytes to be added to the current break. The value of incr may be positive or negative. DESCRIPTION The brk() function sets the lowest data segment location not used by the program (called the break) to addr. In the alternate function sbrk(), incr more bytes are added to the pro- gram's data space, and a pointer to the start of the new area is re- turned. When a program begins execution with the execve() function, the break is set at the highest location defined by the program and data storage areas. Therefore, only programs with growing data areas should need to use sbrk(). The current value of the program break is reliably returned by ``sbrk(0)''. The getrlimit() function may be used to determine the maximum permissible size of the data segment. It is not possible to set the break beyond the value returned from a call to the getrlimit() function. If the data segment was locked at the time of the brk() function, addi- tional memory allocated to the data segment by brk() will also be locked. NOTES Programmers should be aware that the concept of a current break is a historical remnant of earlier UNIX systems. Many existing UNIX pro- grams were designed using this memory model, and these programs typi- cally use the brk() or sbrk() functions to increase or decrease their available memory. The use of the mmap() function is now preferred because it can be used portably with all other memory allocation functions and with any func- tion that uses other allocation functions. RETURN VALUES Upon successful completion, the brk() function returns a value of 0 (zero), and the sbrk function returns the prior break value. If either call fails, a value of -1 is returned and errno is set to indicate the error. ERRORS If the brk() or sbrk() function fails, no additional memory is allo- cated and errno may be set to the following value: The requested change would allocate more space than allowed by the limit as returned by the getrlimit() function. RELATED INFORMATION Functions: exec(2), getrlimit(2), malloc(3), plock(2), mmap(2) Standards: standards(5) delim off brk(2)