What is RLIMIT_DATA?
RLIMIT_DATA limits the maximum size of the process’s data segment in bytes. The data segment is the sum of three memory regions: the initialized data segment (global variables with explicit initial values), the uninitialized data segment (BSS — global variables initialised to zero), and the heap (dynamic memory from malloc/new).
What the Data Segment Contains
int x = 42; at file scope.int arr[1000]; at file scope. The kernel zero-fills these at startup.When the process tries to extend its data segment — via sbrk() or brk() — beyond the soft RLIMIT_DATA limit, the call fails with ENOMEM. In practice, this means malloc() and calloc() return NULL.
Code Example — RLIMIT_DATA and Heap Growth
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/resource.h>
int main(void)
{
struct rlimit rl;
void *ptr;
int alloc_count = 0;
/* Set data segment limit to 8 MB */
rl.rlim_cur = 8 * 1024 * 1024;
rl.rlim_max = 8 * 1024 * 1024;
if (setrlimit(RLIMIT_DATA, &rl) == -1) {
perror("setrlimit"); exit(1);
}
printf("Data segment limit set to 8 MB\n");
/* Keep allocating 1 MB chunks until malloc fails */
while (1) {
ptr = malloc(1024 * 1024); /* 1 MB */
if (ptr == NULL) {
printf("malloc failed after %d MB (errno=%d: %s)\n",
alloc_count, errno,
errno == ENOMEM ? "ENOMEM" : "other");
break;
}
alloc_count++;
printf("Allocated chunk %d (total ~%d MB)\n",
alloc_count, alloc_count);
}
return 0;
}
/* Compile: gcc -o rlimit_data rlimit_data.c
Note: actual failure point may vary due to existing data segment size */Interview Questions
The initialized data segment (.data) — global/static variables with explicit initial values. The uninitialized data segment (.bss) — global/static variables zero-initialized by the kernel. The heap — dynamically allocated memory from malloc()/calloc()/new. All three are summed together for the RLIMIT_DATA check.
sbrk() and brk() fail with ENOMEM. Since malloc(), calloc(), and realloc() use these internally, they return NULL when the data segment limit is hit.
RLIMIT_DATA limits only the data segment (initialized data + BSS + heap). RLIMIT_AS limits the entire virtual address space of the process, including all the above plus text, stack, memory-mapped files, and shared memory. RLIMIT_AS is the broader, more complete constraint.
No. The stack is a separate segment and is governed by RLIMIT_STACK, not RLIMIT_DATA.