Hi,
1. malloc can alloc more memory that Linux really has
2. Try to start program from subsection 6.
You can test that this program can alloc more memory that really kernel
has (memory+swap).
Result of this program is unpredictable.
We saw diagnostics "Killed", "Segmentation fault", "no pages for
init/syslogd/kerneld" and so on.
We tried different releases 2.0.35, 2.5.2-22 and some others.
3. Keywords: memory allocation, paging, swapping
4. For examples:
Linux version 2.0.35 (root@...) (gcc version 2.7.2.3) #6 Wed Sep 16
16:48:13 MSD 1998
Linux version 2.2.14 (root@...) (gcc version egcs-2.91.66
19990314/Linux (egcs-1.1.2 release)) #15 Mon Jul 3 20:25:24 MSD 2000
5. no
6. Test program:
/*
Malloc test
Author: I.V. Oblakov
*/
#include <stdio.h>
#include <malloc.h>
int main(int argc, char **argv)
{
char *p, *p1;
int m = 0, m1;
int size = 4096; /* we begin from request 4í */
/* find maximum possible size for malloc */
while(size){
if(p=malloc((m+size)*1024)){
m+=size; free(p);
}else{
size >>= 1;
}
}
fprintf(stderr," Size is %dKb\n", m);
/* divide our request, but use request many times */
m1 = m/2+m/4;
fprintf(stderr," New size is %dKb\n", m1*2); /* we try to use 150%
of free memory */
/* you can do it more than 2 times !! */
p = malloc(m1*1024); p1 = malloc(m1*1024);
if(p!=NULL && p1 !=NULL){
fprintf(stderr,"Begin memset\n"); /* Yes we see that kernel
give us 150% of "free" memory */
memset(p, 0, m1*1024); /* try to use our memory */
memset(p1, 0, m1*1024); /* you can repeat it with p2... */
}
return 0; /* no chance */
}
7. Different environment. Total free space must be less than malloc can
potentially return (2Gb).
8. Linux has not normal system for paging reservation.
Best regards,
Igor Oblakov
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