Get/Set process resource limits in C
Last Updated :
17 Dec, 2018
The getrlimit() and setrlimit() system calls can be used to get and set the resource limits such as files, CPU, memory etc. associated with a process.
Each resource has an associated soft and hard limit.
- soft limit: The soft limit is the actual limit enforced by the kernel for the corresponding resource.
- hard limit: The hard limit acts as a ceiling for the soft limit.
The soft limit ranges in between 0 and hard limit.
The two limits are defined by the following structure
struct rlimit {
rlim_t rlim_cur;
rlim_t rlim_max;
};
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The signatures of the system calls are
int getrlimit(int resource, struct rlimit *rlim);
int setrlimit(int resource, const struct rlimit *rlim);
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resource refers to the resource limits you want to retrieve or modify.
To set both the limits, set the values with the new values to the elements of rlimit structure.
To get both the limits, pass the address of rlim. Successful call to getrlimit(), sets the rlimit elements to the limits.
On success, both return 0. On error, -1 is returned, and errno is set appropriately.
Here, is a program demonstrating the system calls by changing the value one greater than the maximum file descriptor number to 3.
#include <stdio.h>
#include <sys/resource.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main() {
struct rlimit old_lim, lim, new_lim;
if( getrlimit(RLIMIT_NOFILE, &old_lim) == 0)
printf("Old limits -> soft limit= %ld \t"
" hard limit= %ld \n", old_lim.rlim_cur,
old_lim.rlim_max);
else
fprintf(stderr, "%s\n", strerror(errno));
lim.rlim_cur = 3;
lim.rlim_max = 1024;
if(setrlimit(RLIMIT_NOFILE, &lim) == -1)
fprintf(stderr, "%s\n", strerror(errno));
if( getrlimit(RLIMIT_NOFILE, &new_lim) == 0)
printf("New limits -> soft limit= %ld "
"\t hard limit= %ld \n", new_lim.rlim_cur,
new_lim.rlim_max);
else
fprintf(stderr, "%s\n", strerror(errno));
return 0;
}
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Output:
Old limits -> soft limit= 1048576 hard limit= 1048576
New limits -> soft limit= 3 hard limit= 1024
The Old limits values may vary depending upon the system.
Now, If you try to open a new file, it will show run time error, because maximum 3 files can be opened and that are already being opened by the system(STDIN, STDOUT, STDERR).
#include <stdio.h>
#include <sys/resource.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main() {
struct rlimit old_lim, lim, new_lim;
if( getrlimit(RLIMIT_NOFILE, &old_lim) == 0)
printf("Old limits -> soft limit= %ld \t"
" hard limit= %ld \n", old_lim.rlim_cur,
old_lim.rlim_max);
else
fprintf(stderr, "%s\n", strerror(errno));
lim.rlim_cur = 3;
lim.rlim_max = 1024;
if(setrlimit(RLIMIT_NOFILE, &lim) == -1)
fprintf(stderr, "%s\n", strerror(errno));
if( getrlimit(RLIMIT_NOFILE, &new_lim) == 0)
printf("New limits -> soft limit= %ld \t"
" hard limit= %ld \n", new_lim.rlim_cur,
new_lim.rlim_max);
else
fprintf(stderr, "%s\n", strerror(errno));
if(open("foo.txt", O_WRONLY | O_CREAT, 0) == -1)
fprintf(stderr, "%s\n", strerror(errno));
else
printf("Opened successfully\n");
return 0;
}
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Output:
Old limits -> soft limit= 1048576 hard limit= 1048576
New limits -> soft limit= 3 hard limit= 1024
Too many open files
There is another system call
prlimit() that combines both the system calls.
For more details, check manual by typing
man 2 prlimit
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