Get/Set process resource limits in C

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;  /* Soft limit */
    rlim_t rlim_max;  /* Hard limit (ceiling for rlim_cur) */
};

The signatures of the system calls are

int getrlimit(int resource, struct rlimit *rlim);
int setrlimit(int resource, const struct rlimit *rlim);

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.

// C program to demonstrate working of getrlimit()
// and setlimit()
#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;

    // Get old limits
    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));

    // Set new value
    lim.rlim_cur = 3;
    lim.rlim_max = 1024;

    // Set limits
    if(setrlimit(RLIMIT_NOFILE, &lim) == -1)
        fprintf(stderr, "%s\n", strerror(errno));

    // Get new limits
    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;
}

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).

// C program to demonstrate error when a
// process tries to access resources beyond
// limit.
#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;

    // Get old limits
    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));

    // Set new value
    lim.rlim_cur = 3;
    lim.rlim_max = 1024;


    // Set limits
    if(setrlimit(RLIMIT_NOFILE, &lim) == -1)
        fprintf(stderr, "%s\n", strerror(errno));

    // Get new limits
    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));

    // Try to open a new file
    if(open("foo.txt", O_WRONLY | O_CREAT, 0) == -1)
        fprintf(stderr, "%s\n", strerror(errno));
    else
            printf("Opened successfully\n");

    return 0;
}

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