You could re-invent the wheel, as many other answers suggest. Alternately, you could use someone else's wheel -- I'd suggest Newlib's, which is BSD-licensed and intended for use on embedded systems. It properly handles negative numbers, NaNs, infinities, and cases which are not representable as integers (due to being too large), as well as doing so in an efficient manner that uses exponents and masking rather than generally-costlier floating-point operations. In addition, it's regularly tested, so you know it doesn't have glaring corner-case bugs in it.

The Newlib source can be a bit awkward to navigate, so here are the bits you want:

Float version: https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libm/common/sf_round.c;hb=master

Double version: https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libm/common/s_round.c;hb=master

Word-extraction macros defined here: https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/libm/common/fdlibm.h;hb=master

If you need other files from there, the parent directory is this one: https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;a=tree;f=newlib/libm/common;hb=master

For the record, here's the code for the float version. As you can see, there's a bit of complexity required to deal with all the possible cases correctly.

float roundf(x)
{
  int signbit;
  __uint32_t w;
  /* Most significant word, least significant word. */
  int exponent_less_127;

  GET_FLOAT_WORD(w, x);

  /* Extract sign bit. */
  signbit = w & 0x80000000;

  /* Extract exponent field. */
  exponent_less_127 = (int)((w & 0x7f800000) >> 23) - 127;

  if (exponent_less_127 < 23)
    {
      if (exponent_less_127 < 0)
        {
          w &= 0x80000000;
          if (exponent_less_127 == -1)
            /* Result is +1.0 or -1.0. */
            w |= ((__uint32_t)127 << 23);
        }
      else
        {
          unsigned int exponent_mask = 0x007fffff >> exponent_less_127;
          if ((w & exponent_mask) == 0)
            /* x has an integral value. */
            return x;

          w += 0x00400000 >> exponent_less_127;
          w &= ~exponent_mask;
        }
    }
  else
    {
      if (exponent_less_127 == 128)
        /* x is NaN or infinite. */
        return x + x;
      else
        return x;
    }
  SET_FLOAT_WORD(x, w);
  return x;
}

You need to import <math.h> header :

#include <math.h> //don't forget to import this !

double a;
a = round(5.05286); //will be rounded to 5.00

This function has analog definitions for every type, which means that you can pass the following types and it'll be rounded to the nearest for every one of them :

double round(double a);
float roundf(float a);
long double roundl(long double a);