segmentation fault while reading a FITS file with healpix

I'm trying to open a FITS file with healpix using a C code:

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include "chealpix.h"
#include "fitsio.h"
#include <string.h>
#include "fitsio2.h"

int main(int argc, char **argv){

        char *coordsys="G";
        char *order="RING";
        long nside;
        long *p_nside=&nside;
        nside=512.;
        float *map;
        map=read_healpix_map("file.fits",p_nside,coordsys,order);
         
    return(0);
}

This code returns a segmentation fault (core dumped) when using gdb, I get

Program received signal SIGSEGV, Segmentation fault.
ffgkys (status=0x7fffffffdaf4, comm=<optimized out>, value=0x55555555a004 "G", keyname=<optimized out>, 
    fptr=<optimized out>) at getkey.c:808
808     value[0] = '\0';

This is the first time I am using healpix and I have no idea what is causing the problem. Has anyone any idea what could be wrong?

Per the documentation of read_healpix_map():

ordering and coordsys are output arguments - they are pointers to space into which the function will write results. You have passed pointers to string constants in memory that will normally be marked read-only and may not be sufficiently large to receive the result in any case.

nside is also an output argument, initialising it to 512 serves no purpose. You do not need p_nside; you can simply pass &nside

It is not the most clear of interfaces; specifically it is not clear whether coordsys is a string or a single char. The documentation uses single quotes so you might assume char but then it does that for ordering too and that is clearly a string. It is safest to assume coordsys is a string - it will do no harm.

    char  coordsys[] = "G";    // This will be overwritten
    char  order[] = "NESTED" ; // This will be overwritten, 
                                  initialise to largest expected string
                                  to ensure sufficient space is allocated
    long nside = 0 ;
    float* map = read_healpix_map( "file.fits", &nside, coordsys, order ) ;

    printf( "nside:%ld, coordsys:%s, order:%s", nside, coordsys, order ) ;

Post Script: Looking at the source code both coordsys and ordering are passed to fits_read_key(), so coordsys is treated as a string.