Simple example of reading parameters from a file in plain C (for possible inclusion in QUDA)

example.c

#include "input.h"

typedef struct {
  int Nt;
  int Lx;
  int Ly;
  int xyVol;
  int rVol;
  int latVol;
  double beta;
  double delta; // delta = beta/Nt                                              
  double kappa;
  double r0;    // r0  = old "alpha"                                            
  double e;
  double ma;
  double mb;
  int flip;
  int disc;
  int niter;
  int seed;
  double resEpsilon;
  int hmc_on;
  double dt;
  int nstep;
  int ntr;
  int maxIter;
  int nsigma;
  double *V;
  double *Vinv;
  double complex *VinvFTaa;
  double complex *VinvFTab;
  int verbose;
  int bcV;
} param_t;

void setParams(char *file, param_t *param)
{
  param_list_t *plist;
  param_t p;

  readParamList(file, &plist);

  getParam(p, plist, flip, "%d");
  getParam(p, plist, disc, "%d");
  getParam(p, plist, niter, "%d");
  getParam(p, plist, verbose, "%d");
  getParam(p, plist, Lx, "%d");
  getParam(p, plist, Ly, "%d");
  getParam(p, plist, Nt, "%d");
  getParam(p, plist, delta, "%lg");
  getParam(p, plist, kappa, "%lg");
  getParam(p, plist, r0, "%lg");
  getParam(p, plist, e, "%lg");
  getParam(p, plist, ma, "%lg");
  getParam(p, plist, mb, "%lg");
  getParam(p, plist, resEpsilon, "%lg");
  getParam(p, plist, maxIter, "%d");
  getParam(p, plist, hmc_on, "%d");
  getParam(p, plist, dt, "%lg");
  getParam(p, plist, nstep, "%d");
  getParam(p, plist, ntr, "%d");
  getParam(p, plist, nsigma, "%d");
  getParam(p, plist, seed, "%d");
  getParam(p, plist, bcV, "%d");

  freeParamList(plist);

  // derived parameters

  p.beta = p.delta * p.Nt;
  p.xyVol =  p.Lx * p.Ly;
  p.rVol = 2 *  p.xyVol;
  p.latVol = p.Nt * p.rVol;

  *param = p;
}

input.c

#include "graphene.h"
#include "input.h"
#include <string.h>

#define BUFSIZE (2*FIELDSIZE + 10)

/*!
 * Read the parameter list from a file.  Each line of the file should
 * contain a parameter name and parameter value, separated by
 * whitespace and/or a colon or equals sign.  Blank lines and comments
 * (beginning with "#") are also allowed.
 */
void readParamList(char *filename, param_list_t **plist)
{
  FILE *fp;
  char line[BUFSIZE];
  char *key, *value;
  param_list_t *p, *prev;

  fp = fopen(filename, "r");
  if (!fp) {
    printf("Error: Failed to open parameter file %s\n", filename);
    exit(1);
  }

  *plist = NULL;
  prev = NULL;

  fgets(line, BUFSIZE, fp);
  while (!feof(fp)) {

    key = strtok(line, " \t\n=:");

    // discard blank lines and comments
    if (!key || (*key == '#')) {
      fgets(line, BUFSIZE, fp);
      continue;
    }

    value = strtok(NULL, " \t\n=:");
    if (!value) {
      printf("Error: Bad format in parameter file \"%s\"\n", filename);
      exit(1);
    }

    p = (param_list_t *) malloc(sizeof(param_list_t));
    p->next = NULL;
    p->key[FIELDSIZE-1] = '\0';
    p->value[FIELDSIZE-1] = '\0';
    strncpy(p->key, key, FIELDSIZE-1);
    strncpy(p->value, value, FIELDSIZE-1);

    if (prev) {
      prev->next = p;
    } else {
      *plist = p;
    }
    prev = p;

    fgets(line, BUFSIZE, fp);
  }

  fclose(fp);
}

/*!
 * Clear the parameter list, and free associated memory.
 */
void freeParamList(param_list_t *plist)
{
  param_list_t *next;

  while (plist) {
    next = plist->next;
    free(plist);
    plist = next;
  }
}

/*!
 * Return a string containing the parameter whose name matches "key".
 */
char *findParam(param_list_t *plist, char *key)
{
  while (plist) {
    if (!strcmp(plist->key, key)) break;
    plist = plist->next;
  }
  if (!plist) {
    printf("Error: Parameter \"%s\" not found in input file.\n", key);
    exit(1);
  }
  return plist->value;
}

input.h

#ifndef _INPUT_H
#define _INPUT_H

#define FIELDSIZE 100

typedef struct param_list_s {
  char key[FIELDSIZE];
  char value[FIELDSIZE];
  struct param_list_s *next;
} param_list_t;

/*!
 * The getParam() macro takes the name of a parameter and sets the
 * corresponding member of struct_t based on the value found in the
 * parameter list.  Here "format" is a scanf() format specifier.
 */

#define getParam(p, plist, key, format) \
  sscanf(findParam(plist, #key), format, &(p).key)

// functions defined in input.c:

void readParamList(char *filename, param_list_t **plist);
void freeParamList(param_list_t *plist);
char *findParam(param_list_t *plist, char *key);

#endif // _INPUT_H