load-macho.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mach-o/loader.h>
#include <mach-o/nlist.h>
#include <assert.h>

void *find_function(char *obj, const char *name);

int main(void)
{
  /* load object file into memory */
  FILE *fp = fopen("add.o", "r");
  assert(fp != NULL);
  fseek(fp, 0, SEEK_END);
  long fsize = ftell(fp);
  fseek(fp, 0, SEEK_SET);
  char *obj = malloc(fsize);
  fread(obj, 1, fsize, fp);
  fclose(fp);

  int (*add)(int, int) = (int (*)(int, int))find_function(obj, "_add");
  printf("%d\n", add(2, 4));

  free(obj);
  return 0;
}

void *find_function(char *obj, const char *name)
{
  char *obj_p = obj;

  /* check mach-o header */
  struct mach_header *header = (struct mach_header *)obj_p;
  obj_p += sizeof *header;
  assert(header->magic == MH_MAGIC);

  struct section *sections = NULL;
  struct nlist *nl = NULL;
  int i;
  for (i = 0; i < header->ncmds; ++i) {
    struct load_command *lc = (struct load_command *)obj_p;
    if (lc->cmd == LC_SYMTAB) {
      struct symtab_command *symtab = (struct symtab_command *)obj_p;
      obj_p += sizeof *symtab;

      struct nlist *ns = (struct nlist *)(obj + symtab->symoff);
      char *strtable = obj + symtab->stroff;

      /* find symbol's nlist */
      int j;
      for (j = 0; j < symtab->nsyms; ++j) {
        if (strcmp(name, strtable + ns[j].n_un.n_strx) == 0) {
          assert((ns[j].n_type & N_TYPE) == N_SECT);
          nl = ns + j;
          break;
        }
      }
    }
    else if (lc->cmd == LC_SEGMENT) {
      struct segment_command *segment = (struct segment_command *)obj_p;
      obj_p += sizeof *segment;
      sections = (struct section *)obj_p;
      obj_p += segment->nsects * sizeof *sections;
    }
    else {
      /* skip */
      obj_p += lc->cmdsize;
    }
  }

  assert(sections != NULL);
  assert(nl != NULL);
  struct section *sect = sections + nl->n_sect-1;  /* n_sect is 1-origin */
  char *text = obj + sect->offset + nl->n_value;

  return text;
}

This code cannot run on IOS, I think it is because the memory area where "obj" is located is the data segment