gistfile1.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdlib.h>

#define ENVIRON "/proc/self/environ"

int
main(int argc, char* argv[]) {
  FILE* fp = fopen(ENVIRON, "r");
  struct stat st;
  char buf[BUFSIZ] = {0};
  int i;

  setenv("FOO", "BAR", 1);

  if (stat(ENVIRON, &st) != 0) {
    perror("stat");
    exit(1);
  }
  fgets(buf, sizeof buf, fp);
  fclose(fp);

  for (i = 0; i < sizeof buf; i++) {
    if (buf[i] == 0) {
      if (i > 0 && buf[i-1] == '\n') break;
      buf[i] = '\n';
    }
  }
  puts(buf);
  return 0;
}