/*

Apparently, a buffered write operation after an ungetc() call puts a stream
into a bad state, eventually resulting in a segfault.  While this should
result in undefined behavior according to ANSI/ISO C99, it is not in keeping
with stated glibc policy.  From the glibc manual:

     As you can see, `+' requests a stream that can do both input and
     output.  The ISO standard says that when using such a stream, you
     must call `fflush' (*note Stream Buffering::) or a file positioning
     function such as `fseek' (*note File Positioning::) when switching
     from reading to writing or vice versa.  Otherwise, internal buffers
     might not be emptied properly.  The GNU C library does not have
     this limitation; you can do arbitrary reading and writing
     operations on a stream in whatever order.


OUTPUT:

/a.out
   dump: feof=0  ferror=0  ftell=0
initial hello world succeeded
   dump: feof=0  ferror=0  ftell=12
rewind succeeded
   dump: feof=0  ferror=0  ftell=0
fgetc succeeded
   dump: feof=0  ferror=0  ftell=1
ungetc succeeded
   dump: feof=0  ferror=0  ftell=0
ouch succeeded
   dump: feof=0  ferror=0  ftell=5
Segmentation fault

*/

#define _ISOC99_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <locale.h>
#include <wchar.h>
#include <errno.h>


static void dump(FILE *fp)
{
	fprintf(stderr, "   dump: feof=%d  ferror=%d  ftell=%ld\n",
			feof(fp), ferror(fp), (long) ftell(fp));
}

int main(void)
{
	FILE *fp;
	int r;

	if (!(fp = fopen("tungetc.test", "w+"))) {
		fprintf(stderr, "fopen failed!\n");
		return EXIT_FAILURE;
	}

	dump(fp);

	r = fputs("hello world\n", fp);
	fprintf(stderr, "initial hello world %s\n", (r<0) ? "failed" : "succeeded");
	dump(fp);

	rewind(fp);
	fprintf(stderr, "rewind %s\n", (ferror(fp)) ? "failed" : "succeeded");
	dump(fp);

	r = fgetc(fp);
	fprintf(stderr, "fgetc %s\n", (ferror(fp)) ? "failed" : "succeeded");
	dump(fp);

	r = ungetc(' ', fp);
	fprintf(stderr, "ungetc %s\n", (r<0) ? "failed" : "succeeded");
	dump(fp);

	r = fputs("ouch\n", fp);
	fprintf(stderr, "ouch %s\n", (r<0) ? "failed" : "succeeded");
	dump(fp);

#if 1
	rewind(fp);
	fprintf(stderr, "rewind %s\n", (ferror(fp)) ? "failed" : "succeeded");
	dump(fp);
#elif 1
	r = fseek(fp, 0, SEEK_CUR);
	fprintf(stderr, "fseek %s\n", (r<0) ? "failed" : "succeeded");
	dump(fp);
#else
	fprintf(stderr, "fclose %s\n", (fclose(fp)) ? "failed" : "succeeded");
#endif

	return EXIT_SUCCESS;
}