tests/ansi: add ftell test case

This patch adds a simple test for ftell's functionality which also
acts as a simple reproducible test case for the failing GMP tests
documented in #1221. This test used to fail because our behaviour when
checking ftell after a ungetc doesn't match glibc.

tests/ansi/ftell.c

@@ -0,0 +1,72 @@
+#include <assert.h>
+#include <stdio.h>
+#include <string.h>
+
+#ifdef USE_HOST_LIBC
+#define TEST_FILE "ftell-host-libc.tmp"
+#else
+#define TEST_FILE "ftell.tmp"
+#endif
+
+int main() {
+	FILE *file;
+	char str[] = "mlibc ftell test";
+	size_t str_size = sizeof(str) - 1;
+	char buffer[20];
+
+	// Clear buffer to zero
+	memset(buffer, 0, sizeof(buffer));
+
+	// Open the file for writing in binary mode, because ftell is unspecified
+	// in text mode.
+	file = fopen(TEST_FILE, "wb");
+	assert(file);
+
+	// Write string minus null terminator, flush and close.
+	assert(fwrite(str, 1, str_size, file) == str_size);
+	fflush(file);
+	fclose(file);
+
+	// Open the file for reading in binary mode.
+	file = fopen(TEST_FILE, "rb");
+	assert(file);
+
+	// Check position indicator at the start of the file.
+	assert(ftell(file) == 0);
+
+	// Read 4 bytes and check fread and ftell.
+	assert(fread(buffer, 1, 4, file) == 4);
+	assert(ftell(file) == 4);
+
+	// Rewind and check position indicator at the start of the file.
+	rewind(file);
+	assert(ftell(file) == 0);
+
+	// Read the entire file and check fread and ftell.
+	assert(fread(buffer, 1, str_size, file) == str_size);
+	assert((size_t) ftell(file) == str_size);
+
+	// Rewind and check how ftell interacts with getc.
+	rewind(file);
+	assert(fgetc(file) == 'm');
+	assert(ftell(file) == 1);
+	assert(fgetc(file) == 'l');
+	assert(ftell(file) == 2);
+	assert(fgetc(file) == 'i');
+	assert(ftell(file) == 3);
+
+	// Check whether ftell is decremented after ungetc
+	assert(ungetc('X', file) == 'X');
+	int ftell_after_ungetc = ftell(file);
+	fprintf(stderr, "ftell_after_ungetc: %d\n", ftell_after_ungetc);
+	assert(ftell_after_ungetc == 2);
+	ungetc('Y', file);
+	assert(ftell(file) == 1);
+
+	// Check if rewind undoes ungetc's effects on ftell
+	rewind(file);
+	assert(ftell(file) == 0);
+
+	fclose(file);
+	return 0;
+}

tests/meson.build

@@ -30,6 +30,7 @@ all_test_cases = [
 	'ansi/strxfrm',
 	'ansi/calloc',
 	'ansi/fgetpos',
+	'ansi/ftell',
 	'bsd/ns_get_put',
 	'bsd/reallocarray',
 	'bsd/strl',