C library: add FreeBSD's __srget, __swbuf

Implemented as over-approximations of the implementations found at
https://cgit.freebsd.org/src/tree/lib/libc/stdio/.

src/ansi-c/library/stdio.c

@@ -1749,3 +1749,51 @@ int __stdio_common_vfprintf(
 }
 
 #endif
+
+/* FUNCTION: __srget */
+
+#ifdef __FreeBSD__
+
+#  ifndef __CPROVER_STDIO_H_INCLUDED
+#    include <stdio.h>
+#    define __CPROVER_STDIO_H_INCLUDED
+#  endif
+
+int __VERIFIER_nondet_int(void);
+
+int __srget(FILE *stream)
+{
+  (void)*stream;
+
+  // FreeBSD's implementation returns a character or EOF; __VERIFIER_nondet_int
+  // will capture all these options
+  return __VERIFIER_nondet_int();
+}
+
+#endif
+
+/* FUNCTION: __swbuf */
+
+#ifdef __FreeBSD__
+
+#  ifndef __CPROVER_STDIO_H_INCLUDED
+#    include <stdio.h>
+#    define __CPROVER_STDIO_H_INCLUDED
+#  endif
+
+__CPROVER_bool __VERIFIER_nondet___CPROVER_bool(void);
+
+int __swbuf(int c, FILE *stream)
+{
+  (void)c;
+  (void)*stream;
+
+  // FreeBSD's implementation returns `c` or or EOF in case writing failed; we
+  // just non-deterministically choose between these
+  if(__VERIFIER_nondet___CPROVER_bool())
+    return EOF;
+  else
+    return c;
+}
+
+#endif

src/ansi-c/library_check.sh

@@ -45,6 +45,8 @@ perl -p -i -e 's/^__isoc99_v?sscanf\n//' __functions # sscanf, Linux
 perl -p -i -e 's/^__sigsetjmp\n//' __functions # sigsetjmp, Linux
 perl -p -i -e 's/^__stdio_common_vfscanf\n//' __functions # fscanf, Windows
 perl -p -i -e 's/^__stdio_common_vsscanf\n//' __functions # sscanf, Windows
+perl -p -i -e 's/^__srget\n//' __functions # gets, FreeBSD
+perl -p -i -e 's/^__swbuf\n//' __functions # putc, FreeBSD
 
 # Some functions are covered by existing tests:
 perl -p -i -e 's/^__CPROVER_deallocate\n//' __functions # free-01