Correct the return value of fputc function

According to the man page, the return value of fputc() should be either
the character written as an "unsigned char" cast to an "int" or EOF on
error, but the current implementation doesn't follow the specification.

Therefore, these changes correct the implementation to return the ASCII
code of the written character or -1 if the output fails, thereby
matching the description in the man page.

Additionally, the necessary test cases are also added to validate the
correctness.

Author: DrXiao

lib/c.c

@@ -460,10 +460,9 @@ char *fgets(char *str, int n, FILE *stream)
 
 int fputc(int c, FILE *stream)
 {
-    char buf[1];
-    buf[0] = c;
-    __syscall(__syscall_write, stream, buf, 1);
-    return 0;
+    if (__syscall(__syscall_write, stream, &c, 1) < 0)
+        return -1;
+    return c;
 }
 
 /* Non-portable: Assume page size is 4KiB */

tests/driver.sh

@@ -1674,4 +1674,33 @@ int main() {
 }
 EOF
 
+# test the return value when calling fputc().
+#
+# Since the FILE data type is defined as an int in
+# the built-in C library, and most of the functions
+# such as fputc(), fgetc(), fclose() and fgets() directly
+# treat the "stream" parameter (of type FILE *) as a file
+# descriptor for performing input/output operations, the
+# following test cases define "stdout" as 1, which is the
+# file descriptor for the standard output.
+try_output 0 "awritten = a" << EOF
+#define stdout 1
+int main()
+{
+	int c = fputc('a', stdout);
+	printf("written = %c", c);
+	return 0;
+}
+EOF
+
+try_output 1 "" << EOF
+#define stdout 1
+int main()
+{
+	__syscall(__syscall_close, 1);
+	int c = fputc('a', stdout);
+	return c == -1;
+}
+EOF
+
 echo OK