Add manual error tracing

Error messages and function names/line numbers are traced via
__func__ or __FUNCTION__, __LINE__, etc (or just "unknown" if
not available). A manual stack is kept onto which these values
are pushed which can then be dumped to stderr/stdout.

stderr is used to report this information since no other
facility appears to be provided.

src/err.c

@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2018 Diamond Light Source Ltd.
+ * Author: Charles Mita
+ */
+
+
+#include <stdio.h>
+#include "err.h"
+
+
+struct error_stack_t {
+	char **files;
+	char **funcs;
+	int *lines;
+	int *errors;
+	char **messages;
+	int size;
+};
+
+static char files_buffer[ERR_MAX_FILENAME_LENGTH * ERR_MAX_STACK_SIZE] = {0};
+static char funcs_buffer[ERR_MAX_FUNCNAME_LENGTH * ERR_MAX_STACK_SIZE] = {0};
+static char messages_buffer[ERR_MAX_MESSAGE_LENGTH * ERR_MAX_STACK_SIZE] = {0};
+
+static char *files[ERR_MAX_STACK_SIZE] = {0};
+static char *funcs[ERR_MAX_STACK_SIZE] = {0};
+static int lines[ERR_MAX_STACK_SIZE] = {0};
+static int errors[ERR_MAX_STACK_SIZE] = {0};
+static char *messages[ERR_MAX_STACK_SIZE] = {0};
+
+static struct error_stack_t stack = {files, funcs, lines, errors, messages, 0};
+
+void push_error_stack(const char *file, const char *func, int line, int err, const char *message) {
+	if (stack.size >= ERR_MAX_STACK_SIZE) return; /* unfortunate */
+	int idx = stack.size;
+	stack.files[idx] = files_buffer + (idx * ERR_MAX_FILENAME_LENGTH);
+	stack.funcs[idx] = funcs_buffer + (idx * ERR_MAX_FUNCNAME_LENGTH);
+	stack.messages[idx] = messages_buffer + (idx * ERR_MAX_MESSAGE_LENGTH);
+
+	/* subtract 1 to ensure room for null byte in buffer */
+	sprintf(stack.funcs[idx], "%.*s", ERR_MAX_FUNCNAME_LENGTH - 1, func);
+	sprintf(stack.files[idx], "%.*s", ERR_MAX_FILENAME_LENGTH - 1, file);
+	sprintf(stack.messages[idx], "%.*s", ERR_MAX_MESSAGE_LENGTH - 1, message);
+	stack.lines[idx] = line;
+	stack.errors[idx] = err;
+
+	stack.size++;
+}
+
+
+void reset_error_stack() {
+	stack.size = 0;
+}
+
+
+void dump_error_stack(FILE *out) {
+	int idx = stack.size;
+	if (idx > 0) fprintf(out, "Durin plugin error:\n");
+	while (idx-- > 0) {
+		const char *file = stack.files[idx];
+		const char *func = stack.funcs[idx];
+		const char *message = stack.messages[idx];
+		const int line = stack.lines[idx];
+		if (message[0] != '\0') {
+			fprintf(out, "\t%s - line %d in %s:\n\t\t%s\n", file, line, func, message);
+		} else {
+			fprintf(out, "\t%s - line %d in %s\n", file, line, func);
+		}
+	}
+}