execinfo: Clean up, implement backtrace_symbols_fd

Rename the file to match the header it implements functions from.

Changes to existing code:
* Fix the return value from backtrace (off by one).
* Use __builtin_extract_return_addr as recommended in gcc documentation.
* Document header usage.
* Document where the functions are referenced in the LSB standard.

Signed-off-by: Samuel Holland <samuel@sholland.org>

2 files changed, 97 insertions, 51 deletions

diff --git a/libgcompat/backtrace.c b/libgcompat/backtrace.c
deleted file mode 100644
index c8ded8b..0000000
--- a/libgcompat/backtrace.c
+++ /dev/null
@@ -1,51 +0,0 @@
-#include <dlfcn.h>
-#include <stddef.h>
-#include <stdlib.h>
-
-#define _frame_level(addr_buf, curr, frame, size)                              \
-	do {                                                                   \
-		if (__builtin_frame_address(frame) != NULL                     \
-		    && (curr = __builtin_return_address(frame)) > 0x1000       \
-		    && frame <= size) {                                        \
-			addr_buf[frame] = curr;                                \
-		} else {                                                       \
-			return size;                                           \
-		}                                                              \
-	} while (0)
-
-int backtrace(void **addr_buf, int size)
-{
-	void *curr;
-	_frame_level(addr_buf, curr, 0, size);
-	_frame_level(addr_buf, curr, 1, size);
-	_frame_level(addr_buf, curr, 2, size);
-	_frame_level(addr_buf, curr, 3, size);
-	_frame_level(addr_buf, curr, 4, size);
-	_frame_level(addr_buf, curr, 5, size);
-	_frame_level(addr_buf, curr, 6, size);
-	_frame_level(addr_buf, curr, 7, size);
-	_frame_level(addr_buf, curr, 8, size);
-	_frame_level(addr_buf, curr, 9, size);
-	return 9;
-}
-
-char **backtrace_symbols(void *const *addr_buf, int size)
-{
-	char **result = calloc(sizeof(char *), size);
-	if (result == NULL) {
-		return result;
-	}
-
-	for (int next = 0; next < size; next++) {
-		Dl_info info;
-		int err = dladdr(addr_buf[next], &info);
-
-		if (err != 0) {
-			result[next] = "??:0";
-		} else {
-			result[next] = info.dli_sname;
-		}
-	}
-
-	return result;
-}
diff --git a/libgcompat/execinfo.c b/libgcompat/execinfo.c
new file mode 100644
index 0000000..59441aa
--- /dev/null
+++ b/libgcompat/execinfo.c
@@ -0,0 +1,97 @@
+#include <dlfcn.h>  /* dladdr */
+#include <stddef.h> /* NULL */
+#include <stdint.h> /* uintptr_t */
+#include <stdlib.h> /* calloc */
+#include <string.h> /* strlen */
+#include <unistd.h> /* write */
+
+#define get_frame_level(array, size, n)                                        \
+	do {                                                                   \
+		if (n >= size || __builtin_frame_address(n) == NULL) {         \
+			return n;                                              \
+		}                                                              \
+		void *address = __builtin_return_address(n);                   \
+		array[n] = __builtin_extract_return_addr(address);             \
+		if ((uintptr_t) array[n] < 0x1000) {                           \
+			return n;                                              \
+		}                                                              \
+	} while (0)
+
+/**
+ * Obtain a backtrace for the calling program.
+ *
+ * LSB 5.0: LSB-Core-generic/baselib-backtrace-1.html
+ */
+int backtrace(void **array, int size)
+{
+	get_frame_level(array, size, 0);
+	get_frame_level(array, size, 1);
+	get_frame_level(array, size, 2);
+	get_frame_level(array, size, 3);
+	get_frame_level(array, size, 4);
+	get_frame_level(array, size, 5);
+	get_frame_level(array, size, 6);
+	get_frame_level(array, size, 7);
+	get_frame_level(array, size, 8);
+	get_frame_level(array, size, 9);
+	return 10;
+}
+
+/**
+ * Translate addresses into symbol information.
+ *
+ * LSB 5.0: LSB-Core-generic/baselib-backtrace-1.html
+ */
+const char **backtrace_symbols(void *const *array, int size)
+{
+	const char **result = calloc(size, sizeof(char *));
+
+	if (result == NULL) {
+		return NULL;
+	}
+	for (int i = 0; i < size; ++i) {
+		Dl_info info;
+
+		if (dladdr(array[i], &info) && info.dli_sname != NULL) {
+			result[i] = info.dli_sname;
+		} else {
+			result[i] = "??:0";
+		}
+	}
+
+	return result;
+}
+
+/**
+ * Write symbol information to a file without allocating memory.
+ *
+ * LSB 5.0: LSB-Core-generic/baselib-backtrace-1.html
+ */
+void backtrace_symbols_fd(void *const *array, int size, int fd)
+{
+	for (int i = 0; i < size; ++i) {
+		Dl_info info;
+		const char *line;
+		int len;
+
+		if (dladdr(array[i], &info) && info.dli_sname != NULL) {
+			line = info.dli_sname;
+			len = strlen(line);
+		} else {
+			line = "??:0";
+			len = sizeof("??:0") - 1;
+		}
+		while (len > 0) {
+			int written = write(fd, line, len);
+
+			if (written < 1) {
+				return;
+			}
+			line += written;
+			len -= written;
+		}
+		if (write(fd, "\n", 1) != 1) {
+			return;
+		}
+	}
+}