NAME
backtrace, backtrace_symbols, backtrace_symbols_fd, backtrace_image_offsets, backtrace_from_fp – call stack backtrace and display functions
SYNOPSIS
#include <execinfo.h>
int
backtrace(void** array, int size);
char**
backtrace_symbols(void* const* array, int size);
void
backtrace_symbols_fd(void* const* array, int size, int fd);
void
backtrace_image_offsets(void* const* array, struct image_offset *image_offsets, int size);
int
backtrace_from_fp(void* startfp, void** array, int size);
size_t
backtrace_async(void** array, size_t size, uint32_t* task_id);
DESCRIPTION
These routines provide a mechanism to examine the current thread's call stack.
backtrace() writes the function return addresses of the current call stack to the array of pointers referenced by array. At most, size pointers are written. The number of pointers actually written to array is returned.
backtrace_symbols() attempts to transform a call stack obtained by backtrace() into an array of human-readable strings using dladdr(). The array of strings returned has size elements. It is allocated using malloc() and should be released using free(). There is no need to free the individual strings in the array.
backtrace_symbols_fd() performs the same operation as backtrace_symbols(), but the resulting strings are immediately written to the file descriptor fd, and are not returned.
backtrace_image_offsets() attempts to transform a call stack obtained by backtrace() into an array of image offsets, for deferred symbolication. Each entry in the array has an offset relative to the __TEXT section of the image with the given UUID. The results are written to image_offsets which should be an array of size length.
backtrace_from_fp() takes a backtrace of frames starting from the given frame pointer.
backtrace_async() behaves exactly like backtrace() unless it is invoked from a Swift async context. In that case, instead of writing the return addresses of the OS call stack, the continuation addresses of the async invocations that led to the current state are provided. If unwinding an async stack rather than an OS stack, the value pointed to by task_id will be set to a non-zero identifier that for the current process uniquely identifies the async task currently running. Otherwise, 0 is stored.
Note that the continuation addresses provided by backtrace_async() have an offset of 1 added to them. Most symbolication engines will substract 1 from the call stack return addresses in order to symbolicate the call site rather than the return location. With a Swift async continuation, substracting 1 from its address would result in an address in a different function. This offset allows the returned addresses to be handled correctly by most existing symbolication engines.
EXAMPLE
#include <execinfo.h>
#include <stdio.h>
...
void* callstack[128];
int i, frames = backtrace(callstack, 128);
char** strs = backtrace_symbols(callstack, frames);
for (i = 0; i < frames; ++i) {
printf("%s\n", strs[i]);
}
free(strs);
...
HISTORY
backtrace(), backtrace_symbols(), and backtrace_symbols_fd() first appeared in Mac OS X 10.5. backtrace_image_offsets() and backtrace_from_fp() first appeared macOS 10.14 and iOS 12. backtrace_async() first appeared in macOS 12.