BACKTRACE(3) Linux Programmer's Manual BACKTRACE(3)
NAME
backtrace, backtrace_symbols, backtrace_symbols_fd - support for appli-
cation self-debugging
SYNOPSIS
#include <execinfo.h>
int backtrace(void **buffer, int size);
char **backtrace_symbols(void *const *buffer, int size);
void backtrace_symbols_fd(void *const *buffer, int size, int fd);
DESCRIPTION
backtrace() returns a backtrace for the calling program, in the array
pointed to by buffer. A backtrace is the series of currently active
function calls for the program. Each item in the array pointed to by
buffer is of type void *, and is the return address from the corre-
sponding stack frame. The size argument specifies the maximum number
of addresses that can be stored in buffer. If the backtrace is larger
than size, then the addresses corresponding to the size most recent
function calls are returned; to obtain the complete backtrace, make
sure that buffer and size are large enough.
Given the set of addresses returned by backtrace() in buffer, back-
trace_symbols() translates the addresses into an array of strings that
describe the addresses symbolically. The size argument specifies the
number of addresses in buffer. The symbolic representation of each
address consists of the function name (if this can be determined), a
hexadecimal offset into the function, and the actual return address (in
hexadecimal). The address of the array of string pointers is returned
as the function result of backtrace_symbols(). This array is mal-
loc(3)ed by backtrace_symbols(), and must be freed by the caller. (The
strings pointed to by the array of pointers need not and should not be
freed.)
backtrace_symbols_fd() takes the same buffer and size arguments as
backtrace_symbols(), but instead of returning an array of strings to
the caller, it writes the strings, one per line, to the file descriptor
fd. backtrace_symbols_fd() does not call malloc(3), and so can be
employed in situations where the latter function might fail, but see
NOTES.
RETURN VALUE
backtrace() returns the number of addresses returned in buffer, which
is not greater than size. If the return value is less than size, then
the full backtrace was stored; if it is equal to size, then it may have
been truncated, in which case the addresses of the oldest stack frames
are not returned.
On success, backtrace_symbols() returns a pointer to the array mal-
loc(3)ed by the call; on error, NULL is returned.
VERSIONS
backtrace(), backtrace_symbols(), and backtrace_symbols_fd() are pro-
vided in glibc since version 2.1.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
+-----------------------+---------------+---------+
|Interface | Attribute | Value |
+-----------------------+---------------+---------+
|backtrace(), | Thread safety | MT-Safe |
|backtrace_symbols(), | | |
|backtrace_symbols_fd() | | |
+-----------------------+---------------+---------+
CONFORMING TO
These functions are GNU extensions.
NOTES
These functions make some assumptions about how a function's return
address is stored on the stack. Note the following:
* Omission of the frame pointers (as implied by any of gcc(1)'s
nonzero optimization levels) may cause these assumptions to be vio-
lated.
* Inlined functions do not have stack frames.
* Tail-call optimization causes one stack frame to replace another.
* backtrace() and backtrace_symbols_fd() don't call malloc() explic-
itly, but they are part of libgcc, which gets loaded dynamically
when first used. Dynamic loading usually triggers a call to mal-
loc(3). If you need certain calls to these two functions to not
allocate memory (in signal handlers, for example), you need to make
sure libgcc is loaded beforehand.
The symbol names may be unavailable without the use of special linker
options. For systems using the GNU linker, it is necessary to use the
-rdynamic linker option. Note that names of "static" functions are not
exposed, and won't be available in the backtrace.
EXAMPLE
The program below demonstrates the use of backtrace() and back-
trace_symbols(). The following shell session shows what we might see
when running the program:
$ cc -rdynamic prog.c -o prog
$ ./prog 3
backtrace() returned 8 addresses
./prog(myfunc3+0x5c) [0x80487f0]
./prog [0x8048871]
./prog(myfunc+0x21) [0x8048894]
./prog(myfunc+0x1a) [0x804888d]
./prog(myfunc+0x1a) [0x804888d]
./prog(main+0x65) [0x80488fb]
/lib/libc.so.6(__libc_start_main+0xdc) [0xb7e38f9c]
./prog [0x8048711]
Program source
#include <execinfo.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define BT_BUF_SIZE 100
void
myfunc3(void)
{
int j, nptrs;
void *buffer[BT_BUF_SIZE];
char **strings;
nptrs = backtrace(buffer, BT_BUF_SIZE);
printf("backtrace() returned %d addresses\n", nptrs);
/* The call backtrace_symbols_fd(buffer, nptrs, STDOUT_FILENO)
would produce similar output to the following: */
strings = backtrace_symbols(buffer, nptrs);
if (strings == NULL) {
perror("backtrace_symbols");
exit(EXIT_FAILURE);
}
for (j = 0; j < nptrs; j++)
printf("%s\n", strings[j]);
free(strings);
}
static void /* "static" means don't export the symbol... */
myfunc2(void)
{
myfunc3();
}
void
myfunc(int ncalls)
{
if (ncalls > 1)
myfunc(ncalls - 1);
else
myfunc2();
}
int
main(int argc, char *argv[])
{
if (argc != 2) {
fprintf(stderr, "%s num-calls\n", argv[0]);
exit(EXIT_FAILURE);
}
myfunc(atoi(argv[1]));
exit(EXIT_SUCCESS);
}
SEE ALSO
addr2line(1), gcc(1), gdb(1), ld(1), dlopen(3), malloc(3)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 BACKTRACE(3)
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