backtrace(3) Library Functions Manual backtrace(3)
NAME
backtrace, backtrace_symbols, backtrace_symbols_fd - support for appli-
cation self-debugging
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <execinfo.h>
int backtrace(void *buffer[.size], int size);
char **backtrace_symbols(void *const buffer[.size], int size);
void backtrace_symbols_fd(void *const buffer[.size], 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 correspond-
ing stack frame. The size argument specifies the maximum number of ad-
dresses 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 ad-
dress consists of the function name (if this can be determined), a hexa-
decimal 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 back-
trace_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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ backtrace(), backtrace_symbols(), │ Thread safety │ MT-Safe │
│ backtrace_symbols_fd() │ │ │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
GNU.
HISTORY
glibc 2.1.
NOTES
These functions make some assumptions about how a function's return ad-
dress 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 violated.
• 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 malloc(3).
If you need certain calls to these two functions to not allocate mem-
ory (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.
EXAMPLES
The program below demonstrates the use of backtrace() and backtrace_sym-
bols(). The following shell session shows what we might see when run-
ning 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 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 (size_t 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)
Linux man-pages 6.9.1 2024-06-15 backtrace(3)
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