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NAME
backtrace, backtrace_symbols, backtrace_symbols_fd - support for application 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 corresponding 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, backtrace_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 malloc(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 malloc(3)ed by the call; on
error, NULL is returned.
VERSIONS
backtrace(), backtrace_symbols(), and backtrace_symbols_fd() are provided since glibc 2.1.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│backtrace(), backtrace_symbols(), backtrace_symbols_fd() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
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 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() explicitly, 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 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.
EXAMPLES
The program below demonstrates the use of backtrace() and backtrace_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 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)