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backtrace_symbols_fd(3) [freebsd man page]

BACKTRACE(3)						   BSD Library Functions Manual 					      BACKTRACE(3)

backtrace -- fill in the backtrace of the currently executing thread LIBRARY
library ``libexecinfo'' SYNOPSIS
#include <execinfo.h> size_t backtrace(void **addrlist, size_t len); char ** backtrace_symbols(void * const *addrlist, size_t len); int backtrace_symbols_fd(void * const *addrlist, size_t len, int fd); char ** backtrace_symbols_fmt(void * const *addrlist, size_t len, const char *fmt); int backtrace_symbols_fmt_fd(void * const *addrlist, size_t len, const char *fmt, int fd); DESCRIPTION
The backtrace() function places into the array pointed by addrlist the array of the values of the program counter for each frame called up to len frames. The number of frames found (which can be fewer than len) is returned. The backtrace_symbols_fmt() function takes an array of previously filled addresses from backtrace() in addrlist of len elements, and uses fmt to format them. The formatting characters available are: a The numeric address of each element as would be printed using %p. n The name of the nearest function symbol (smaller than the address element) as determined by dladdr(3) if the symbol was dynamic, or looked up in the executable if static and the /proc filesystem is available to determine the executable path. d The difference of the symbol address and the address element printed using 0x%tx. D The difference of the symbol addresss and the address element printed using +0x%tx if non-zero, or nothing if zero. f The filename of the symbol as determined by dladdr(3). The array of formatted strings is returned as a contiguous memory address which can be freed by a single free(3). The backtrace_symbols() function is equivalent of calling backtrace_symbols_fmt() with a format argument of %a <%n%D> at %f The backtrace_symbols_fd() and backtrace_symbols_fmt_fd() are similar to the non _fd named functions, only instead of returning an array or strings, they print a new-line separated array of strings in fd, and return 0 on success and -1 on failure. RETURN VALUES
The backtrace() function returns the number of elements that were filled in the backtrace. The backtrace_symbols() and backtrace_symbols_fmt() return a string array on success, and NULL on failure, setting errno. Diagnostic output may also be produced by the ELF symbol lookup functions. SEE ALSO
dladdr(3), elf(3) HISTORY
The backtrace() library of functions first appeared in NetBSD 7.0 and FreeBSD 10.0. BUGS
1. Errors should not be printed but communicated to the caller differently. 2. Because these functions use elf(3) this is a separate library instead of being part of libc/libutil so that no library dependencies are introduced. 3. The Linux versions of the functions (there are no _fmt variants) use int instead of size_t arguments. BSD
August 23, 2013 BSD

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BACKTRACE(3)						     Linux Programmer's Manual						      BACKTRACE(3)

backtrace, backtrace_symbols, backtrace_symbols_fd - support for application 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, 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. 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 in glibc since version 2.1. 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 violated. * Inlined functions do not have stack frames. * Tail-call optimization causes one stack frame to replace another. 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 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/ [0xb7e38f9c] ./prog [0x8048711] Program source #include <execinfo.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> void myfunc3(void) { int j, nptrs; #define SIZE 100 void *buffer[100]; char **strings; nptrs = backtrace(buffer, SIZE); printf("backtrace() returned %d addresses ", 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 ", 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 ", argv[0]); exit(EXIT_FAILURE); } myfunc(atoi(argv[1])); exit(EXIT_SUCCESS); } SEE ALSO
gcc(1), ld(1), dlopen(3), malloc(3) COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at GNU
2008-06-14 BACKTRACE(3)
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