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Copyright (c) 2012 The NetBSD Foundation, Inc.
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by Christos Zoulas
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.Dd August 23, 2013 .Dt BACKTRACE 3 .Os .Sh NAME .Nm backtrace .Nd fill in the backtrace of the currently executing thread .Sh LIBRARY .Lb libexecinfo .Sh SYNOPSIS n execinfo.h .Ft size_t .Fn backtrace "void **addrlist" "size_t len" .Ft "char **" .Fn backtrace_symbols "void * const *addrlist" "size_t len" .Ft int .Fn backtrace_symbols_fd "void * const *addrlist" "size_t len" "int fd" .Ft "char **" .Fn backtrace_symbols_fmt "void * const *addrlist" "size_t len" "const char *fmt" .Ft int .Fn backtrace_symbols_fmt_fd "void * const *addrlist" "size_t len" "const char *fmt" "int fd" .Sh DESCRIPTION The .Fn backtrace function places into the array pointed by .Fa addrlist the array of the values of the program counter for each frame called up to .Fa len frames. The number of frames found (which can be fewer than .Fa len ) is returned.
p The .Fn backtrace_symbols_fmt function takes an array of previously filled addresses from .Fn backtrace in .Fa addrlist of .Fa len elements, and uses .Fa fmt to format them. The formatting characters available are: l -tag -width a -offset indent t Dv a The numeric address of each element as would be printed using %p. t Dv n The name of the nearest function symbol (smaller than the address element) as determined by .Xr 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. t Dv d The difference of the symbol address and the address element printed using 0x%tx. t Dv D The difference of the symbol addresss and the address element printed using +0x%tx if non-zero, or nothing if zero. t Dv f The filename of the symbol as determined by .Xr dladdr 3 . .El
p The array of formatted strings is returned as a contiguous memory address which can be freed by a single .Xr free 3 .
p The .Fn backtrace_symbols function is equivalent of calling .Fn backtrace_symbols_fmt with a format argument of .Dv "%a <%n%D> at %f"
p The .Fn backtrace_symbols_fd and .Fn 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 .Dv 0 on success and .Dv -1 on failure. .Sh RETURN VALUES The .Fn backtrace function returns the number of elements that were filled in the backtrace. The .Fn backtrace_symbols and .Fn backtrace_symbols_fmt return a string array on success, and .Dv NULL on failure, setting .Va errno . Diagnostic output may also be produced by the ELF symbol lookup functions. .Sh SEE ALSO .Xr dladdr 3 , .Xr elf 3 .Sh HISTORY The .Fn backtrace library of functions first appeared in .Nx 7.0 and .Fx 10.0 . .Sh BUGS l -enum t Errors should not be printed but communicated to the caller differently. t Because these functions use .Xr elf 3 this is a separate library instead of being part of libc/libutil so that no library dependencies are introduced. t The Linux versions of the functions (there are no _fmt variants) use .Ft int instead of .Ft size_t arguments. .El