man3/backtrace.3

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  26 .\"   glibc manual and source
  27 .TH BACKTRACE 3 2019-03-06 GNU "Linux Programmer's Manual"
  28 .SH NAME
  29 backtrace, backtrace_symbols, backtrace_symbols_fd \- support
  30 for application self-debugging
  31 .SH SYNOPSIS
  32 .B #include <execinfo.h>
  33 .PP
  34 .B int backtrace(void
  35 .BI ** buffer ,
  36 .B int
  37 .IB size );
  38 .PP
  39 .B char **backtrace_symbols(void *const
  40 .BI * buffer ,
  41 .B int
  42 .IB size );
  43 .PP
  44 .B void backtrace_symbols_fd(void *const
  45 .BI * buffer ,
  46 .B int
  47 .IB size ,
  48 .B int
  49 .IB fd );
  50 .SH DESCRIPTION
  51 .BR backtrace ()
  52 returns a backtrace for the calling program,
  53 in the array pointed to by
  54 .IR buffer .
  55 A backtrace is the series of currently active function calls for
  56 the program.
  57 Each item in the array pointed to by
  58 .I buffer
  59 is of type
  60 .IR "void\ *" ,
  61 and is the return address from
  62 the corresponding stack frame.
  63 The
  64 .I size
  65 argument specifies the maximum number of addresses
  66 that can be stored in
  67 .IR buffer .
  68 If the backtrace is larger than
  69 .IR size ,
  70 then the addresses corresponding to the
  71 .I size
  72 most recent function calls are returned;
  73 to obtain the complete backtrace, make sure that
  74 .I buffer
  75 and
  76 .I size
  77 are large enough.
  78 .PP
  79 Given the set of addresses returned by
  80 .BR backtrace ()
  81 in
  82 .IR buffer ,
  83 .BR backtrace_symbols ()
  84 translates the addresses into an array of strings that describe
  85 the addresses symbolically.
  86 The
  87 .I size
  88 argument specifies the number of addresses in
  89 .IR buffer .
  90 The symbolic representation of each address consists of the function name
  91 (if this can be determined), a hexadecimal offset into the function,
  92 and the actual return address (in hexadecimal).
  93 The address of the array of string pointers is returned
  94 as the function result of
  95 .BR backtrace_symbols ().
  96 This array is
  97 .BR malloc (3)ed
  98 by
  99 .BR backtrace_symbols (),
 100 and must be freed by the caller.
 101 (The strings pointed to by the array of pointers
 102 need not and should not be freed.)
 103 .PP
 104 .BR backtrace_symbols_fd ()
 105 takes the same
 106 .I buffer
 107 and
 108 .I size
 109 arguments as
 110 .BR backtrace_symbols (),
 111 but instead of returning an array of strings to the caller,
 112 it writes the strings, one per line, to the file descriptor
 113 .IR fd .
 114 .BR backtrace_symbols_fd ()
 115 does not call
 116 .BR malloc (3),
 117 and so can be employed in situations where the latter function might
 118 fail, but see NOTES.
 119 .SH RETURN VALUE
 120 .BR backtrace ()
 121 returns the number of addresses returned in
 122 .IR buffer ,
 123 which is not greater than
 124 .IR size .
 125 If the return value is less than
 126 .IR size ,
 127 then the full backtrace was stored; if it is equal to
 128 .IR size ,
 129 then it may have been truncated, in which case the addresses of the
 130 oldest stack frames are not returned.
 131 .PP
 132 On success,
 133 .BR backtrace_symbols ()
 134 returns a pointer to the array
 135 .BR malloc (3)ed
 136 by the call;
 137 on error, NULL is returned.
 138 .SH VERSIONS
 139 .BR backtrace (),
 140 .BR backtrace_symbols (),
 141 and
 142 .BR backtrace_symbols_fd ()
 143 are provided in glibc since version 2.1.
 144 .SH ATTRIBUTES
 145 For an explanation of the terms used in this section, see
 146 .BR attributes (7).
 147 .TS
 148 allbox;
 149 lbw22 lb lb
 150 l l l.
 151 Interface       Attribute       Value
 152 T{
 153 .BR backtrace (),
 154 .br
 155 .BR backtrace_symbols (),
 156 .br
 157 .BR backtrace_symbols_fd ()
 158 T}      Thread safety   MT-Safe
 159 .TE
 160 .SH CONFORMING TO
 161 These functions are GNU extensions.
 162 .SH NOTES
 163 These functions make some assumptions about how a function's return
 164 address is stored on the stack.
 165 Note the following:
 166 .IP * 3
 167 Omission of the frame pointers (as
 168 implied by any of
 169 .BR gcc (1)'s
 170 nonzero optimization levels) may cause these assumptions to be
 171 violated.
 172 .IP *
 173 Inlined functions do not have stack frames.
 174 .IP *
 175 Tail-call optimization causes one stack frame to replace another.
 176 .IP *
 177 .BR backtrace ()
 178 and
 179 .BR backtrace_symbols_fd ()
 180 don't call
 181 .BR malloc ()
 182 explicitly, but they are part of
 183 .IR libgcc ,
 184 which gets loaded dynamically when first used.
 185 Dynamic loading usually triggers a call to
 186 .BR malloc (3).
 187 If you need certain calls to these two functions to not allocate memory
 188 (in signal handlers, for example), you need to make sure
 189 .I libgcc
 190 is loaded beforehand.
 191 .PP
 192 The symbol names may be unavailable without the use of special linker
 193 options.
 194 For systems using the GNU linker, it is necessary to use the
 195 .I \-rdynamic
 196 linker option.
 197 Note that names of "static" functions are not exposed,
 198 and won't be available in the backtrace.
 199 .SH EXAMPLE
 200 The program below demonstrates the use of
 201 .BR backtrace ()
 202 and
 203 .BR backtrace_symbols ().
 204 The following shell session shows what we might see when running the
 205 program:
 206 .PP
 207 .in +4n
 208 .EX
 209 .RB "$" " cc \-rdynamic prog.c \-o prog"
 210 .RB "$" " ./prog 3"
 211 backtrace() returned 8 addresses
 212 \&./prog(myfunc3+0x5c) [0x80487f0]
 213 \&./prog [0x8048871]
 214 \&./prog(myfunc+0x21) [0x8048894]
 215 \&./prog(myfunc+0x1a) [0x804888d]
 216 \&./prog(myfunc+0x1a) [0x804888d]
 217 \&./prog(main+0x65) [0x80488fb]
 218 \&/lib/libc.so.6(__libc_start_main+0xdc) [0xb7e38f9c]
 219 \&./prog [0x8048711]
 220 .EE
 221 .in
 222 .SS Program source
 223 \&
 224 .EX
 225 #include <execinfo.h>
 226 #include <stdio.h>
 227 #include <stdlib.h>
 228 #include <unistd.h>
 229
 230 #define BT_BUF_SIZE 100
 231
 232 void
 233 myfunc3(void)
 234 {
 235     int j, nptrs;
 236     void *buffer[BT_BUF_SIZE];
 237     char **strings;
 238
 239     nptrs = backtrace(buffer, BT_BUF_SIZE);
 240     printf("backtrace() returned %d addresses\en", nptrs);
 241
 242     /* The call backtrace_symbols_fd(buffer, nptrs, STDOUT_FILENO)
 243        would produce similar output to the following: */
 244
 245     strings = backtrace_symbols(buffer, nptrs);
 246     if (strings == NULL) {
 247         perror("backtrace_symbols");
 248         exit(EXIT_FAILURE);
 249     }
 250
 251     for (j = 0; j < nptrs; j++)
 252         printf("%s\en", strings[j]);
 253
 254     free(strings);
 255 }
 256
 257 static void   /* "static" means don\(aqt export the symbol... */
 258 myfunc2(void)
 259 {
 260     myfunc3();
 261 }
 262
 263 void
 264 myfunc(int ncalls)
 265 {
 266     if (ncalls > 1)
 267         myfunc(ncalls \- 1);
 268     else
 269         myfunc2();
 270 }
 271
 272 int
 273 main(int argc, char *argv[])
 274 {
 275     if (argc != 2) {
 276         fprintf(stderr, "%s num\-calls\en", argv[0]);
 277         exit(EXIT_FAILURE);
 278     }
 279
 280     myfunc(atoi(argv[1]));
 281     exit(EXIT_SUCCESS);
 282 }
 283 .EE
 284 .SH SEE ALSO
 285 .BR addr2line (1),
 286 .BR gcc (1),
 287 .BR gdb (1),
 288 .BR ld (1),
 289 .BR dlopen (3),
 290 .BR malloc (3)