GDB break if variable equal value

Question

I like to make GDB set a break point when a variable equal some value I set  I tried this example    include  lt stdio h gt  main           int i   0       for i 0 i lt 7   i          printf   d n   i         return 0      Output from GDB    gdb  break if   int i    5  No default breakpoint address now   gdb  run Starting program   home SIFE run  0 1 2 3 4 5 6  Program exited normally   gdb    Like you see  GDB didn t make any break point  is this possible with GDB

User · Answer

There are hardware and software watchpoints. They are for reading and for writing a variable. You need to consult a tutorial:

http://www.unknownroad.com/rtfm/gdbtut/gdbwatch.html

To set a watchpoint, first you need to break the code into a place where the varianle i is present in the environment, and set the watchpoint.

watch command is used to set a watchpoit for writing, while rwatch for reading, and awatch for reading/writing.

User · Answer

First  you need to compile your code with appropriate flags  enabling debug into code     gcc -Wall -g -ggdb -o ex1 ex1 c   then just run you code with your favourite debugger    gdb   ex1   show me the code    gdb  list 1    include  lt stdio h gt  2   int main void  3      4     int i   0  5     for i 0 i lt 7   i  6       printf   d n   i   7    8     return 0  9       break on lines 5 and looks if i    5    gdb  b 5 Breakpoint 1 at 0x4004fb  file ex1 c  line 5   gdb  rwatch i if i  5 Hardware read watchpoint 5  i   checking breakpoints   gdb  info b Num     Type           Disp Enb Address            What 1       breakpoint     keep y   0x00000000004004fb in main at ex1 c 5     breakpoint already hit 1 time 5       read watchpoint keep y                      i     stop only if i  5   running the program   gdb  c Continuing  0 1 2 3 4 Hardware read watchpoint 5  i  Value   5 0x0000000000400523 in main    at ex1 c 5 5     for i 0 i lt 7   i

User · Answer

You can use a watchpoint for this  A breakpoint on data instead of code    You can start by using watch i  Then set a condition for it using condition  lt breakpoint num gt  i    5  You can get the breakpoint number by using info watch

User · Answer

in addition to a watchpoint nested inside a breakpoint you can also set a single breakpoint on the  filename line number  and use a condition  I find it sometimes easier    gdb  break iter c 6 if i    5 Breakpoint 2 at 0x4004dc  file iter c  line 6   gdb  c Continuing  0 1 2 3 4  Breakpoint 2  main    at iter c 6 6           printf   d n   i     If like me you get tired of line numbers changing  you can add a label then set the breakpoint on the label like so    include  lt stdio h gt  main           int i   0       for i 0 i lt 7   i           looping          printf   d n   i               return 0      gdb  break main looping if i    5

[c] GDB: break if variable equal value

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