How can I set a breakpoint in C or C++ code programatically that will work for gdb on Linux?
I.e.:
int main(int argc, char** argv)
{
/* set breakpoint here! */
int a = 3;
a++; /* In gdb> print a; expect result to be 3 */
return 0;
}
One way is to signal an interrupt:
#include <csignal>
// Generate an interrupt
std::raise(SIGINT);
In C:
#include <signal.h>
raise(SIGINT);
UPDATE: Microsoft Docs says that Windows doesn't really support SIGINT, so if portability is a concern, you're probably better off using SIGABRT.
SIGINT is not supported for any Win32 application. When a CTRL+C interrupt occurs, Win32 operating systems generate a new thread to specifically handle that interrupt. This can cause a single-thread application, such as one in UNIX, to become multithreaded and cause unexpected behavior.
Disappointing to see so many answers not using the dedicated signal for software breakpoints, SIGTRAP:
#include <signal.h>
raise(SIGTRAP); // At the location of the BP.
On MSVC/MinGW, you should use DebugBreak, or the __debugbreak intrinsic. A simple #ifdef can handle both cases (POSIX and Win32).
SIGTRAP defined.
raise(SIGTRAP); raise(SIGTRAP);) , that seemed to work reliably!
By looking here, I found the following way:
void main(int argc, char** argv)
{
asm("int $3");
int a = 3;
a++; // In gdb> print a; expect result to be 3
}
This seems a touch hackish to me. And I think this only works on x86 architecture.
cl.exe) does not support this syntax, but uses a different syntax.
In a project I work on, we do this:
raise(SIGABRT); /* To continue from here in GDB: "signal 0". */
(In our case we wanted to crash hard if this happened outside the debugger, generating a crash report if possible. That's one reason we used SIGABRT. Doing this portably across Windows, Mac, and Linux took several attempts. We ended up with a few #ifdefs, helpfully commented here: http://hg.mozilla.org/mozilla-central/file/98fa9c0cff7a/js/src/jsutil.cpp#l66 .)
breakpoint() function in your program (it can be empty or just contain a print statement) and add break breakpoint to your ~/.gdbinit.
Commented
Sep 21, 2016 at 0:00
__asm__("int $3"); should work:
int main(int argc, char** argv)
{
/* set breakpoint here! */
int a = 3;
__asm__("int $3");
a++; /* In gdb> print a; expect result to be 3 */
return 0;
}
#define this, so that I don't have toi remember the syntax. I have it sprinkled throughout my code, sometimes in place of assert(), since stopping the debiugger let's me examine all variables & the stack. And, of course, like assert, I don't have to remove it for production code
raise() instead, but it is valuable to see what goes behind it.
int $3 is only "what goes behind it" on x86 systems, which the question doesn't specify...
Commented
Sep 9, 2021 at 22:11
On OS X you can just call std::abort() (it might be the same on Linux)
<debugging> will be available in C++26: std::breakpoint, std::breakpoint_if_debugging and std::is_debugger_present.
The cppreference documentations for std::breakpoint and std::is_debugger_present also mention some of the current solutions.
for ARM there is asm("BKPT");
https://developer.arm.com/documentation/dui0473/m/arm-and-thumb-instructions/bkpt
int mainrather thanvoid main.return 0is not necessary, though, and is just noise!mainis special in all versions of C++ and also in C since C99. Reaching the final}ofmainwithout returning is equivalent toreturn 0;so the compiler should not warn, and it cannot corrupt anything. In C89 it's undefined, but that's the exception to the rule, not the general case as you imply by "100% necessary". C++98 and C99 are not new, time to update your knowledge ;-)