<\/p>\n
I figured I’ll write a mini-tutorial on how to use gdb, because there’s not that many places where they teach you how to use gdb effectively.<\/p>\n
<\/p>\n
Let’s say you have a program and it crashes, what do you do?<\/p>\n
<\/p>\n
Example code:<\/p>\n
<\/p>\n
“`<\/p>\n
void func() { char *p = 0; *p = 0x69; }<\/span><\/p>\n int main() { func(); }<\/span><\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n Next:<\/p>\n <\/p>\n “`<\/p>\n gdb a.out<\/span><\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n Followed by the `run` command:<\/p>\n <\/p>\n “`<\/p>\n (gdb) run<\/span><\/p>\n Starting program: \/home\/d\/a.out <\/span><\/p>\n [Thread debugging using libthread_db enabled]<\/span><\/p>\n Using host libthread_db library “\/lib\/x86_64-linux-gnu\/libthread_db.so.1”.<\/span><\/p>\n Program received signal SIGSEGV, Segmentation fault.<\/span><\/p>\n 0x000055555555513d in func () at test.c:1<\/span><\/p>\n 1\tvoid func() { char *p = 0; *p = 0x69; }<\/span><\/p>\n (gdb) <\/span><\/p>\n <\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n You can see where it crashes, but you’d like a stacktrace…<\/p>\n <\/p>\n “`<\/p>\n gdb) bt<\/span><\/p>\n #0 0x000055555555513d in func () at test.c:1<\/span><\/p>\n #1 0x0000555555555155 in main () at test.c:3<\/span><\/p>\n (gdb) <\/span><\/p>\n <\/p>\n <\/p>\n “`<\/p>\n <\/p>\n (NB: `bt` stands for backtrace)<\/p>\n <\/p>\n You can go up a frame and check local variables:<\/p>\n <\/p>\n “`<\/p>\n (gdb) up<\/span><\/p>\n #1 0x0000555555555155 in main () at test.c:3<\/span><\/p>\n 3\tint main() { func(); }<\/span><\/p>\n (gdb) info local<\/span><\/p>\n No locals.<\/span><\/p>\n <\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n Or down a frame and check local variables:<\/p>\n <\/p>\n “`<\/p>\n (gdb) down<\/span><\/p>\n #0 0x000055555555513d in func () at test.c:1<\/span><\/p>\n 1\tvoid func() { char *p = 0; *p = 0x69; }<\/span><\/p>\n (gdb) info local<\/span><\/p>\n p = 0x0<\/span><\/p>\n <\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n <\/p>\n You can continue after the segfault:<\/p>\n <\/p>\n “`<\/p>\n (gdb) cont<\/span><\/p>\n Continuing.<\/span><\/p>\n Program terminated with signal SIGSEGV, Segmentation fault.<\/span><\/p>\n The program no longer exists.<\/span><\/p>\n (gdb) <\/span><\/p>\n <\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n Now we can re-run it, but before we do, we can set a breakpoint:<\/p>\n <\/p>\n “`<\/p>\n (gdb) break func<\/span><\/p>\n Breakpoint 1 at 0x555555555131: file test.c, line 1.<\/span><\/p>\n (gdb) <\/span><\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n Now we run it again, it will stop at the breakpoint:<\/p>\n <\/p>\n “`<\/p>\n (gdb) run<\/span><\/p>\n Starting program: \/home\/d\/a.out <\/span><\/p>\n [Thread debugging using libthread_db enabled]<\/span><\/p>\n Using host libthread_db library “\/lib\/x86_64-linux-gnu\/libthread_db.so.1”.<\/span><\/p>\n Breakpoint 1, func () at test.c:1<\/span><\/p>\n 1\tvoid func() { char *p = 0; *p = 0x69; }<\/span><\/p>\n (gdb) <\/span><\/p>\n <\/p>\n “`<\/p>\n <\/p>\n We can single step through it:<\/p>\n <\/p>\n “`<\/p>\n (gdb) step<\/span><\/p>\n Program received signal SIGSEGV, Segmentation fault.<\/span><\/p>\n 0x000055555555513d in func () at test.c:1<\/span><\/p>\n 1\tvoid func() { char *p = 0; *p = 0x69; }<\/span><\/p>\n (gdb) <\/span><\/p>\n <\/p>\n “`<\/p>\n <\/p>\n <\/p>\n We can also try the command `next`, which is similar to `step` but skips over subroutines.<\/p>\n <\/p>\n We can get help from gdb at any time using `help`.<\/p>\n <\/p>\n Another useful `info` command is `info reg`, which shows CPU registers.<\/p>\n <\/p>\n Also useful is `disas` command, which disassembles the code.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Using gdb I figured I’ll write a mini-tutorial on how to use gdb, because there’s not that many places where they teach you how to use gdb effectively. Let’s say you have a program and it crashes, what do you do? Example code: “` void func() { char *p = 0; *p = 0x69; } […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-56","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts\/56","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=56"}],"version-history":[{"count":1,"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts\/56\/revisions"}],"predecessor-version":[{"id":57,"href":"https:\/\/davidshoon.com\/index.php?rest_route=\/wp\/v2\/posts\/56\/revisions\/57"}],"wp:attachment":[{"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=56"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=56"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/davidshoon.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=56"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n
<\/span><\/p>\n