gcc - Neither ld wrap nor LD_PRELOAD working to intercept system call -

i trying use -wl,-wrap=sendto -wl,-wrap,sendto in final g++ link command links app replace standard sendto function own.

i compile following source code gcc -c -o wrap.o wrap.c , include wrap.o in final g++ command links app (the rest of app c++ hence use of g++)

#include <stdio.h> #include <sys/types.h> #include <sys/socket.h>  ssize_t __real_sendto(int, const void *, size_t, int, const struct sockaddr *, socklen_t);  ssize_t __wrap_sendto (     int sockfd,     const void *buf,     size_t len,     int flags,     const struct sockaddr *dest_addr,     socklen_t addrlen ) {     printf("my wrap sendto ...\n");     return __real_sendto(sockfd, buf, len, flags, dest_addr, addrlen); } 

when use sendto in own source code, wrapper in fact used ok 3rd party shared objects linked against in final g++ command use sendto still use system sendto i.e. not wrapper. how can sendto wrapper used throughout ?

i have tried ld_preload approach sendto , dlsym(rtld_next) inside did not work either.

how can figure out why 3rd party library keeps on using libc sendto directly ?

when use ldd find shared object dependencies of compiled app , objdump -t on each 1 of them grepping sendto, und (undefined) 3rd party shared objects. shared objects define is:

/lib64/libpthread.so.0 000000000000ed80  w   df .text  0000000000000064  glibc_2.2.5 sendto /lib64/libc.so.6 00000000000e98d0  w   df .text  0000000000000064  glibc_2.2.5 sendto 

i see in glibc sendto.c on git following:

weak_alias (__libc_sendto, sendto) weak_alias (__libc_sendto, __sendto) 

can of in getting solution ?

the --wrap sendto option not define sendto symbol in binary. instead, replaces references symbols __wrap_sendto , leaves sendto undefined.

in other words, executable not provide sendto, run-time symbol resolution picks 1 glibc.

to fix need define sendto in executable. try dlsym once again, time without ld_preload/shim library:

ssize_t sendto (     int sockfd,     const void *buf,     size_t len,     int flags,     const struct sockaddr *dest_addr,     socklen_t addrlen ) {     ssize_t (*libc_sendto)(int, const void *, size_t, int, const struct sockaddr *, socklen_t)         = dlsym(rtld_next, "sendto");     printf("my wrap sendto ...\n");     return libc_sendto(sockfd, buf, len, flags, dest_addr, addrlen); } 

if third-party libraries keep finding wrong sendto after this, see 1 (not particularly likely) possibility. shared libraries linked -bsymbolic/-bsymbolic-functions , provide own sendto.

also, since you've tagged question g++, make sure symbol names don't mangled - use extern "c".