c - Why does LD_PRELOAD not seem to work for write with wc -

i playing around ld_preload intercept libc calls, appears write call doesn't intercepted wc, though seem work cat. stripped down version of problem appears below.

redhat linux 2.6.9-42.elsmp

makefile

writelib:         gcc -wall -rdynamic -fpic -c write.c         gcc -shared -wl,-soname,libwrite.so -wl,-export-dynamic -o libwrite.so write.o -ldl 

write.c:

#include <stdio.h> #include <string.h> #ifndef __use_gnu  #define __use_gnu  #define __use_gnu_defined #endif #include <dlfcn.h> #ifdef __use_gnu_defined  #undef __use_gnu  #undef __use_gnu_defined #endif #include <unistd.h> #include <stdlib.h>  static ssize_t (*libc_write)(int fd, const void *buf, size_t len);  ssize_t write(int fd, const void *buf, size_t len) {     static int already;     ssize_t ret;      if (!already) {             if ((libc_write = dlsym(rtld_next, "write")) == null) {                     exit(1);             }             = 1;     }       ret = (*libc_write)(fd,"ld_preload\n",11);     return len; // not ret cat doesn't take forever } 

output:

prompt: make gcc -wall -rdynamic -fpic -c write.c gcc -shared -wl,-soname,libwrite.so -wl,-export-dynamic -o libwrite.so write.o -ldl prompt: ld_preload=./libwrite.so /bin/cat write.c ld_preload prompt: ld_preload=./libwrite.so /usr/bin/wc write.c  32  70 572 write.c 

any explanations ?

that's because while cat uses write, wc uses printf, using either inlined version of write, or reference write bound libc, cannot interposed.

this can seen using ltrace:

$ echo foo | ltrace wc 2>&1 | grep 'write\|print' printf("%*s", 7, "1")                            = 7 printf(" %*s", 7, "1")                           = 8 printf(" %*s", 7, "4")                           = 8   $ echo foo | ltrace cat 2>&1 | grep 'write\|print' write(1, "foo\n", 4foo