Valgrind showing memory leak for printf and unused blocks

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]){
  char *str = malloc(sizeof(char)*5);
  str = strcpy(str, "test");
  printf("%s\n", str);
  free(str);
  return 0;
}

When I use Valgrind on my Mac (OS X, 10.9.5) I get the following message:

==77215== HEAP SUMMARY:
==77215==     in use at exit: 29,211 bytes in 374 blocks
==77215==   total heap usage: 451 allocs, 77 frees, 35,160 bytes allocated
==77215== 
==77215== 4,096 bytes in 1 blocks are still reachable in loss record 76 of 76
==77215==    at 0x66CB: malloc (in /usr/local/Cellar/valgrind/3.10.0/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==77215==    by 0x182855: __smakebuf (in /usr/lib/system/libsystem_c.dylib)
==77215==    by 0x197217: __swsetup (in /usr/lib/system/libsystem_c.dylib)
==77215==    by 0x1B0158: __v2printf (in /usr/lib/system/libsystem_c.dylib)
==77215==    by 0x1B06AF: __xvprintf (in /usr/lib/system/libsystem_c.dylib)
==77215==    by 0x187B29: vfprintf_l (in /usr/lib/system/libsystem_c.dylib)
==77215==    by 0x18596F: printf (in /usr/lib/system/libsystem_c.dylib)
==77215==    by 0x100000F2B: main (test.c:8)
==77215== 
==77215== LEAK SUMMARY:
==77215==    definitely lost: 0 bytes in 0 blocks
==77215==    indirectly lost: 0 bytes in 0 blocks
==77215==      possibly lost: 0 bytes in 0 blocks
==77215==    still reachable: 4,096 bytes in 1 blocks
==77215==         suppressed: 25,115 bytes in 373 blocks
==77215== 
==77215== For counts of detected and suppressed errors, rerun with: -v
==77215== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 15 from 15)

Does printf allocate memory by itself? If I remove the printf I only get the following:

==77237== HEAP SUMMARY:
==77237==     in use at exit: 25,115 bytes in 373 blocks
==77237==   total heap usage: 450 allocs, 77 frees, 31,064 bytes allocated
==77237== 
==77237== LEAK SUMMARY:
==77237==    definitely lost: 0 bytes in 0 blocks
==77237==    indirectly lost: 0 bytes in 0 blocks
==77237==      possibly lost: 0 bytes in 0 blocks
==77237==    still reachable: 0 bytes in 0 blocks
==77237==         suppressed: 25,115 bytes in 373 blocks
==77237== 
==77237== For counts of detected and suppressed errors, rerun with: -v
==77237== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 15 from 15)

Where does the 373 blocks come from?

Until the Valgrind team prioritizes OS X, you can safely assume that it will not give correct results on Apple systems running OS X versions newer than 10.7.

On my Mavericks (10.9.5) machine, I still get the following warning from Valgrind (3.9.0)

WARNING: Support on MacOS 10.8/10.9 is experimental and mostly broken.
WARNING: Expect incorrect results, assertions and crashes.
WARNING: In particular, Memcheck on 32-bit programs will fail to
WARNING: detect any errors associated with heap-allocated data.

For what it's worth, Valgrind 3.10.0 shows no leaks on my Debian Jessie installation.

abligh

It isn't telling you that there is a leak:

==77215== LEAK SUMMARY:
==77215==    definitely lost: 0 bytes in 0 blocks
==77215==    indirectly lost: 0 bytes in 0 blocks
==77215==      possibly lost: 0 bytes in 0 blocks
==77215==    still reachable: 4,096 bytes in 1 blocks
==77215==         suppressed: 25,115 bytes in 373 blocks

it's telling you that there is a block which is still reachable; whether or not there is a leak is dependent on your definition of 'leak'. What it does mean is that there is a pointer to the block somewhere.

Refer to Still Reachable Leak detected by Valgrind for more information.

来源：https://stackoverflow.com/questions/26409611/valgrind-showing-memory-leak-for-printf-and-unused-blocks

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