C Language Most common errors encountered while using Valgrind


Example

Valgrind provides you with the lines at which the error occurred at the end of each line in the format (file.c:line_no). Errors in valgrind are summarised in the following way:

ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

The most common errors include:

  1. Illegal read/write errors
==8451== Invalid read of size 2
==8451==    at 0x4E7381D: getenv (getenv.c:84)
==8451==    by 0x4EB1559: __libc_message (libc_fatal.c:80)
==8451==    by 0x4F5256B: __fortify_fail (fortify_fail.c:37)
==8451==    by 0x4F5250F: __stack_chk_fail (stack_chk_fail.c:28)
==8451==    by 0x40059C: main (valg.c:10)
==8451==  Address 0x700000007 is not stack'd, malloc'd or (recently) free'd

This happens when the code starts to access memory which does not belong to the program. The size of the memory accessed also gives you an indication of what variable was used.

  1. Use of Uninitialized Variables
==8795== 1 errors in context 5 of 8:
==8795== Conditional jump or move depends on uninitialised value(s)
==8795==    at 0x4E881AF: vfprintf (vfprintf.c:1631)
==8795==    by 0x4E8F898: printf (printf.c:33)
==8795==    by 0x400548: main (valg.c:7)

According to the error, at line 7 of the main of valg.c, the call to printf() passed an uninitialized variable to printf.

  1. Illegal freeing of Memory
==8954== Invalid free() / delete / delete[] / realloc()
==8954==    at 0x4C2EDEB: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==8954==    by 0x4005A8: main (valg.c:10)
==8954==  Address 0x5203040 is 0 bytes inside a block of size 240 free'd
==8954==    at 0x4C2EDEB: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==8954==    by 0x40059C: main (valg.c:9)
==8954==  Block was alloc'd at
==8954==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==8954==    by 0x40058C: main (valg.c:7)

According to valgrind, the code freed the memory illegally (a second time) at line 10 of valg.c, whereas it was already freed at line 9, and the block itself was allocated memory at line 7.