Valgrind memory access errors on trivial files

[charles-randall]

Sorting the attached 3-line file under valgrind gives the attached errors on my system. I don't think that there's anything special about the input file as it's trivial.

$ gcc --version
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0

$ valgrind --version
valgrind-3.15.0

Msort compiled with "gcc -ggdb" or "gcc -g" as /tmp/msort-g,

$ valgrind --log-file=abc.valgrind.txt /tmp/msort-g abc.txt
a
b
c

This produces the correct output but please see the valgrind log.

Can you reproduce this? Is this real?

The first error is,

==3408920== Invalid read of size 4
==3408920==    at 0x4A337D7: fgets (iofgets.c:47)
==3408920==    by 0x10AB49: fillBuffer (msort.c:666)
==3408920==    by 0x10B32D: handleMerges (msort.c:846)
==3408920==    by 0x10BE19: main (msort.c:1131)
==3408920==  Address 0x4ba5b70 is 0 bytes inside a block of size 472 free'd
==3408920==    at 0x483CA3F: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==3408920==    by 0x4A33042: _IO_deallocate_file (libioP.h:863)
==3408920==    by 0x4A33042: fclose@@GLIBC_2.2.5 (iofclose.c:74)
==3408920==    by 0x10B391: handleMerges (msort.c:856)
==3408920==    by 0x10BE19: main (msort.c:1131)
==3408920==  Block was alloc'd at
==3408920==    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==3408920==    by 0x4A33AAD: __fopen_internal (iofopen.c:65)
==3408920==    by 0x4A33AAD: fopen@@GLIBC_2.2.5 (iofopen.c:86)
==3408920==    by 0x10B28E: handleMerges (msort.c:827)
==3408920==    by 0x10BE19: main (msort.c:1131)

abc.txt
abc.valgrind.txt

[charles-randall]

I believe that you're using a filehandle after it has been closed. The easy way to find this is to set the filehandle to NULL after calling fclose() and the program will crash when you try to use a filehandle that's already been closed.

E.g., replace,
fclose(f1);

with,
fclose(f1); f1=NULL;

everywhere.