Ben Palmer Ben Palmer - 1 month ago 7
C Question

Why isn't C freeing memory from malloc'd char* set by strtok?

This function reads the values from a text file with space separated values, into a 2d array. When I run it, works just fine - but a memory leak check through Valgrind confirms Xcode's suspicions that "char *splitString" is never freed, the two times which it is called. I don't understand this, considering my "char *buffer" seems to be freed just fine. Any help is super appreciated!

int** readMatrixFile(char* inFileName, int** matrix, int sizeY, int sizeX)
{
FILE* matrixFP;
int ii=0, jj=0, fileValid = 1;
char *buffer, *splitString;
const char delim[]=" \n\r";

matrixFP = fopen(inFileName, "r");
if(matrixFP != NULL)
{
/*Check if file is the same size as specified by the command line
*assumed valid until the file is checked*/
splitString = malloc(100*sizeof(char)); <------where allocated
buffer = malloc(5000*sizeof(char));
do
{
fgets(buffer, 5000, matrixFP);
jj=0;
splitString = strtok(buffer, delim);
while(splitString != NULL)
{
jj++;
splitString = strtok(NULL, delim);
}
if(jj!=sizeX)
{
fileValid = 0;
}
ii++;
} while(!feof(matrixFP));
if(ii != sizeY || buffer==NULL)
{
fileValid = 0;
}

free(splitString); <-----Appears to do nothing?
free(buffer);

if(fileValid) /*Files match to specified command line values*/
{
ii=0;
rewind(matrixFP);
matrix = (int**)malloc(sizeY * sizeof(int *));
do
{
matrix[ii] = (int*)malloc(sizeX * sizeof(int));
jj=0;
do
{
fscanf(matrixFP, "%d", &matrix[ii][jj]);
jj++;
} while(jj<sizeX);
ii++;
} while(ii<sizeY && !feof(matrixFP));
}
else
{
printf("Error: File does not match size specified by the command line\n");
}
fclose(matrixFP);
}
else
{
perror("Error: File does not exist or is invalid");
matrix = NULL;
}

return matrix;
}


And Valgrind output:

==14087== Memcheck, a memory error detector
==14087== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==14087== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==14087== Command: ./pmms a b 10 3 10
==14087==
/*irrelevent program output*/
==14087==
==14087== HEAP SUMMARY:
==14087== in use at exit: 200 bytes in 2 blocks
==14087== total heap usage: 21 allocs, 19 frees, 11,680 bytes allocated
==14087==
==14087== 100 bytes in 1 blocks are definitely lost in loss record 1 of 2
==14087== at 0x4A06A2E: malloc (vg_replace_malloc.c:270)
==14087== by 0x400B55: readMatrixFile (matrix_reader.c:35)
==14087== by 0x40095E: main (pmms.c:23)
==14087==
==14087== 100 bytes in 1 blocks are definitely lost in loss record 2 of 2
==14087== at 0x4A06A2E: malloc (vg_replace_malloc.c:270)
==14087== by 0x400B55: readMatrixFile (matrix_reader.c:35)
==14087== by 0x400982: main (pmms.c:24)
==14087==
==14087== LEAK SUMMARY:
==14087== definitely lost: 200 bytes in 2 blocks
==14087== indirectly lost: 0 bytes in 0 blocks
==14087== possibly lost: 0 bytes in 0 blocks
==14087== still reachable: 0 bytes in 0 blocks
==14087== suppressed: 0 bytes in 0 blocks
==14087==
==14087== For counts of detected and suppressed errors, rerun with: -v
==14087== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 6)

Answer

First, you iterate

    while(splitString != NULL)
    {
          ...
    }

so after the loop completion splitString is NULL, and free(NULL) does nothing.

Second, once you allocated a hundred-byte memory block

    splitString = malloc(100*sizeof(char));   // (*)

the next reference to the splitString variable is an assignment:

    splitString = strtok(buffer, delim);

So, whatever was stored in the variable, it got overwritten! splitString points somewhere into the buffer block now and the address of the block allocated at (*) is lost forever. That, by the way, indicates you never use that block, and you don't actually need to allocate it.

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