malloc with variable length

Hi, I'm using arm cortex, KEIL, FreeRTOS. The problem I met is:
after many times of running the following malloc function, the heap was used out, couldn't allocat 1KB space any more.

char Str = (char )myMalloc(strLen); // strLen is a variable, value between 0 and 1024
if(Str != NULL){
......
myFree(Str);
}

Here are the definitions of myMalloc and myFree:

void myMalloc( size_t xWantedSize )
{ void pvReturn;

pvReturn = NULL;
if(xWantedSize) {
vTaskSuspendAll();
pvReturn = malloc( xWantedSize );
xTaskResumeAll();
} return pvReturn;
} void myFree( void *pv )
{ if( pv ){
vTaskSuspendAll();
free( pv );
xTaskResumeAll();
} }

After the parameter strLen was changed to a fixed value, the problem was gone. Why variable length malloc doesn't work properly? Anything wrong in functions above?

Any help will be appreciated.

Thank you!