使用malloc和fgetc的内存泄漏

Question

I'm having some trouble reading a like from user using malloc and getchar. I get the result, however, I get memory leaks using valgrind. I am pretty clueless in this and have asked my classmates and mentors, but none seem to find out why.

char *ReadLineFile(FILE *infile){
   int i=0;
   char c;
   char *newStringLine;
   newStringLine = (char *) malloc(sizeof(char));
   while( (c = fgetc(infile)) != '\n' ){
        newStringLine[i++] = c;
        realloc(newStringLine, (sizeof(char) * (i+1)));
   }
   newStringLine[i] = '\0';
   return newStringLine;
}

Valgrind gives me several errors, including Invalid write/read of 1, and invalid realloc.

Answer 1

Your usage of realloc() is erroneous.

realloc() , if successful, frees the passed pointer and returns a new pointer with the allocated memory. You need to

• catch the return value of realloc() in a temporary pointer,

• check against NULL to ensure success and then

  • If returned pointer is not NULL, ie, reallocation is successful, use the new pointer.
  • If the returned pointer is NULL, make some decisions and you can keep using the old pointer (passed as argument).

Related, quoting C11 , chapter §7.22.3.5

The realloc function deallocates the old object pointed to by ptr and returns a pointer to a new object that has the size specified by size . [....]

and,

[...] If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.

Otherwise, in case, realloc() is successful, you are ( most probably ) trying to use an already free-d memory, which , of course, causes undefined behavior .

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Uh-oh, and did I mention, please see this discussion on why not to cast the return value of malloc() and family in C ?