The problem with realloc () in C. Always freezes, but compiles fine

Question

I am having problems with a program that is designed for the String buffer, especially this function is designed to reset the buffer with the string cstr. If cstr is null, the contents should be reset to an empty char '\ 0'. It always hangs in the second realloc set, where it resizes buf-> content. I do not know why this is so. Any help would be awesome.

Structure:

typedef struct strbuf {
     char   *contents;
     size_t  length;  
} StringBuffer;

It is called from

strbuf_reset(sb, NULL)

Here is the strbuf_reset function that has the problem.

StringBuffer *strbuf_reset(StringBuffer *buf, const char *cstr)
{
if(buf == NULL)
    return NULL;

StringBuffer *tempBuf = NULL ;

if(cstr == NULL)
    tempBuf = (StringBuffer*)realloc(buf,sizeof(StringBuffer) + sizeof(char));
else
    tempBuf = (StringBuffer*)realloc(buf,sizeof(buf) + strlen(cstr)*sizeof(char));

if(tempBuf == NULL)
    return NULL;

if(cstr == NULL)
    tempBuf->contents = (char*)realloc(buf->contents,sizeof(char));
else
    tempBuf->contents = (char*)realloc(buf->contents,(sizeof(buf->contents) + strlen(cstr)*sizeof(char) + 1));

if(tempBuf->contents == NULL){
    free(tempBuf);
    return NULL;
}
buf = tempBuf;

if(cstr == NULL)
   buf->contents = '\0';
else
   strcat(buf->contents,cstr);

buf->length = strlen(buf->contents);    

return buf;
 }

With what I consider the proposed changes ...

StringBuffer *strbuf_reset(StringBuffer *buf, const char *cstr)
{
if(buf == NULL)
    return NULL;

StringBuffer *tempBuf = NULL ;

if(cstr == NULL)
    tempBuf = (StringBuffer*)realloc(buf,sizeof(StringBuffer) + sizeof(char) + 10);
else
    tempBuf = (StringBuffer*)realloc(buf,sizeof(buf) + strlen(cstr)*sizeof(char)+ 1);

if(tempBuf != NULL)
    buf = tempBuf;
else
    return NULL;    

if(cstr == NULL)
    tempBuf->contents = (StringBuffer*)realloc(buf->contents,sizeof(StringBuffer) + sizeof(char) + 10);
else
    tempBuf->contents = (StringBuffer*)realloc(buf->contents,sizeof(buf) + strlen(cstr)*sizeof(char)+ 1);

if(tempBuf != NULL)
    buf->contents = tempBuf->contents;
else
    return NULL;

if(cstr == NULL)
   buf->contents = '\0';
else
   strcat(buf->contents,cstr);

buf->length = strlen(buf->contents);    

return buf;
 }

Answer 1

StringBuffer, realloc . , , . malloc .

typedef struct strbuf {
  char   *contents;
  size_t  length;  
} StringBuffer;

strbuf_reset buf cstr. buf, NULL.

StringBuffer *strbuf_reset(StringBuffer *buf, const char *cstr)
{
  if (!buf)  return NULL;      
  if (!cstr)  cstr = "";

  size_t len = strlen(cstr);
  if (len > buf->length) {
    char *new_contents = malloc(len + 1);
    if (!new_contents)  return NULL;
    free(buf->contents);
    buf->contents = new_contents;
  }
  memcpy(buf->contents, cstr, len + 1);
  buf->length = len;

  return buf;
}

Answer 2

, , realloc.

( , ), , , .

, , .

, .

Answer 3

StringBuffer. , buf- > . , StringBuffer?

buf- > , StringBuffer, , // , , .

, buf- > , , :

struct StringBuffer {
    size_t length;
    char contents[1];
};

buf- > , realloc StringBuffer .