Question 7.30

comp.lang.c FAQ list · Question 7.30

Q: Is it legal to pass a null pointer as the first argument to realloc? Why would you want to?

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A: ANSI C sanctions this usage (and the related realloc(..., 0), which frees), although several earlier implementations do not support it, so it may not be fully portable. Passing an initially-null pointer to realloc can make it easier to write a self-starting incremental allocation algorithm.
Here is an example--this function reads an arbitrarily-long line into dynamically-allocated memory, reallocating the input buffer as necessary. (The caller must freethe returned pointer when it is no longer needed.)

#include <stdio.h>
#include <stdlib.h>

/* read a line from fp into malloc'ed memory */
/* returns NULL on EOF or error */
/* (use feof or ferror to distinguish) */

char *agetline(FILE *fp)
{
 char *retbuf = NULL;
 size_t nchmax = 0;
 register int c;
 size_t nchread = 0;
 char *newbuf;

 while((c = getc(fp)) != EOF) {
  if(nchread >= nchmax) {
   nchmax += 20;
   if(nchread >= nchmax) { /* in case nchmax overflowed */
    free(retbuf);
    return NULL;
   }
#ifdef SAFEREALLOC
   newbuf = realloc(retbuf, nchmax + 1);
#else
   if(retbuf == NULL) /* in case pre-ANSI realloc */
    newbuf = malloc(nchmax + 1);
   else newbuf = realloc(retbuf, nchmax + 1);
#endif
      /* +1 for \0 */
   if(newbuf == NULL) {
    free(retbuf);
    return NULL;
   }

   retbuf = newbuf;
  }

  if(c == '\n')
   break;

  retbuf[nchread++] = c;
 }

 if(retbuf != NULL) {
  retbuf[nchread] = '\0';

  newbuf = realloc(retbuf, nchread + 1);
  if(newbuf != NULL)
   retbuf = newbuf;
 }

 return retbuf;
}

(In production code, a line like nchmax += 20 can prove troublesome, as the function may do lots of reallocating. Many programmers favor multiplicative reallocation, e.g. nchmax *= 2, although it obviously isn't quite as self-starting, and can run into problems if it has to allocate a huge array but memory is limited.)
References: ISO Sec. 7.10.3.4
H&S Sec. 16.3 p. 388