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

April 26, 2017Legal null pass pointer

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

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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 free the returned pointer when it is no longer needed.) #include #include /* 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);