c - Realloc setting pointer to empty -

void test(){    char *c = malloc(strlen("i coffe") + 1);    strcpy(c, "i coffe");    char **s = &c;    while(strlen(*s) < 25)       my_function(s); }  void my_function(char **s){   char *w  = *s;       char *tmp = realloc(w, len + 2);//error here. *s gets = ""   if(tmp != null)   w = tmp;   for(i= len; i>=p; i--){       w[i+1] = w[i];   }   w[p] = c;     }   

this function used insert new character inside char *.
also, function inside while loop. works fine 3rd time loop runs, sets *s = "".
thought using char *tmp keep data if wrong thing happen. can't understand why p *s been setted empty string.

you've forgotten assign new value *s in function contains realloc().

void test(void)  // unaltered; still broken! {     char *c = malloc(strlen("i coffe") + 1);     strcpy(c, "i coffe");     char **s = &c;     while (strlen(*s) < 25)         my_function(s); }  void my_function(char **s)  // fixed 1 way {     char *w  = *s;        size_t len = strlen(w) + 1;  // define , initialize len      char *tmp = realloc(w, len + 2);     if (tmp != null)         w = tmp;     *s = w;  // reassign `*s` } 

or, more simply:

void my_function(char **s)  // fixed way {     char *w  = *s;        size_t len = strlen(w);  // define , initialize len      char *tmp = realloc(w, len + 2);     if (tmp != null)         *s = tmp;  // reassign `*s` } 

assigning w sets local variable copy of *s; not reset pointer in calling code.

note fix, loop in test() going run long time because nothing changes length of string in c. there's another problem: don't pass address of s my_function(), my_function() can't modify s.

void test(void) {     char *c = malloc(strlen("i coffe") + 1);     strcpy(c, "i coffe");     while (strlen(c) < 25)     {         my_function(&c);         strcat(c, "az");  // grow string — not in real code         printf("%2zu: <<%s>>\n", strlen(c), c);     } }  void my_function(char **s) {     char *w = *s;        size_t len = strlen(w) + 1;  // define , initialize len      char *tmp = realloc(w, len + 2);     if (tmp != null)         *s = tmp;  // reassign `*s` } 

this away pointer pointer char in test(). if that's crucial, there's more thinking done.

^{code not formally tested yet!}

code tested — can "pig's ear"? copy'n'paste of wrong material made test code fail. here's instrumented working version — valgrind gives clean bill of health.

#include <stdio.h> #include <stdlib.h> #include <string.h>  static void my_function(char **s) {     char *w = *s;     size_t len = strlen(w) + 1;  // define , initialize len     printf("m1: %p: %2zu: <<%s>>\n", (void *)w, len, w);      char *tmp = realloc(w, len + 2);     if (tmp != null)         *s = tmp;  // reassign `*s`     printf("m2: %p: %2zu: <<%s>>\n", (void *)*s, strlen(*s), *s); }  static void test(void) {     char *c = malloc(strlen("i coffe") + 1);     if (c == 0)     {         fprintf(stderr, "out of memory\n");         exit(exit_failure);     }     strcpy(c, "i coffe");     printf("t1: %p: %2zu: <<%s>>\n", (void *)c, strlen(c), c);     while (strlen(c) < 25)     {         my_function(&c);         printf("t2: %p: %2zu: <<%s>>\n", (void *)c, strlen(c), c);         if (c == null)         {             fprintf(stderr, "out of memory\n");             exit(exit_failure);         }         strcat(c, "az");  // grow string — not in real code         printf("t3: %p: %2zu: <<%s>>\n", (void *)c, strlen(c), c);     }     free(c); }  int main(void) {     test();     return 0; }