c - Size of allocated memory to a pointer changes when increment the pointer -

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• program crashes when deleting pointer pointing heap? 2 answers

 char *ptr = (char*)malloc(10);      if(null == ptr)     {         printf("\n malloc failed \n");         return -1;     }     else if(argc == 1)     {         printf("\n usage  \n");     }     else     {         memset(ptr, 0, 10);          strncpy(ptr, argv[1], 9);          while(*ptr != 'z')         {             ptr++;         }          if(*ptr == 'z')         {             printf("\n string contains 'z'\n");             /* more processing */         }         free(ptr);     } 

in previous code, lets arguments program is: mixx, program gives segmentation error.

and question is:
when in while loop:

ptr++; 

does mean size of memory allocated pointer ptr changes , that's why when call free() function crashes.

your hunch correct: behaviour of program undefined.

you must pass original value of ptr free.

(also, while(*ptr != 'z') vulnerable overrunning input. consider checking \0 too.)