realloc: invalid next size and malloc: memory corruption (fast)
c, realloc
Solution
You´re reallocating on copied addresses (because parameters). A parameter in C is a copy of the original value everytime; in case of a pointer it will point to the same location but the address itself is copied.
`realloc` resizes the buffer asociated with the address, everything fine so far. But it can relocate the whole thing and assign a completely new address, and this new address (if it happens) will be lost after the function returns to main.
Use a double pointer: Pass a `char **s` instead of `char *s` (==`char s[]`) as formal parameter, pass `&xyz` intead of `xyz` as actual value, and inside the function, use `*xyz` and `**xyz` (or `(*xyz)[index]`) for address and value.
Other things: Global variables are ugly (and confusing when named same as parameters), multiplying with `sizeof(char)` is nonsense because it´s be 1 everytime, and names in capitals should be used for `#define`´s rather than variables.
Problem
I am doing an exercise for fun from K and R C programming book. The program is for finding the longest line from a set of lines entered by the user and then prints it. Inputs: ``` This is a test This is another long test this is another long testthis is another long test ``` Observation: It runs fine for the first two inputs but fails for the larger string (3rd input) Errors: ``` Error in `./longest': realloc(): invalid next size: 0x000000000246e010 *** Error in `./longest': malloc(): memory corruption (fast): 0x000000000246e030 *** ``` My efforts: I have been trying to debug this since 2 days (rubber duck debugging) but the logic seems fine. GDB points to the realloc call in the _getline function and shows a huge backtrace with glibc.so memory allocation calls at the top. Here is what I have written (partially, some part is taken from the book directly):- ``` #include <stdio.h> #include <stdlib.h> int MAXLINE = 10; int INCREMENT = 10; char* line = NULL, *longest = NULL; void _memcleanup(){ free(line); free(longest); } void copy(char longest[], char line[]){ int i=0; char* temp = realloc(longest,(MAXLINE)*sizeof(char)); if(temp == NULL){ printf("%s","Unable to allocate memory"); _memcleanup(); exit(1); } longest = temp; while((longest[i] = line[i]) != '\0'){ ++i; } } int _getline(char s[]){ int i,c; for(i=0; ((c=getchar())!=EOF && c!='\n'); i++){ if(i == MAXLINE - 1){ char* temp = realloc(s,(MAXLINE + INCREMENT)*sizeof(char)); if(temp == NULL){ printf("%s","Unable to allocate memory"); _memcleanup(); exit(1); } s= temp; MAXLINE += INCREMENT; } s[i] = c; } if(c == '\n'){ s[i++] = c; } s[i]= '\0'; return i; } int main(){ int max=0, len; line = malloc(MAXLINE*sizeof(char)); longest = malloc(MAXLINE*sizeof(char)); while((len = _getline(line)) > 0){ printf("%d%d", len, MAXLINE); if(len > max){ max = len; copy(longest, line); } } if(max>0){ printf("%s",longest); } _memcleanup(); return 0; } ```