C程序中的分段错误（核心已转储）

Question

I have the following program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 255

char * decrypt(char *p, int key){
    char *tmp;
    for(int i = 0; p[i] != '\0'; i++){
        tmp[i] = p[i]-key;
    }
    return tmp;
}

int main(void){
    printf("Hallo Welt!");
    printf("%s\n", decrypt("ibmmp", 1));
    return EXIT_SUCCESS;
}

When I compile it with gcc -Wall i get the Warning tmp could get uninitialized in this function [-Wmaybe-uninitialized] tmp[i] = p[i]-key (translated from german) and segmentation fault (core dumped) ./crypto when i run it

What is causing that error?

I know this quesion has been asked many times, but i could not fix this warning, because other people had different sourcecodes and i couldn't adapt it to my problem.

Answer 1

You need to allocate 'tmp' and then, keeping with good 'c' coding, check that the allocation was successful. I assume you have MAX defined so you can set an upper-bound on the length of your string, so I use that below. If MAX is intended to be the number of characters without a null, then you need to 'malloc(MAX +1)'. If it is intended to include NULL, then just leave the code as defined below. You also want to decide what to return on failure of the malloc. I return NULL, but you may want to do something different depending on your needs.

Also be aware, that this function is returning allocated memory, so someone needs to free it so you aren't leaking memory.

char * decrypt(char *p, int key){
    char *tmp;
    tmp = (char *) malloc(MAX);
    if(!tmp)
        return NULL;
    for(int i = 0; p[i] != '\0'; i++){
        tmp[i] = p[i]-key;
    }
    return tmp;
} 

Answer 2

Allocate memory for tmp before using it. Make sure you null-terminate the string before returning it.

// Make the input a const string.
// char * decrypt(char *p, int key){
char * decrypt(char const* p, int key){
    char *tmp = malloc(strlen(p) + 1); // Allocate memory
    int i = 0;
    for( ; p[i] != '\0'; i++){
        tmp[i] = p[i]-key;
    }
    tmp[i] = '\0';                     // null terminate.
    return tmp;
}

Make sure you deallocate the memory. Just using

printf("%s\n", decrypt("ibmmp", 1));

will result in a memory leak.

int main(void){
    printf("Hallo Welt!");
    char* dec = decrypt("ibmmp", 1)
    printf("%s\n", dec);
    free(dec);                        // Deallocate memory.
    return EXIT_SUCCESS;
}