Added extra characters to the beginning of the line?

Question

I have two extra characters that are added to the beginning of my line, and I cannot understand why. Characters are not even displayed in the code. I'm at a loss here. This is my code:

#include <stdlib.h> #include <stdio.h> #include <string.h> char *chars; char* vector(char input, char *newlist); int main(){ char *input, *out = "Input: "; printf("Enter characters: "); while(1){ char i = getchar(); //get input if(i == '\n'){ break; //detect a return key } else{ input = vector(i, input); //call vector } } char * print = (char *)malloc(1 + strlen(input) + strlen(out)); strcpy(print, out); //concat the strings strcat(print, input); printf("\n%s", print); //print array free(print); free(input); free(chars); return 0; //exit } char* vector(char in, char *newlist){ int length = strlen(newlist); //determine length of newlist(input) chars = (char*)calloc(length+2, sizeof(char)); //allocate more memory strcpy(chars, newlist); //copy the array to chars chars[length] = in; //appened new character chars[length + 1] = '\0'; //append end character return chars; } 

For some reason, the code produces this:

 Enter characters: gggg Input: PEgggg 

When should this be done:

 Enter characters: gggg Input: gggg 

Answer 1

You passed the uninitialized input to vector() and used it, so you called undefined behavior.

Try changing char *input to char *input = "" .

Also remove free(chars); , or you will encounter a dual access problem.

Answer 2

All @MikeCat points are correct, just add that the memory allocated by calloc is not freed, which leads to a memory leak. You can free it, as @MM said, but next time, to avoid memory leaks, you can use valgrind :

Let your program like hash.c Go to the command line and compile it, for example:

 gcc hash.c -Wall 

If your program compiles successfully, an executable or out will appear. Since we did not specify the name of the executable file, it will default to a.out . So, let it start with valgrind :

 valgrind -- leak-check=full ./a.out 

This will run the executable along with valgrind, and if a memory leak occurs, it will show it when the execution finishes.

---

If you do not have valgrind , you can install it from.

Answer 3

I think you have one or more uninitialized fields. I get these warnings when I try to compile:

 $ clang -Weverything vector.c vector.c:15:18: warning: implicit conversion loses integer precision: 'int' to 'char' [-Wconversion] char i = getchar(); //get input ~ ^~~~~~~~~ vector.c:19:31: warning: variable 'input' may be uninitialized when used here [-Wconditional-uninitialized] input = vector(i, input); //call vector ^~~~~ vector.c:11:16: note: initialize the variable 'input' to silence this warning char *input, *out = "Input: "; ^ = NULL vector.c:40:33: warning: implicit conversion changes signedness: 'int' to 'unsigned long' [-Wsign-conversion] chars = (char*)calloc(length+2, sizeof(char)); //allocate more memory ~~~~~~ ~~~~~~^~ vector.c:38:18: warning: implicit conversion loses integer precision: 'unsigned long' to 'int' [-Wshorten-64-to-32] int length = strlen(newlist); //determine length of newlist(input) ~~~~~~ ^~~~~~~~~~~~~~~ vector.c:5:7: warning: no previous extern declaration for non-static variable 'chars' [-Wmissing-variable-declarations] char *chars; ^ 5 warnings generated. 

When I use ASan ^{What is ASan ?} I get the following error:

 $ echo 1 2 3 | ./a.out Enter characters: ================================================================= ==23718==ERROR: AddressSanitizer: attempting double-free on 0x60200000ef70 in thread T0: #0 0x4a5f4b in free /home/development/llvm/3.7.0/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:30:3 #1 0x4cd631 in main (/home/brian/src/so/a.out+0x4cd631) #2 0x7f3b94ef5a3f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x20a3f) #3 0x4174c8 in _start (/home/brian/src/so/a.out+0x4174c8) 0x60200000ef70 is located 0 bytes inside of 7-byte region [0x60200000ef70,0x60200000ef77) freed by thread T0 here: #0 0x4a5f4b in free /home/development/llvm/3.7.0/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:30:3 #1 0x4cd5fa in main (/home/brian/src/so/a.out+0x4cd5fa) #2 0x7f3b94ef5a3f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x20a3f) previously allocated by thread T0 here: #0 0x4a63b4 in calloc /home/development/llvm/3.7.0/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:56:3 #1 0x4cd67c in vector (/home/brian/src/so/a.out+0x4cd67c) #2 0x4cd57b in main (/home/brian/src/so/a.out+0x4cd57b) #3 0x7f3b94ef5a3f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x20a3f) SUMMARY: AddressSanitizer: double-free /home/development/llvm/3.7.0/final/llvm.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:30:3 in free ==23718==ABORTING 

Answer 4

The initialization of char * is absent and therefore leads to undefined behavior. Pls initializes char *

Answer 5

You need to remove the free (print) and assign to pointers. First called double free and the last kernel called dumped. I am working on ubuntu and my version of gcc 4.8.4