Frist few bytes of a char* are corrupted after creating it with malloc()

Question

Whenever I create a char* with malloc(), the first few bytes get random data, which is different every time I compile the code. In this case, I wanted to concatenate 2 char* to create 1 char*.

char *JoinStrings(char *str1, char *str2) { int length = strlen(str1) + strlen(str2) + 1; char *newString = malloc(length); if (newString == NULL) { printf("malloc failed\n"); return NULL; } newString[length] = 0; // add the 0 byte at the end printf("String 1: %s\n", str1); printf("String 2: %s\n", str2); printf("New String: %s\n", newString); strcat(newString, str1); strcat(newString, str2); printf("Joined String: %s\n", newString); return newString; }

I will call the function using this line:

char *join1 = JoinStrings("Hello,", " World");

I would expect it to print out "Joined String: Hello, World" as the last printed message, but it instead returns this:

String 1: Hello, String 2: World New String: p‼√ΓJ☻ Joined String: p‼√ΓJ☻Hello, World

I have no idea where it gets the random bytes from, and they are randomized every time I compile it. And because it gets randomized when I compile it, here is the GCC version I'm using: gcc.exe (Rev1, Built by MSYS2 project) 11.2.0.

Jean-Baptiste Yunès · Accepted Answer · 2022-07-16 20:12:00Z

2

Malloc does not initialise the allocated memory content to any value. As strcat tries to locate the end of the string to concat, anything may happen. So don't use strcat for the first but strcpy:

strcpy(newString,str1); strcat(newString,str2);

and:

newString[length] = 0;

should be:

newString[length-1] = 0;

Last element has index length-1.

answered Jul 16, 2022 at 20:12

Jean-Baptiste Yunès

36.6k4 gold badges51 silver badges76 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

TonyB Over a year ago

The last element (i.e. index) of a string is indeed "length-1", however the location of the null-terminator is [length]... which I believe was what he was trying to provide. with " newString[length] = 0; // add the 0 byte at the end "

Jean-Baptiste Yunès Over a year ago

@TonyB No. The number of allocated byte is length, all are indexed through 0..length-1. The terminator NUL char must be in one of these memory cells. Accessing a cell outside these is "Undefined behavior".

TonyB Over a year ago

You of course are correct. I missed the fact that "length" included the null terminator.

Collectives™ on Stack Overflow

Frist few bytes of a char* are corrupted after creating it with malloc()

1 Answer 1

3 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Related