String *char concatenate error with malloc dynamic memory allocation
I want to make a function to concatenate strings, it works but it gives an error and the processor restarts, I think there's something wrong with pointers, but I do not know what it is, a problem of memory allocation.
Thanks in advance!
char *buf;
int main(void) {
// ...
WriteString("#INIT.\r\n"); //serial output
buf = "";
while(1)
{
char *str1 = "qwe";
char *str2 = "asd";
char *str3 = "zxc";
char *str4 = "123";
buf = my_strcat(buf,str1);
buf = my_strcat(buf,str2);
buf = my_strcat(buf,str3);
buf = my_strcat(buf,str4);
WriteString(buf); //serial output
free(buf);
}
}
char *my_strcat(const char *str1, const char *str2) {
char *new_str;
new_str = malloc(strlen(str1)+strlen(str2)+1);
new_str[0] = '\0';
strcat(new_str,str1);
strcat(new_str,str2);
return new_str;
}
Serial output...
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
#INIT.
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
qweasdzxc123
Your first call to my_strcat
has undefined behavior because buf
was not initialized before. Precisely this line is the problem
new_str = malloc(strlen(str1)+strlen(str2)+1);
strlen(str1)
where str1
is uninitialized.
Suggestion, use realloc
char *my_strcat(char *str1, const char *str2)
{
char *new_str;
size_t length;
size_t str1length;
if (str2 == NULL)
return str1;
str1length = 0;
if (str1 != NULL)
str1length = strlen(str1);
length = strlen(str2) + str1length;
new_str = realloc(str1, 1 + length);
if (new_str == NULL)
return str1;
new_str[str1length] = '\0';
strcat(new_str, str2);
return new_str;
}
and
char *buf;
char *str1 = "qwe";
char *str2 = "asd";
char *str3 = "zxc";
char *str4 = "123";
buf = NULL;
buf = my_strcat(buf, str1);
buf = my_strcat(buf, str2);
buf = my_strcat(buf, str3);
buf = my_strcat(buf, str4);