C Buffer Allocation, Copying, and Concatenation: A Comprehensive Guide
In this quick tutorial, I will show you how to allocate new buffer and copy source buffer to target buffer in C programming language.
TLDR
// π Allocate new buffer
char *buff = (char *)calloc(LEN, sizeof(char));
// π Copy source buffer to target buffer
char buff[6];
strncpy(buff, "helllo is extra hello", sizeof(buff) - 1);
buff[sizeof(buff) - 1] = '\0';If you want to learn more about the basic of buffer in C programming languge, continue below.
Brief overview
Buffer or String in C is represented as an array of char terminated by null
character \0.
char buff[6] = {'h', 'e', 'l', 'l', 'o', '\0'};
/* or */
char *buff = "hello"; /* \0 implicitly appended */
/* or */
char buff[] = "hello";Null character is not the part of buff length but it does requires a memory
space.
The problem is in the function for handling a string operation like allocation
malloc, copy strcpy(dest, src) and concatination strcat(dest, src) is
performed in unsafe way.
malloc only allocate memory for us without initialize it which potentially
have a garbage values (a values from previous operation that are not cleared).
Where strcpy and strcat is assume that dest is big enough which lead us
buffer overflow problem if the src is larger than dest.
So how to safely allocate and copy buffer in C programming language?
Solution
Use calloc instead of malloc to allocate new buffer:
char *buff = (char *)calloc(LEN, sizeof(char));calloc will allocate LEN contiguos block of memory with size
sizeof(char)bytes for each block and initialize it to zero. This will make
sure that buff is not contains any garbage values or sensitive information
from the previous operation.
Use strncpy and terminate to copy source buffer to target buffer:
char buff[6];
strncpy(buff, "helllo is extra hello", sizeof(buff) - 1);
buff[sizeof(buff) - 1] = '\0';strncpy is copy all the first sizeof(buff) - 1 from src. This help us
prevent buffer overflow and corrupting the heap.
Use strncat to concat two or more buffers:
char path[11];
char fname = "hello";
char *ext = ".txt";
strncat(path, fname, sizeof(path) - strlen(fname) - 1);
strncat(path, ext, sizeof(path) - strlen(ext) - 1);This will make sure that the concat operation is not write to more than
allocated memory for path buffer.
Thatβs it.