Appending strings in C

Question

How do I combine multiple strings. For example,

char item[32];
scanf(\"%s\", item);
printf(\"Shopping list: %s\\n\", item); //I want to combine this string          


        

Answer 1

Processing raw strings in C is always ugly. You have to do two steps to concatenate strings:

1. Allocate memory for the new string.

2. Copy the source strings into your new buffer.

As a lesson, please note that there are already three different answers to this question with three different buffer overrun errors.

Here is a function which concatenates two strings and returns a new string:

char *astrcat(const char *x, const char *y)
{
    size_t nx = strlen(x), ny = strlen(y);
    char *z = malloc(nx + ny + 1);
    if (!z)
        return NULL;
    memcpy(z, x, nx);
    memcpy(z + nx, y, ny + 1);
    return z;
}

You can expand this to work with three strings, or you can just call it twice. Sane folk tend to use a library or make extensive use of fixed-size buffers.

Alternative: With fixed-size buffers and snprintf, you get safety and ease of use but you're stuck with fixed size buffers.

const char *x = ..., *y = ...;
char buf[100];
snprintf(buf, sizeof(buf), "%s%s", x, y);

If you don't have snprintf, then you're probably using MSVC, which has _snprintf which is almost the same but doesn't always nul-terminate the output.

Don't use strcat or strcpy: There are very few circumstances in which strcat is acceptable:

char buf[100];
strcpy(buf, x); // buffer overflow
strcat(buf, y); // buffer overflow

Don't use sprintf: It also overflows:

char buf[100];
sprintf(buf, "%s%s", x, y); // buffer overflow