How to concat strings in for loop with snprintf or similar function (C)?

Question

I have the following code, which is my attempt to make a single string containing the stored HTTP header fields:

typedef struct _header {
    char* name;
    char* value;
} header;

const header headers[] = {
    { "Content-Type", "text/html" },
    { "Server", "testServer" }
};

int headerStringSize = sizeof(char) * 1024 + 1;
char* headerString = malloc(headerStringSize);
int i, headersLength = sizeof(headers) / sizeof(headers[0]);
for (i = 0; i < headersLength; ++i) {
    header h = headers[i];
    snprintf(headerString, headerStringSize, "%s: %s\r\n", h.name, h.value);
}

However, it doesn't work as snprintf simply overwrites the contents of headerString on each iteration, rather than appending at the correct char index. I am used to higher-level languages than C, so my problems are entirely down to my own ignorance. I would, therefore, greatly appreciate it if someone could show me the best way to achieve what I want.

Answer 1

You should initialize your string:

headerString[0]=0;

before the loop.

Now you can calculate the end of the string and give snprintf a pointer to the end of the string:

snprintf(headerString + strlen(headerString), headerStringSize - strlen(headerString), "%s: %s\r\n", h.name, h.value);

Also note the changed maximum length parameter. You should also add a check on the return value of malloc.