String concatenation without altering original values - C

Question

I am making a dynamic PUT request through the use of string concatenation with C. My problem is that after the first request, a string that I need to remain static putEndpoint , is altered with the string concatenation I'm using it for.

char putEndpoint[] = "PUT /api/v1/products/";
char http[] = " HTTP/1.1";
char productID[idLen];

for(int i = 0; i < 13; i++) {
  productID[i] = newTag[i];
}

// going into this strcat, putEndpoint correctly = "PUT /api/v1/products/"

char *putRequestID = strcat(putEndpoint,productID);

// putEndpoint now = "PUT /api/v1/products/xxxxxxxxxxx"

char *putRequestEndpoint = strcat(putRequestID,http);

Now if I were to make a 2nd call (which I will need to do), putEndpoint initializes as "PUT /api/v1/products/xxxxxxxxxxx" .

EDIT: Is there an alternative to strcat() that could accomplish this concatenation? I now understand that strcat() is meant to alter values.

Answer 1

You can use sprintf .

A simple working example -

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

 int main(void) {

      char putEndpoint[] = "PUT /api/v1/products/";
      char http[] = " HTTP/1.1";
      char *putRequestEndpoint;

      putRequestEndpoint=malloc(strlen(putEndpoint)+strlen(http)+1); //allocating memory

      sprintf(putRequestEndpoint,"%s%s",putEndpoint,http); // formatted output is stored in putRequestEndpoint
      printf("%s\n",putRequestEndpoint);
      printf("%s",putEndpoint); // will print original string unchanged.
      free(putRequestEndpoint);  //freeing memory
      return 0;
   }

Answer 2

I ended up changing putEndpoint to a constant and created a buffer array and that I then copied putEndpoint into. This array then resets after each request.

const char putEndpoint[] = "PUT /api/v1/products/";
char http[] = " HTTP/1.1";
char productID[idLen];

char putRequestBuffer[100];
strcpy(putRequestBuffer, putEndpoint);
strcat(putRequestBuffer, productID);

char *putRequestEndpoint = strcat(putRequestBuffer,http);

Answer 3

具有固定大小和适当范围检查的memcpy和静态数组是您的朋友，我的朋友。

Answer 4

You would have to reset it with code. If you change putEndpoint[] to a const, which will make your first line look like

const char putEndpoint[] = "PUT /api/v1/products/";

the compiler will give you an error the first time you strcat(putEndpoint,...) because strcat will be trying to write to the constant variable. This will force you to find an alternative solution.

The code below cleanly allocates temporary memory for the endpoint string as needed, copies the first string into it, concatenates the next two onto it, uses it, and finally de-allocates it and sets the pointer back to NULL.

int lengthEndpoint = 0;
char* putRequestEndpoint = NULL;

lengthEndpoint = strlen(putEndpoint) + strlen(productID) + strlen(http);
lengthEndpoint += 1; // add room for null terminator

putRequestEndpoint = malloc(lengthEndpoint);
strcpy(putRequestEndpoint, putEndpoint);
strcat(putRequestEndpoint, productID);
strcat(putRequestEndpoint, http);

// do something with putRequestEndpoint

free(putRequestEndpoint);
putRequestEndpoint = NULL;

Before answering this, I refreshed my memory of C string manipulation using this WIKIBOOKS site . I'd recommend it for further reading on this subject.