I receive a char * buffer which have the lenght of 10. But I want to concat the whole content in my struct which have an variable char *.
typedef struct{ char *buffer; //.. }file_entry; file_entry real[128]; int fs_write(char *buffer, int size, int file) { //every time this function is called buffer have 10 of lenght only // I want to concat the whole text in my char* in my struct } Something like this :
real[i].buffer += buffer; How can I do this in C ?
In general, do the following (adjust and add error checking as you see fit)
// real[i].buffer += buffer; // Determine new size int newSize = strlen(real[i].buffer) + strlen(buffer) + 1; // Allocate new buffer char * newBuffer = (char *)malloc(newSize); // do the copy and concat strcpy(newBuffer,real[i].buffer); strcat(newBuffer,buffer); // or strncat // release old buffer free(real[i].buffer); // store new pointer real[i].buffer = newBuffer; You can use strcat(3) to concatenate strings. Make sure you have allocated enough space at the destination!
Note that just calling strcat() a bunch of times will result in a Schlemiel the Painter's algorithm. Keeping track of the total length in your structure (or elsewhere, if you prefer) will help you out with that.
strncat (also included in the link above) and not strcat.
strcat, which is because the two buffers are under pre-determined guarantees, then you should always use strncat. Someone starting out in C will always be better off using strncat over strcat simply because the risk is not worth the unnoticeable reward. Considering what the benefit actually is, I would even argue that you are never going to actually benefit from it unless you're writing something incredibly low level and the pre-requisite checks have already been done.I am not clear. Do you want:
real[0].buffer, orreal[i].buffer, orYou will need to allocate enough space for the copy of the buffer:
#include <stdlib.h> //... int size = 10+1; // need to allocate enough space for a terminating '\0' char* buff = (char *)malloc(size); if (buff == NULL) { fprintf(stderr, "Error: Failed to allocate %d bytes in file: %s, line %d\n", size, __FILE__, __LINE__ ); exit(1); } buff[0] = '\0'; // terminate the string so that strcat can work, if needed //... real[i].buffer = buff; // now buffer points at some space //... strncpy(real[i].buffer, buffer, size-1); 
