C: How to append/concatenate 'x' spaces to a string

I want to add a variable number of spaces to a string in C, and wanted to know if there is a standard way to do it, before I implement it by myself.

Until now I used some ugly ways to do it:

• Please assume that before I called any of the below functions, I took care to have enough memory allocated for the spaces I want to concatenate

This is one way I used:

add_spaces(char *dest, int num_of_spaces) { int i; for (i = 0 ; i < num_of_spaces ; i++) { strcat(dest, " "); }
}

This one is a better in performance, but also doesn't look standard:

add_spaces(char *dest, int num_of_spaces) { int i; int len = strlen(dest); for (i = 0 ; i < num_of_spaces ; i++) { dest[len + i] = ' '; } dest[len + num_of_spaces] = '\0';
}

So, do you have any standard solution for me, so I don't reinvent the wheel?

3 Answers

I would do

add_spaces(char *dest, int num_of_spaces) { int len = strlen(dest); memset( dest+len, ' ', num_of_spaces ); dest[len + num_of_spaces] = '\0';
}

But as @self stated, a function that also gets the maximum size of dest (including the '\0' in that example) is safer:

add_spaces(char *dest, int size, int num_of_spaces) { int len = strlen(dest); // for the check i still assume dest tto contain a valid '\0' terminated string, so len will be smaller than size if( len + num_of_spaces >= size ) { num_of_spaces = size - len - 1; } memset( dest+len, ' ', num_of_spaces ); dest[len + num_of_spaces] = '\0';
}

void add_spaces(char *dest, int num_of_spaces) { sprintf(dest, "%s%*s", dest, num_of_spaces, "");
}

Please assume that before I called any of the below functions, I took care to have enough memory allocated for the spaces I want to concatenate

So in main suppose you declared your array like char dest[100] then initialize your string with speces.

like

char dest[100];
memset( dest,' ',sizeof(dest)); 

So you not need even add_spaces(char *dest, int num_of_spaces).