Copying string using strncpy cause garbage value in output

Question

I have a file name "test.mf" as a character pointer. I want to change extension of this file into "test.pfb". I am writing a simple code for this but it is resulting in printing some garbage characters. Running same code on online C compiler results in no garbage values but running it using GCC results in garbage value. If I can achieve the same thing (changing file extension of .mf file to .pfb) in a much better, fast way please suggest. I am new in this so please ignore silly mistakes.

Code

char *Get_PFB_font_file_name(char *TTF_Font_name) 
{
    size_t alen = strlen(TTF_Font_name);
    alen = alen+1; 
    char mystring[alen];
    const char* extension = ".pfb";
    char *PFB_File= malloc(alen+1);
    strncpy(mystring, TTF_Font_name, alen-4);
    strcat( mystring, extension );
    strncpy(PFB_File, mystring, alen);

    if(!PFB_File)
    {
        printf("error in get TTX conversion function");
        exit(1);
    } 
    return PFB_File;
}

int main ( void )
{
    char *MF_Font_file_name = "test.mf";
    char *PFB_Font_file_name;
    PFB_Font_file_name = Get_PFB_font_file_name(MF_Font_file_name);
    printf("PFB font file name is %s \n", PFB_Font_file_name); // garbage value here (test�.p)

    return 0;
}

Answer 1

about strncpy:

The strncpy() function is similar (to strcpy), except that at most n bytes of src are copied. Warning: If there is no null byte among the first n bytes of src, the string placed in dest will not be null-terminated.

strncpy(mystring, TTF_Font_name, alen-4);
strcat( mystring, extension );

this has undefined behaviour because TTF_Font_name is longer than alen-4 so the warning/limitation above applies, and strcat cannot compute the size of mystring properly.

One way to fix it: just insert this between the two lines:

mystring[alen-4] = '\0';