C 中的 Fprintf 实现

Question

Hi I want to write base implementation of fprintf function in C. I have problem with the void cast. It says that it is incompatible type for argument 2 of write . I tried several ways to do it but nothing changes. Thats the function:

void myfprintf(int fd, char *format, void *value) {
    if(strcmp(format,"%d")){
        write(fd,(*(int*)value),sizeof(int));
    }
    if(strcmp(format,"%f")){
        write(fd,(*(float*)value),sizeof(float));
    }
    if(strcmp(format,"%lf")){
        write(fd,(*(double*)value),sizeof(double));
    }
}

For start I want to print int double and float types.

Answer 1

To implement an fprintf-like function you should declare your function with ellipsis,

int myprintf(int fd, char *format, ...);

then use the so called 'var_args' (variable arguments) tools to access argument. See the manual pages on va_start(), va_arg(), va_end(). These tools are macros that properly walk the values and pointers on your stack. The stack arrangement is machine-architecture and implementation dependent, and those dependencies are hidden in the var args macro implementation.

You'll find examples here: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/ieee754/ldbl-opt/nldbl-compat.c;h=2b1261acc7505d9a6ac3f25b76bafb9118854ef8;hb=HEAD