redirect stdout/stderr to function in C -

i want redirect stdout , stderr function , write simple code. fprintf ok when use printf without \r, stream passed terminal!

#include <stdio.h> static ssize_t file_write(void *c, const char *buf, size_t size) {   file * pfile;   pfile = fopen ("output.txt","w");   fprintf(pfile, "%s", buf);   fclose (pfile);  return size; }  void redirect() {   file *stream;   stream=fopencookie(null, "w", (cookie_io_functions_t) {(ssize_t) 0,  file_write, 0, 0});   setbuf(stdout, null);   stdout = stream;   setbuf(stderr, null);   stderr = stream; }  int main() {   redirect();   fprintf(stderr,"1-stderr test\n");   fprintf(stdout, "2-stdout test\n");   printf("3-printf r test\n\r");   printf("4-printf without r test\n");  return 0; } 

file "output.txt":

1-stderr test
2-stdout test
3-printf r test

terminal output:

$ ./sample
4-printf without r test

you can't "redirect" standard input from, or standard output function in program.

you can create pair of pipes , replace standard input , output these pipes, , have function (possibly in thread or child process) use other end of pipes communicate function.

you can of course use e.g. setvbuf change buffers of stdin , stdout pair of buffers provide, can read directly from, or write directly buffers other memory buffer (i.e. array). not recommend though, it's frail , prone errors.

however in case seems want writing stdout should go file. in case (like use of pipes) there no standard c way of handling it, need use operating system specific functionality. if you're on posix platform (like linux or osx) can tell operating system duplicate underlying file descriptor file, , stdout_filno (which file descriptor used stdout) duplicate file descriptor of file opened. writes stdout written file. need use the dup2 system call.