Question

I want to wget in C and I am using popen to do so.

FILE *stdoutPtr = popen(command,"r");
fseek(stdoutPtr, 0L, SEEK_END);
long pos = ftell(stdoutPtr);

Here I want to get the size of stdout so that I can initialize buffer. But pos variable is always -1.
pos is supposed to tell me current position of read pointer.

Please help….

Answer 1

The FILE returned by popen is not a regular file, but a thing called a pipe. (That’s what the p stands for.) Data flows through the pipe from the stdout of the command you invoked to your program. Because it’s a communications channel and not a file on disk, a pipe does not have a definite size, and you cannot seek to different locations in the data stream. Therefore, fseek and ftell will both fail when applied to this FILE, and that’s what a -1 return value means. If you inspect errno immediately after the call to ftell you will discover that it has the value ESPIPE, which means “You can’t do that to a pipe.”

If you’re trying to read all of the output from the command into a single char* buffer, the only way to do it is to repeatedly call one of the read functions until it indicates end-of-file, and enlarge the buffer as necessary using realloc. If the output is potentially large, it would be better to change your program to process the data in chunks, if there’s any way to do that.