Difference between fprintf, printf and sprintf?

Question

Can anyone explain in simple English about the differences between printf, fprintf, and sprintf with examples?

What stream is it in?

I'm really confused between the three of these while reading about "File Handling in C".

Bedlamite

Experiment with them by writing some sample code. That is very easy way to clearly understand the difference between them. – Imparity

Prefer snprintf() to sprintf() to avoid silly buffer overflows. – Keramic

Prefer streams or Boost formatters to avoid silly buffer overflows and nasty type-unsafety bugs – Reposeful

Streams and boost formatters may be too slow. – Keramic

@Maxim, whilst you raise a valid point I'll take the safety in knowledge that my buffers aren't going to overflow and explode my app in to pieces. I'd only ever look at these functions if the streams/boost formatters were shown to be causing noticeable bottlenecks. :) – Jaunitajaunt

I've dropped the C tag, those are C functions and while you can call them from C++, it's better not to, this way you'll avoid grumpy programmers (in C++, use streams) – Pleura

@Maxim: Almost never. I use streams in production financial servers, processing 5 million messages per second, all day every day and they have never been an issue for me. – Reposeful

@Matthieu: I think you actually dropped the [c++] tag. Was this intended? – Reposeful

@John: type safety of streams is a bit overrated. Modern compilers check the arguments against the format string, just make sure you don't ignore these warnings or make them a hard error. And in the end of the day streams do call snprintf under the hood, so by not using streams you avoid layers of code. – Keramic

@John: 5M/sec messages is 200ns/msg. I bet you could do better than that if you drop using IOStreams. lol – Keramic

@Maxim: Point is, there's no need. My code isn't a bottleneck. 5m/sec is how many messages the exchange sends, not how many we're capable of processing. So why prematurely optimize? – Reposeful

@John: With market data and high-frequency trading the goal is often not throughput, but rather latency. – Keramic

@Maxim: I'm well-aware of this. Again it's not an issue in my experience. – Reposeful

Answer 1

In C, a "stream" is an abstraction; from the program's perspective it is simply a producer (input stream) or consumer (output stream) of bytes. It can correspond to a file on disk, to a pipe, to your terminal, or to some other device such as a printer or tty. The FILE type contains information about the stream. Normally, you don't mess with a FILE object's contents directly, you just pass a pointer to it to the various I/O routines.

There are three standard streams: stdin is a pointer to the standard input stream, stdout is a pointer to the standard output stream, and stderr is a pointer to the standard error output stream. In an interactive session, the three usually refer to your console, although you can redirect them to point to other files or devices:

$ myprog < inputfile.dat > output.txt 2> errors.txt

In this example, stdin now points to inputfile.dat, stdout points to output.txt, and stderr points to errors.txt.

fprintf writes formatted text to the output stream you specify.

printf is equivalent to writing fprintf(stdout, ...) and writes formatted text to wherever the standard output stream is currently pointing.

sprintf writes formatted text to an array of char, as opposed to a stream.

Cardiovascular

"it is simply a producer (input stream) or consumer (output stream) of bytes." Are these backwards? Wouldn't a producer create (output) something? Asking because I genuinely do not know. – Exaction

@DaveVoyles: These are from the perspective of your program. An input stream produces bytes for your program to read; an output stream consumes the bytes produced from your program. – Cardiovascular

for anyone thinking what the f prefix/suffix is for. I initially thought the f in printf / sprintf / scanf means file. But it just means format. – Crossbones

Answer 2

printf outputs to the standard output stream (stdout)

fprintf goes to a file handle (FILE*)

sprintf goes to a buffer you allocated. (char*)

Jaunitajaunt

Answer 3

• printf(const char *format, ...) is used to print the data onto the standard output which is often a computer monitor.
• sprintf(char *str, const char *format, ...) is like printf. Instead of displaying the formated string on the standard output i.e. a monitor, it stores the formated data in a string pointed to by the char pointer (the very first parameter). The string location is the only difference between printf and sprint syntax.
• fprintf(FILE *stream, const char *format, ...) is like printf again. Here, instead of displaying the data on the monitor, or saving it in some string, the formatted data is saved on a file which is pointed to by the file pointer which is used as the first parameter to fprintf. The file pointer is the only addition to the syntax of printf.

If stdout file is used as the first parameter in fprintf, its working is then considered equivalent to that of printf.

Bangweulu

Answer 4

printf(...) is equivalent to fprintf(stdout,...).

fprintf is used to output to stream.

sprintf(buffer,...) is used to format a string to a buffer.

Note there is also vsprintf, vfprintf and vprintf

Lyingin

The v flavors are non-standard i believe – Reposeful

The v flavors are very certainly in the C standard. – Shuck

@larsmans: ah, ok. thanks for the correction. This question was at one point tagged [c++] – Reposeful

Answer 5

You can also do very useful things with vsnprintf() function:

$ cat test.cc
#include <exception>
#include <stdarg.h>
#include <stdio.h>

struct exception_fmt : std::exception
{
    exception_fmt(char const* fmt, ...) __attribute__ ((format(printf,2,3)));
    char const* what() const throw() { return msg_; }
    char msg_[0x800];
};

exception_fmt::exception_fmt(char const* fmt, ...)
{
    va_list ap;
    va_start(ap, fmt);
    vsnprintf(msg_, sizeof msg_, fmt, ap);
    va_end(ap);
}

int main(int ac, char** av)
{
    throw exception_fmt("%s: bad number of arguments %d", *av, ac);
}

$ g++ -Wall -o test test.cc

$ ./test
terminate called after throwing an instance of 'exception_fmt'
  what():  ./test: bad number of arguments 1
Aborted (core dumped)

Keramic

+1 useful indeed, but be aware that vsnprintf is a non-Standard function. Most implementations I've seen implement this or something like it, but it is implementation-specific. – Reposeful

@larsmans mentions above that this is, in fact, a part of the C standard, so I take that back – Reposeful

What does this have to do with C? – Leicester

Answer 6

printf

1. printf is used to perform output on the screen.
2. syntax = printf("control string ", argument );
3. It is not associated with File input/output

fprintf

1. The fprintf it used to perform write operation in the file pointed to by FILE handle.
2. The syntax is fprintf (filename, "control string ", argument );
3. It is associated with file input/output

Wriggler

If this is quoted from somewhere else it's best to cite the source with a link, but definitely still keep the text you've quoted here. – Myocardium

Answer 7

sprintf: Writes formatted data to a character string in memory instead of stdout

Syntax of sprintf is:

#include <stdio.h>
int sprintf (char *string, const char *format
[,item [,item]…]);

Here,

String refers to the pointer to a buffer in memory where the data is to be written.

Format refers to pointer to a character string defining the format.

Each item is a variable or expression specifying the data to write.

The value returned by sprintf is greater than or equal to zero if the operation is successful or in other words the number of characters written, not counting the terminating null character is returned and returns a value less than zero if an error occurred.

printf: Prints to stdout

Syntax for printf is:

printf format [argument]…

The only difference between sprintf() and printf() is that sprintf() writes data into a character array, while printf() writes data to stdout, the standard output device.

Demagogy

Answer 8

fprintf This is related with streams where as printf is a statement similar to fprintf but not related to streams, that is fprintf is file related

Polyhymnia

Answer 9

Detailed explanations have already been provided by others; I'll limit my answer to a practical discussion on print vs sprintf, by means of a very basic example.

Suppose you want your program to output both, the current line number and the file name. Specifically, you wish to: (i) print this on your screen and, (ii) save it in a variable, for future use. You can use printf for (i) and sprintf for (ii). Here is the code.

/* saves file name and current line in a string and prints it on the screen*/

#include <stdio.h>

int main(void) {
  
  /* note the use of a marco to save the line nr. */
  int line_n= __LINE__; 
  
  /* note the use of a marco to save the file name */
  char file_name[]= __FILE__;
       
  /* Some text you wish to print/save */
  char line[] = "Line ";
  char file[]= " of file ";

  char my_str[100];

  /* expand everything and save it in my_str for future use */
  sprintf(my_str, "%s%d%s%s", line, line_n, file, file_name);

  /* or just print it out on the screen */
  printf("%s", my_str);

  return 0;
}

Harvison