# c – Difference between fprintf, printf and sprintf?

## The Question :

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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”.

• Experiment with them by writing some sample code. That is very easy way to clearly understand the difference between them.
• Prefer snprintf() to sprintf() to avoid silly buffer overflows.
• Prefer streams or Boost formatters to avoid silly buffer overflows and nasty type-unsafety bugs
• @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. 🙂
• @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?

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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. ## The Answer 2 162 people think this answer is useful printf outputs to the standard output stream (stdout) fprintf goes to a file handle (FILE*) sprintf goes to a buffer you allocated. (char*) ## The Answer 3 42 people think this answer is useful printf(“format”, args) is used to print the data onto the standard output which is often a computer monitor. sprintf(char *, “format”, args) 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 *fp, “format”, args) 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. ## The Answer 4 26 people think this answer is useful 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 ## The Answer 5 2 people think this answer is useful 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)



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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

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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

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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.

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