C++ <cstdio> - fprintf() Function
The C++ <cstdio> fprintf() function writes the C string pointed by format to the stream. If format includes format specifiers (sub-sequences beginning with %), the additional arguments following format are formatted and inserted in the resulting string replacing their respective specifiers.
After the format parameter, the function expects additional arguments at least as many as the number of format specifiers in the format string.
Syntax
int fprintf ( FILE * stream, const char * format, ... );
Parameters
stream |
Specify a pointer to a FILE object that specifies an output stream. | ||||||||||||||||||||||||||||||||||||||||
format |
Specify the format string. The possible values of %specifier are:
Additional format values can be placed between the % and the specifier (e.g. %.3f) also known as sub-specifier. These sub-specifier are:
Note: If multiple additional format values are provided, they must be in %[flags][width][.precision][length]specifier order. | ||||||||||||||||||||||||||||||||||||||||
... (additional arguments) |
Depending on the format string, a sequence of additional arguments should be passed in the function, each containing a value to replace a format specifiers in the format string. Number of arguments should be at least equal to the number of format specifiers in the format string. Additional arguments will be ignored by this function. |
Return Value
Returns the total number of characters written on success (not including the terminating null character). If a writing error occurs, sets the error indicator ferror() and a negative number is returned. If a multibyte character encoding error occurs while writing wide characters, errno is set to EILSEQ and a negative number is returned.
Example: Different specifiers
Lets assume that we have a file called test.txt in the current working directory. In the example below, the fprintf() function is used to write the formatted string in this file.
#include <cstdio> int main (){ //open the file in write and read mode FILE *pFile = fopen("test.txt", "w+"); fprintf(pFile, "Characters: %c %c \n", 'b', 66); fprintf(pFile, "Decimals: %d %i \n", 200, 300); fprintf(pFile, "More Decimals: %ld %li \n", 20000L, 30000L); fprintf(pFile, "Octals: %o %#o \n", 100, 100); fprintf(pFile, "Hexadecimals: %x %#x %X %#X \n", 100, 100, 100, 100); fprintf(pFile, "Strings: %s \n", "Hello"); fprintf(pFile, "Scientific notation: %e %E \n", 123.45, 123.45); fprintf(pFile, "Floats: %2.0f %2.2f %2.4f \n", 3.1416, 3.1416, 3.1416); fprintf(pFile, "Positive signed number = %+.2f \n", 3.1416); fprintf(pFile, "Padded number = %05d \n", 89); fprintf(pFile, "Number with Width = %*d \n", 5, 89); //set the position to the start rewind(pFile); //display the content of the file int c = getc(pFile); while(c != EOF) { putchar(c); c = getc(pFile); } //close the file fclose (pFile); return 0; }
The output of the above code will be:
Characters: b B Decimals: 200 300 More Decimals: 20000 30000 Octals: 144 0144 Hexadecimals: 64 0x64 64 0X64 Strings: Hello Scientific notation: 1.234500e+02 1.234500E+02 Floats: 3 3.14 3.1416 Positive signed number = +3.14 Padded number = 00089 Number with Width = 89
Example: Format date string
In the example below, this function is used to write the formatted date string in the stream.
#include <cstdio> int main (){ //open the file in write and read mode FILE *pFile = fopen("test.txt", "w+"); int year = 2015; int month = 5; int day = 1; //each format specifier specifies padding //with 0 and width of the field fprintf(pFile, "Date is: %04d-%02d-%02d", year, month, day); //set the position to the start rewind(pFile); //display the content of the file int c = getc(pFile); while(c != EOF) { putchar(c); c = getc(pFile); } //close the file fclose (pFile); return 0; }
The output of the above code will be:
Date is: 2015-05-01
Example: String specifiers
Consider one more example to see how to use string specifiers with a given character string.
#include <cstdio> int main (){ //open the file in write and read mode FILE *pFile = fopen("test.txt", "w+"); char x[] = "catfish"; const char* y = "many catfishes"; fprintf(pFile, "[%s]\n", x); //standard string fprintf(pFile, "[%10s]\n", x); //right-justified with spaces fprintf(pFile, "[%*s]\n", 10, x); //right-justified with spaces fprintf(pFile, "[%-10s]\n", x); //left-justified with spaces fprintf(pFile, "[%-*s]\n", 10, x); //left-justified with spaces fprintf(pFile, "[%10.8s]\n", y); //right-justified (8 characters cutoff) fprintf(pFile, "[%-10.8s]\n", y); //left-justified (8 characters cutoff) fprintf(pFile, "[%-*.*s]\n", 10, 8, y); //left-justified (8 characters cutoff) //set the position to the start rewind(pFile); //display the content of the file int c = getc(pFile); while(c != EOF) { putchar(c); c = getc(pFile); } //close the file fclose (pFile); return 0; }
The output of the above code will be:
[catfish] [ catfish] [ catfish] [catfish ] [catfish ] [ many cat] [many cat ] [many cat ]
Length sub-specifier
Specifiers | |||||||
---|---|---|---|---|---|---|---|
length | d i | u o x X | f F e E g G a A | c | s | p | n |
(none) | int | unsigned int | double | int | char* | void* | int* |
hh | signed char | unsigned char | signed char* | ||||
h | short int | unsigned short int | short int* | ||||
l | long int | unsigned long int | double | wint_t | wchar_t* | long int* | |
ll | long long int | unsigned long long int | long long int* | ||||
j | intmax_t | uintmax_t | intmax_t* | ||||
z | size_t | size_t | size_t* | ||||
t | ptrdiff_t | ptrdiff_t | ptrdiff_t* | ||||
L | long double |
See <cinttypes> for the specifiers for extended types.
❮ C++ <cstdio> Library