sprintf

Learn to build formatted strings with sprintf.

sprintf: formatted writing to a string

The sprintf function works like printf but writes the result into a string buffer instead of the screen. It comes with the <stdio.h> header. Its signature is int sprintf(char *buf, const char *fmt, ...).

char buf[64];
int age = 25;
sprintf(buf, "Age: %d", age);
// buf = "Age: 25"

The first argument is the target buffer, the second is the format string, and the rest are values. The return value is the number of characters written. You must make sure the buffer is large enough; otherwise a memory overflow occurs.

Joining text with sprintf

With sprintf you can join several pieces into a single string. It is like printf writing into a buffer instead of the screen.

char buf[128];
char *name = "Ayse";
int score = 95;
sprintf(buf, "Player: %s, Score: %d", name, score);
printf("%s\n", buf);

Here name and score become a single string. You can also use the return value of sprintf to learn the length written. This is the cleanest way to join many pieces in one line.

Exercise Fill a buffer with the age info

Define a char buf[64] buffer. Use sprintf to write int age = 25 into the buffer as "Age: 25". Then print the buffer with printf.

Show example solution

char buf[64];
sprintf(buf, "Age: %d", age);
printf("%s\n", buf);

Test yourself

Fill in the blank: sprintf call

char buf[64];
int age = 25;
____(buf, "Age: %d", age);

Show answer

char buf[64];
int age = 25;
sprintf(buf, "Age: %d", age);

Match format specifier to type

%d

int
%s

string
%f

double
%c

char
What does the sprintf call return?
- The written string
- A pointer
- An error code
- The number of characters written
Show answer

The number of characters written — sprintf returns the number of characters written.

Next lesson: sscanf

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