printf-like function
- C course
- Intermediate
- Lesson 78/116
- 5 min
Learn to write your own printf-like function.
printf-like functions with vprintf
If you want to write your own function that behaves like printf, you use vprintf. This function takes a va_list and prints according to the format string.
Signature: void log_msg(const char *level, const char *fmt, ...). Inside, first print the level (printf("%s: ", level)), then va_list args; va_start(args, fmt); vprintf(fmt, args); va_end(args); to print the formatted part. vprintf is the va_list-taking variant of printf; it uses the same format specifiers (%d, %s, etc.). This pattern is the basis of logging functions. vfprintf and vsprintf similarly write to a file and to a string.
The log_msg example
A real log_msg function combines a level and a formatted message. The output might be INFO: Sum 42.
Definition: void log_msg(const char *level, const char *fmt, ...) { printf("%s: ", level); va_list args; va_start(args, fmt); vprintf(fmt, args); va_end(args); printf("\n"); }. First the level and colon are printed, then the formatted part with vprintf, finally a newline. The second argument to va_start is fmt because that's the last fixed parameter. Call: log_msg("INFO", "Sum %d", 42). This pattern is the basis of libraries that log at different levels; you can change the level and reuse the same format.
Practice The log_msg function
Write void log_msg(const char *level, const char *fmt, ...). Start with printf("%s: ", level);. Call va_list args; va_start(args, fmt); vprintf(fmt, args); va_end(args);. Finally printf("\n");. In main call log_msg("INFO", "Sum %d", 42);.
Starter code
#include <stdio.h>
#include <stdarg.h>
// log_msg: print a message with level and format
// print level with printf; then va_list, va_start, vprintf, va_end, newline
// main: call log_msg("INFO", "Sum %d", 42)
int main(void) {
// write here
return 0;
}Sample solution
#include <stdio.h>
#include <stdarg.h>
void log_msg(const char *level, const char *fmt, ...) {
printf("%s: ", level);
va_list args;
va_start(args, fmt);
vprintf(fmt, args);
va_end(args);
printf("\n");
}
int main(void) {
log_msg("INFO", "Sum %d", 42);
return 0;
}Expected output
INFO: Sum 42
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 The vprintf family
| Term | Meaning |
|---|---|
vprintf |
print to screen |
vfprintf |
print to file |
vsprintf |
print to string |
vsnprintf |
print to bounded string |
2 The vprintf call
vprintf(fmt, ____);vprintf(fmt, args);Explanation: vprintf(fmt, args) produces the formatted output.
3 What's the difference between vprintf and printf?
- vprintf is slower
- printf takes a va_list
- vprintf takes a va_list Correct answer
- vprintf takes no format
Explanation: vprintf is the va_list-taking variant of printf.