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Project Notes

printf partial strings

Learning how printf can be instructed to select a limited number of characters from a string.

Notes

I learned of this from the Embedded Artistry blog. It could be useful when dealing with string buffers that are not null terminated.

Basically: the width parameter of the format string applies also the string.

e.g. %.5s limits output to a maximum of 5 characters:

const char * mystr = "... long string ...";
printf("Just 5 characters: %.5s\n", mystr);

Or the * width specifier can be used to take the length as an argument:

printf("Just 5 characters: %.*s\n", 5, mystr);

This also works with related functions such as sprintf.

Demo

demo.c exercises various string format combinations. Use make to compile and run:

$ make
gcc -Wall -O0    demo.c   -o demo
./demo

===== test_explicit_printf_format
> printf("Prints just 5 characters (should print 'ABCDE'): %.5s\n", long_string);
Prints just 5 characters (should print 'ABCDE'): ABCDE

===== test_printf_width_as_argument
> printf("Prints just %d characters (should print 'AB'): %.*s\n", 2, 2, long_string);
Prints just 2 characters (should print 'AB'): AB

===== test_explicit_printf_format
> printf("Asking for 5 characters, but only 4 available (should print 'ABCD'): %.5s\n", short_string);
Asking for 5 characters, but only 4 available (should print 'ABCD'): ABCD

===== test_explicit_sprintf_format
> sprintf(result, "%.5s", long_string);

===== test_sprintf_with_width_as_argument
> sprintf(result, "%.*s", 2, long_string);

Credits and References

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This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

LittleCodingKata is my collection of programming exercises, research and code toys broadly spanning things that relate to programming and software development (languages, frameworks and tools).

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