Standard String Library Functions | Programming in C

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Standard String Library Functions

Programming in C · Chapter 7: Arrays, Strings, Input Validation, and Formatted OutputSection 7.6 · lesson 45 of 787 min · free with a free account

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In this lesson

Measuring, copying, joining, and comparing strings with string.h and ctype.h.

  • Compare what strlen and sizeof report for a string
  • Copy and join strings with strcpy and strcat, and explain their overflow risk
  • Test two strings for equality with strcmp instead of ==
  • Use snprintf or strncpy plus a null terminator to copy safely

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Transcript

Everything said in the video, section by section (890 words). Read along, or search it.

Opening Question

Everyday Jobs With Names at the Desk

At the arcade desk, Priya works with names all day. She counts letters, copies a name onto a badge, checks whether two names match, and glues first and last names together. Writing a loop for each job would be tiring. C already ships the tools, in a toolbox called string dot h. You only need to know what each tool does, and one big danger that comes with them.

String Length

Strlen Counts Letters, Sizeof Counts Room

Every string function here lives in the header string dot h, so include it at the top. The first tool is strlen, string length. It counts the letters from the start up to, but not including, the marker. Run it. Strlen says nine, and sizeof says twenty. Strlen asks how long the text is right now. Sizeof asks how much room the array has. Both matter: the first tells you what's stored, and the second tells you what's safe. A string of n letters always needs n plus one boxes.

Common Mistake

Arrays Cannot Be Assigned With Equals

With a number, you can write score equals fifty any time. With a string array, you can't assign the whole array with equals. Try it: tag equals Sam. Build it, and clang refuses: array type char thirty is not assignable.

Copy and Glue

Strcpy and Strcat Trust the Destination

Instead, copy the letters with strcpy, destination first, then source. It copies every letter, and the marker too. To glue text onto the end, use strcat, string concatenate. It finds the marker, writes the new letters over it, and puts a fresh marker at the end. Run it. Player colon Priya, thirteen letters. Then strcpy replaces the text with Sam. Both tools trust you completely: the destination must be big enough for the result.

Common Mistake

Double Equals Compares Addresses

How do you check whether two names match? The obvious double equals doesn't compare letters. It asks whether the two arrays live at the same place in memory, which two separate arrays never do. Clang even warns: array comparison always evaluates to false. Run it anyway, and two names that are both Sam come out different.

Comparing Strings

Strcmp Returns Negative, Zero, or Positive

The right tool is strcmp, string compare. It walks both strings together, and stops at the first box where they differ, or at the markers. It returns a negative number when the first string comes first in alphabetical order, zero when they're exactly the same, and a positive number when the first comes later. The exact negative or positive number can differ between systems, so test only the sign. Run it. Apple before banana, negative. Apple and apple, zero. Pear after banana, positive. And Zebra comes before apple! Strcmp compares letter numbers, and every capital has a smaller number than every small letter. Z is ninety, a is ninety-seven.

Quick Check

Zero Means the Names Match

Quick check. The player typed Sam, and this test uses strcmp to decide if it matches Sam. What does the program print? Pause and predict. No match, though the names are the same! Strcmp returns zero when they match, and zero counts as false, so the else branch runs. Write strcmp equals equals zero, and the code says what you mean.

Buffer Overflow

Strcpy Never Checks the Size

Now the big danger. An array has a fixed size, but strcpy never checks it. It keeps copying until it has copied the source's marker. Copy fifteen letters into an eight-box array: eight letters fit. The other seven, and the marker, land in memory that the array doesn't own. That's a buffer overflow, one of the most famous causes of crashes and security holes in C. It's undefined behavior, so we won't run one.

Safe Copies

Snprintf, or Strncpy Plus a Marker

Instead, use a copy that knows the size. Snprintf writes at most size boxes, and always ends with a marker. Strncpy copies at most n letters. But if the source is that long, it adds no marker, so you add one yourself. The size travels in as a separate number, because inside a function sizeof can't see the array's real size. Sam types StarlightArcade, fifteen letters. Both tools keep seven, S T A R L I G, plus the marker, and nothing spills out.

Character Tools

Testing and Changing Single Letters

One more toolbox, ctype dot h, works on single letters. Isdigit says whether a character is a digit, and isalpha whether it's a letter. Toupper gives the capital version. Pass each one the letter with unsigned char in front, to keep them safe. Put them in a loop that stops at the marker, and you can check or change every letter. Run it: A B 1 2 X, three letters and two digits.

Quick Check

Gluing Changes Text, Not Room

Quick check. A twenty-box tag starts as Star, and strcat glues light onto it. What do strlen and sizeof print? Pause and predict. Nine, then twenty. Starlight has nine letters, and strlen never counts the marker that strcat moved to the end. Sizeof still says twenty: gluing changes the text, not the array's room.

Story Resolved

A Short but Safe Badge

Back at the desk, Priya's badge printer had an eight-box name array. Test it with the longest name you can think of, she told Sam. He typed StarlightArcade. With her safe copy, the badge showed Starlig, and the printer kept running. Short, but safe, Sam said. Priya made a note to widen the array to twenty boxes.

Key Takeaways

The String Toolbox at a Glance

To recap. Strlen counts letters before the marker; sizeof counts the array's boxes. Strcpy and strcat copy and glue, destination first, and never check the size. Strcmp returns zero for a match, so test for equals equals zero. And for safe copies, use snprintf, or strncpy plus a marker.

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FAQ

Questions students ask us

About Standard String Library Functions

What is the difference between strlen and sizeof for a C string?
strlen counts the characters before the null terminator, while sizeof counts the elements of the whole array. For char name[20] = "Starlight";, strlen(name) is 9 and sizeof name is 20. strlen tells you how long the text is right now, and sizeof tells you how much room the array has, which is what you need to know before copying or joining. strlen is declared in string.h.
How do you compare two strings in C?
Use strcmp from string.h, not ==. strcmp returns zero when the strings match, a negative number when the first comes earlier, and a positive number when it comes later, so test strcmp(a, b) == 0 for equality. Using == on two arrays compares their addresses, which differ for separate arrays. The comparison uses character codes, so "Zebra" comes before "apple": Z is 90 and a is 97.
How do you copy a string into a char array in C without overflowing it?
Use a copy that knows the destination's size, such as snprintf(dest, size, "%s", src), which writes at most size characters including the null terminator. Plain strcpy(dest, src) copies until the source's terminator and never checks the size, so a long source causes a buffer overflow. strncpy(dest, src, n) copies at most n characters but adds no terminator when the source is that long, so set one yourself.
What do isdigit, isalpha, and toupper do in C?
These functions from ctype.h work on single characters. isdigit(c) returns nonzero if c is a digit, isalpha(c) returns nonzero if c is a letter, and toupper(c) returns the capital version of a lowercase letter. Cast the argument to unsigned char, as in toupper((unsigned char)s[i]), to keep it safe. In a loop that stops at the null terminator, they check or change every character of a string.

About the Programming in C course

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Programming in C (Computer Science 240) carries a NCCRS credit recommendation for 3 semester credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.
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