C-Style Stringsugweb.cs.ualberta.ca/~c201/labs/lab6/c-strings.pdfFlushing an I/O stream with fflush....

Introduction to printf and scanfA conceptual model of a C-string

String handling functions in the C standard libraryString parsing functions

Stuff to learn on your own

C-Style Strings

Mike Closson

Dept of Computing ScienceUniversity of Alberta

Small modifications: Michael Buro Feb.2006

22nd February 2006

Mike Closson C-Style Strings




Introduction to printf, fprintf.

Printing Strings:

/* Print to standard output */printf( "Hello, World!\n" );

/* Print to file associated with filepointer fp */fprintf( fp, "Hello, World!\n" );





Flushing an I/O stream with fflush.

For performance reasons, data written with the stream functions(fwrite, printf, fprintf) may not always appear on theterminal or file after the printf function returns. To force thedata to be written, use the fflush function.

printf("Enter your password: ");fflush( stdout );

Usually, fflush is not needed.





Reading data with scanf.

Data is written with printf, and data is read with scanf.

char password[100];

printf( "Enter your password: " ); fflush( stdout );if (scanf( "%99s", password ) != 1) // Error ...

Note that we asked scanf to read at most 99 characters, thisprevents a potential buffer overflow.





Reading integers and doubles with scanf.

The scanf family of functions can also be used to read integersand doubles. The number of successfully read data items isreturned.

int i;double d;

if (scanf( "%d", &i ) != 1) // Error ...if (scanf( "%lf", &d ) != 1) // Error ...

Note that for ints and doubles, use &i instead of just i (likewise,&d instead of just d). When reading strings with scanf, you don’tneed the &.





Reading a whole line with fgets.

You can use scanf to read a whitespace delimited string. If youwant to read a whole line (i.e., a newline delimited string) usefgets.

char string[100];FILE *f;f = fopen("data.txt", "r" );if (!f) { perror("Error"); exit(10); }

while( fgets( string, 100, f ) != NULL ) {/* process line */....

}fclose( f );





The Gory Details.

C assumes that the programmer means what he/she says.

in other words

C makes it easy for the programmer to shoot him/herselfin the foot.





Introduction

C-strings can be thought of as a bounded array of characters.

The compiler and run-time environment don’t do automaticbounds checking or automatic overwrite detection.

The programmer must take care to avoid memory corruption!

C-strings (and more generally, memory management) are afrequent source of bizarre programming errors.

Seemingly unrelated bugs are often caused by improper stringhandling.

Intel CPU’s and Unix use byte addressable memory. (So dopretty much all CPU/OS combinations.)





A Simple String.

Note: This isn’t exactly how thingswork in memory, but it works for ourpurposes.

char buf[10] = "Hello!\n";

The variable buf is actually a”pointer” to where, in memory,the string is located.

In C, a string ends with a NULL(’\0’) character. Some stringoperations, like the stringdeclaration above, willautomatically append the NULLterminator. Sometimes though,we have to NULL-terminate astring ourselves.

0x1000

Variable Name

0x1004buf

Hello!\n\0

0x5000

0x5000

Memory Address Memory Contents





Copying a string.

You must use a function like strncpy to copy a string. Theassignment operator will do a ”pointer copy”, it won’t copy thecontents of the string.

char *buf1 = "Hello";char buf2[100];

buf2 = buf1; /* incorrect */strncpy( buf2, buf1, 100 ); /* correct */





Multiple Strings that use the same memory.

Consider the following code:

char buf[20] = "Hello, World!\n";char *buf2 = buf + 7;

printf("buf: %s\n", buf);printf("buf2: %s\n", buf2);

buf2[0] = ’M’;printf("buf: %s\n", buf);

What is the output?

Variable Name

5000


buf210001004

5000500150025003

500650075008

501050115012501350145015501650175018

5009

50055004

H

o,

el

’ ’World!\n\0

l

buf5007





’char *’ versus ’char string[100]’.

(1) char *no_space_buffer;(2) char buffer_with_space[100];(3) char *class_name = "C201: Advanced Programming ...";

Often you will see strings declared in one of these ways. Thedifference is that ’char buffer with space[100]’ explicitlytells the compiler to allocate 100 bytes of storage. With the ’char*’ declaration, storage must come from somewhere else.Declaration (3) will cause the compiler to allocate space for thisstring on the string table.





Access a single character in a string.

Use the [] operator to access a single character in a string.

char *str = "Hello";printf( "The second character of %s is %c.\n",

str, str[1] );

Notes:

Use %c to print a single character with printf.

Note that in C, like in Java, arrays start at 0, not 1.





Character literals.

Character literal are enclosed in single quotes (i.e., ’, not ”).

char buf[10];

buf[0] = ’A’; /* correct */buf[0] = "A"; /* incorrect */buf[1] = 0; /* NULL terminator */





Multiple strings and Buffer overflow.

Strings in C don’t ”grow” automatically. In-stead, the programmer must always be awareof how much space is available. Consider thisexample.

char s1[10]; char s2[10];

strcpy( s1,

"This string is to long!\n" );

We are copying 25 bytes into a 10 byte buffer!The compiler and run-time environment will notdetect this! We have overwritten string s2!Since the string is 25 characters and s1 and s2

are 20 characters total, we have written off theend of our own memory and have potentiallycorrupted the program’s call stack!

Variable Name

5000


s1s2

10001004

5000500150025003

500650075008

501050115012501350145015501650175018

5009

50055004

5010

’ ’

g’ ’

s

This

strin

i

’ ’

ot

’ ’l





strncpy

To make string handling safer, the C string library provides afunction called strncpy.

char global_name[100];...voidsave_name( const char *n ){

strncpy( global_name, n, 100 );global_name[99] = 0;

}





More about strncpy

If you carefully read the strncpy man-page, there are two(not-so-obvious) things to keep in mind.

1 If there is not enough space in the buffer, strncpy may notNULL terminate the string.

2 If you write 8 bytes to a 10 byte buffer, the remaining 2 bytesare set to the NULL character. The caveat is that if you havea 10 bytes buffer and you do a

strncpy( buffer, stuff, 20 );

you will overflow 10 bytes off the end of buffer even ifstuff will fit in buffer.





The string library

Some useful string handling functions in the C string library.





strcpy, strncpy: Copy in-memory string data.

strcmp, strncmp: String comparison. Returns zero if stringsare identical.

strlen: Get the length of a string.





More on strncmp.

strncmp is also useful for testing the prefix of a string.

char *string = "Hello, World!";

if( strncmp( string, "Hello", 5 ) == 0 ) {printf("This string begins with Hello.\n");

}





String parsing with scanf.

Say you have some data in a string buffer that was read withfgets. And you want to grab the data out of it.

char *str = "100 feet 3.14159 Pi";int i; double pi;

sscanf( str, "%d feet %lf", &i, &pi );





Another function that is useful for string parsing is strchr. Thisfunction will search for a character in a string. strchr will returna pointer to the character. If the character cannot be found, NULLis returned.

char *buf = "Hello, World!\n";char *pc;

pc = strchr( buf, ’ ’ );if( pc == NULL ) {

/* space not found. */}else {

printf("pc: %s\n", pc );}

What is the output?Mike Closson C-Style Strings




More string library functions:

String concatenation: strcat, strncat

Finding a character in a string: strrchr, index, rindex

Find a substring: strstr

More string parsing functions: strsep, strspn, strtok

For more information: man string


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