[c] How do I concatenate const/literal strings in C?

I'm working in C, and I have to concatenate a few things.

Right now I have this:

message = strcat("TEXT ", var);

message2 = strcat(strcat("TEXT ", foo), strcat(" TEXT ", bar));

Now if you have experience in C I'm sure you realize that this gives you a segmentation fault when you try to run it. So how do I work around that?

Also malloc and realloc are useful if you don't know ahead of time how many strings are being concatenated.

#include <stdio.h>
#include <string.h>

void example(const char *header, const char **words, size_t num_words)
{
    size_t message_len = strlen(header) + 1; /* + 1 for terminating NULL */
    char *message = (char*) malloc(message_len);
    strncat(message, header, message_len);

    for(int i = 0; i < num_words; ++i)
    {
       message_len += 1 + strlen(words[i]); /* 1 + for separator ';' */
       message = (char*) realloc(message, message_len);
       strncat(strncat(message, ";", message_len), words[i], message_len);
    }

    puts(message);

    free(message);
}

int main()
{
    char input[100];
    gets(input);

    char str[101];
    strcpy(str, " ");
    strcat(str, input);

    char *p = str;

    while(*p) {
       if(*p == ' ' && isalpha(*(p+1)) != 0)
           printf("%c",*(p+1));
       p++;
    }

    return 0;
}

Also malloc and realloc are useful if you don't know ahead of time how many strings are being concatenated.

#include <stdio.h>
#include <string.h>

void example(const char *header, const char **words, size_t num_words)
{
    size_t message_len = strlen(header) + 1; /* + 1 for terminating NULL */
    char *message = (char*) malloc(message_len);
    strncat(message, header, message_len);

    for(int i = 0; i < num_words; ++i)
    {
       message_len += 1 + strlen(words[i]); /* 1 + for separator ';' */
       message = (char*) realloc(message, message_len);
       strncat(strncat(message, ";", message_len), words[i], message_len);
    }

    puts(message);

    free(message);
}

Assuming you have a char[fixed_size] rather than a char*, you can use a single, creative macro to do it all at once with a <<cout<<like ordering ("rather %s the disjointed %s\n", "than", "printf style format"). If you are working with embedded systems, this method will also allow you to leave out malloc and the large *printf family of functions like snprintf() (This keeps dietlibc from complaining about *printf too)

#include <unistd.h> //for the write example
//note: you should check if offset==sizeof(buf) after use
#define strcpyALL(buf, offset, ...) do{ \
    char *bp=(char*)(buf+offset); /*so we can add to the end of a string*/ \
    const char *s, \
    *a[] = { __VA_ARGS__,NULL}, \
    **ss=a; \
    while((s=*ss++)) \
         while((*s)&&(++offset<(int)sizeof(buf))) \
            *bp++=*s++; \
    if (offset!=sizeof(buf))*bp=0; \
}while(0)

char buf[256];
int len=0;

strcpyALL(buf,len,
    "The config file is in:\n\t",getenv("HOME"),"/.config/",argv[0],"/config.rc\n"
);
if (len<sizeof(buf))
    write(1,buf,len); //outputs our message to stdout
else
    write(2,"error\n",6);

//but we can keep adding on because we kept track of the length
//this allows printf-like buffering to minimize number of syscalls to write
//set len back to 0 if you don't want this behavior
strcpyALL(buf,len,"Thanks for using ",argv[0],"!\n");
if (len<sizeof(buf))
    write(1,buf,len); //outputs both messages
else
    write(2,"error\n",6);

• Note 1, you typically wouldn't use argv[0] like this - just an example
• Note 2, you can use any function that outputs a char*, including nonstandard functions like itoa() for converting integers to string types.
• Note 3, if you are already using printf anywhere in your program there is no reason not to use snprintf(), since the compiled code would be larger (but inlined and significantly faster)

This was my solution

#include <stdlib.h>
#include <stdarg.h>

char *strconcat(int num_args, ...) {
    int strsize = 0;
    va_list ap;
    va_start(ap, num_args);
    for (int i = 0; i < num_args; i++) 
        strsize += strlen(va_arg(ap, char*));

    char *res = malloc(strsize+1);
    strsize = 0;
    va_start(ap, num_args);
    for (int i = 0; i < num_args; i++) {
        char *s = va_arg(ap, char*);
        strcpy(res+strsize, s);
        strsize += strlen(s);
    }
    va_end(ap);
    res[strsize] = '\0';

    return res;
}

but you need to specify how many strings you're going to concatenate

char *str = strconcat(3, "testing ", "this ", "thing");

Avoid using strcat in C code. The cleanest and, most importantly, the safest way is to use snprintf:

char buf[256];
snprintf(buf, sizeof buf, "%s%s%s%s", str1, str2, str3, str4);

Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile, but most compilers already issue a warning if this is the case.

Do not forget to initialize the output buffer. The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string:

char out[1024] = ""; // must be initialized
strcat( out, null_terminated_string ); 
// null_terminated_string has less than 1023 chars

If you have experience in C you will notice that strings are only char arrays where the last character is a null character.

Now that is quite inconvenient as you have to find the last character in order to append something. strcat will do that for you.

So strcat searches through the first argument for a null character. Then it will replace this with the second argument's content (until that ends in a null).

Now let's go through your code:

message = strcat("TEXT " + var);

Here you are adding something to the pointer to the text "TEXT" (the type of "TEXT" is const char*. A pointer.).

That will usually not work. Also modifying the "TEXT" array will not work as it is usually placed in a constant segment.

message2 = strcat(strcat("TEXT ", foo), strcat(" TEXT ", bar));

That might work better, except that you are again trying to modify static texts. strcat is not allocating new memory for the result.

I would propose to do something like this instead:

sprintf(message2, "TEXT %s TEXT %s", foo, bar);

Read the documentation of sprintf to check for it's options.

And now an important point:

Ensure that the buffer has enough space to hold the text AND the null character. There are a couple of functions that can help you, e.g., strncat and special versions of printf that allocate the buffer for you. Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs.

Do not forget to initialize the output buffer. The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string:

char out[1024] = ""; // must be initialized
strcat( out, null_terminated_string ); 
// null_terminated_string has less than 1023 chars

Best way to do it without having a limited buffer size is by using asprintf()

char* concat(const char* str1, const char* str2)
{
    char* result;
    asprintf(&result, "%s%s", str1, str2);
    return result;
}

int main()
{
    char input[100];
    gets(input);

    char str[101];
    strcpy(str, " ");
    strcat(str, input);

    char *p = str;

    while(*p) {
       if(*p == ' ' && isalpha(*(p+1)) != 0)
           printf("%c",*(p+1));
       p++;
    }

    return 0;
}

It is undefined behaviour to attempt to modify string literals, which is what something like:

strcat ("Hello, ", name);

will attempt to do. It will try to tack on the name string to the end of the string literal "Hello, ", which is not well defined.

Try something this. It achieves what you appear to be trying to do:

char message[1000];
strcpy (message, "TEXT ");
strcat (message, var);

This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it. Just be careful with buffer overflows. If you control the input data (or check it before-hand), it's fine to use fixed length buffers like I have.

Otherwise, you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it. In other words, something like:

const static char TEXT[] = "TEXT ";

// Make *sure* you have enough space.

char *message = malloc (sizeof(TEXT) + strlen(var) + 1);
if (message == NULL)
     handleOutOfMemoryIntelligently();
strcpy (message, TEXT);
strcat (message, var);

// Need to free message at some point after you're done with it.

Also malloc and realloc are useful if you don't know ahead of time how many strings are being concatenated.

#include <stdio.h>
#include <string.h>

void example(const char *header, const char **words, size_t num_words)
{
    size_t message_len = strlen(header) + 1; /* + 1 for terminating NULL */
    char *message = (char*) malloc(message_len);
    strncat(message, header, message_len);

    for(int i = 0; i < num_words; ++i)
    {
       message_len += 1 + strlen(words[i]); /* 1 + for separator ';' */
       message = (char*) realloc(message, message_len);
       strncat(strncat(message, ";", message_len), words[i], message_len);
    }

    puts(message);

    free(message);
}

If you have experience in C you will notice that strings are only char arrays where the last character is a null character.

Now that is quite inconvenient as you have to find the last character in order to append something. strcat will do that for you.

So strcat searches through the first argument for a null character. Then it will replace this with the second argument's content (until that ends in a null).

Now let's go through your code:

message = strcat("TEXT " + var);

Here you are adding something to the pointer to the text "TEXT" (the type of "TEXT" is const char*. A pointer.).

That will usually not work. Also modifying the "TEXT" array will not work as it is usually placed in a constant segment.

message2 = strcat(strcat("TEXT ", foo), strcat(" TEXT ", bar));

That might work better, except that you are again trying to modify static texts. strcat is not allocating new memory for the result.

I would propose to do something like this instead:

sprintf(message2, "TEXT %s TEXT %s", foo, bar);

Read the documentation of sprintf to check for it's options.

And now an important point:

Ensure that the buffer has enough space to hold the text AND the null character. There are a couple of functions that can help you, e.g., strncat and special versions of printf that allocate the buffer for you. Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs.

You are trying to copy a string into an address that is statically allocated. You need to cat into a buffer.

Specifically:

...snip...

destination

Pointer to the destination array, which should contain a C string, and be large enough to contain the concatenated resulting string.

...snip...

http://www.cplusplus.com/reference/clibrary/cstring/strcat.html

There's an example here as well.

Try something similar to this:

#include <stdio.h>
#include <string.h>

int main(int argc, const char * argv[])
{
  // Insert code here...
  char firstname[100], secondname[100];
  printf("Enter First Name: ");
  fgets(firstname, 100, stdin);
  printf("Enter Second Name: ");
  fgets(secondname,100,stdin);
  firstname[strlen(firstname)-1]= '\0';
  printf("fullname is %s %s", firstname, secondname);

  return 0;
}

Strings can also be concatenated at compile time.

#define SCHEMA "test"
#define TABLE  "data"

const char *table = SCHEMA "." TABLE ; // note no + or . or anything
const char *qry =               // include comments in a string
    " SELECT * "                // get all fields
    " FROM " SCHEMA "." TABLE   /* the table */
    " WHERE x = 1 "             /* the filter */ 
                ;

If you have experience in C you will notice that strings are only char arrays where the last character is a null character.

Now that is quite inconvenient as you have to find the last character in order to append something. strcat will do that for you.

So strcat searches through the first argument for a null character. Then it will replace this with the second argument's content (until that ends in a null).

Now let's go through your code:

message = strcat("TEXT " + var);

Here you are adding something to the pointer to the text "TEXT" (the type of "TEXT" is const char*. A pointer.).

That will usually not work. Also modifying the "TEXT" array will not work as it is usually placed in a constant segment.

message2 = strcat(strcat("TEXT ", foo), strcat(" TEXT ", bar));

That might work better, except that you are again trying to modify static texts. strcat is not allocating new memory for the result.

I would propose to do something like this instead:

sprintf(message2, "TEXT %s TEXT %s", foo, bar);

Read the documentation of sprintf to check for it's options.

And now an important point:

Ensure that the buffer has enough space to hold the text AND the null character. There are a couple of functions that can help you, e.g., strncat and special versions of printf that allocate the buffer for you. Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs.

Folks, use strncpy(), strncat(), or snprintf().
Exceeding your buffer space will trash whatever else follows in memory!
(And remember to allow space for the trailing null '\0' character!)

If you have experience in C you will notice that strings are only char arrays where the last character is a null character.

Now that is quite inconvenient as you have to find the last character in order to append something. strcat will do that for you.

So strcat searches through the first argument for a null character. Then it will replace this with the second argument's content (until that ends in a null).

Now let's go through your code:

message = strcat("TEXT " + var);

Here you are adding something to the pointer to the text "TEXT" (the type of "TEXT" is const char*. A pointer.).

That will usually not work. Also modifying the "TEXT" array will not work as it is usually placed in a constant segment.

message2 = strcat(strcat("TEXT ", foo), strcat(" TEXT ", bar));

That might work better, except that you are again trying to modify static texts. strcat is not allocating new memory for the result.

I would propose to do something like this instead:

sprintf(message2, "TEXT %s TEXT %s", foo, bar);

Read the documentation of sprintf to check for it's options.

And now an important point:

Ensure that the buffer has enough space to hold the text AND the null character. There are a couple of functions that can help you, e.g., strncat and special versions of printf that allocate the buffer for you. Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs.

Assuming you have a char[fixed_size] rather than a char*, you can use a single, creative macro to do it all at once with a <<cout<<like ordering ("rather %s the disjointed %s\n", "than", "printf style format"). If you are working with embedded systems, this method will also allow you to leave out malloc and the large *printf family of functions like snprintf() (This keeps dietlibc from complaining about *printf too)

#include <unistd.h> //for the write example
//note: you should check if offset==sizeof(buf) after use
#define strcpyALL(buf, offset, ...) do{ \
    char *bp=(char*)(buf+offset); /*so we can add to the end of a string*/ \
    const char *s, \
    *a[] = { __VA_ARGS__,NULL}, \
    **ss=a; \
    while((s=*ss++)) \
         while((*s)&&(++offset<(int)sizeof(buf))) \
            *bp++=*s++; \
    if (offset!=sizeof(buf))*bp=0; \
}while(0)

char buf[256];
int len=0;

strcpyALL(buf,len,
    "The config file is in:\n\t",getenv("HOME"),"/.config/",argv[0],"/config.rc\n"
);
if (len<sizeof(buf))
    write(1,buf,len); //outputs our message to stdout
else
    write(2,"error\n",6);

//but we can keep adding on because we kept track of the length
//this allows printf-like buffering to minimize number of syscalls to write
//set len back to 0 if you don't want this behavior
strcpyALL(buf,len,"Thanks for using ",argv[0],"!\n");
if (len<sizeof(buf))
    write(1,buf,len); //outputs both messages
else
    write(2,"error\n",6);

• Note 1, you typically wouldn't use argv[0] like this - just an example
• Note 2, you can use any function that outputs a char*, including nonstandard functions like itoa() for converting integers to string types.
• Note 3, if you are already using printf anywhere in your program there is no reason not to use snprintf(), since the compiled code would be larger (but inlined and significantly faster)

You are trying to copy a string into an address that is statically allocated. You need to cat into a buffer.

Specifically:

...snip...

destination

Pointer to the destination array, which should contain a C string, and be large enough to contain the concatenated resulting string.

...snip...

http://www.cplusplus.com/reference/clibrary/cstring/strcat.html

There's an example here as well.

Avoid using strcat in C code. The cleanest and, most importantly, the safest way is to use snprintf:

char buf[256];
snprintf(buf, sizeof buf, "%s%s%s%s", str1, str2, str3, str4);

Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile, but most compilers already issue a warning if this is the case.

Best way to do it without having a limited buffer size is by using asprintf()

char* concat(const char* str1, const char* str2)
{
    char* result;
    asprintf(&result, "%s%s", str1, str2);
    return result;
}

You are trying to copy a string into an address that is statically allocated. You need to cat into a buffer.

Specifically:

...snip...

destination

Pointer to the destination array, which should contain a C string, and be large enough to contain the concatenated resulting string.

...snip...

http://www.cplusplus.com/reference/clibrary/cstring/strcat.html

There's an example here as well.

Folks, use strncpy(), strncat(), or snprintf().
Exceeding your buffer space will trash whatever else follows in memory!
(And remember to allow space for the trailing null '\0' character!)

You can write your own function that does the same thing as strcat() but that doesn't change anything:

#define MAX_STRING_LENGTH 1000
char *strcat_const(const char *str1,const char *str2){
    static char buffer[MAX_STRING_LENGTH];
    strncpy(buffer,str1,MAX_STRING_LENGTH);
    if(strlen(str1) < MAX_STRING_LENGTH){
        strncat(buffer,str2,MAX_STRING_LENGTH - strlen(buffer));
    }
    buffer[MAX_STRING_LENGTH - 1] = '\0';
    return buffer;
}

int main(int argc,char *argv[]){
    printf("%s",strcat_const("Hello ","world"));    //Prints "Hello world"
    return 0;
}

If both strings together are more than 1000 characters long, it will cut the string at 1000 characters. You can change the value of MAX_STRING_LENGTH to suit your needs.

As people pointed out string handling improved much. So you may want to learn how to use the C++ string library instead of C-style strings. However here is a solution in pure C

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

void appendToHello(const char *s) {
    const char *const hello = "hello ";

    const size_t sLength     = strlen(s);
    const size_t helloLength = strlen(hello);
    const size_t totalLength = sLength + helloLength;

    char *const strBuf = malloc(totalLength + 1);
    if (strBuf == NULL) {
        fprintf(stderr, "malloc failed\n");
        exit(EXIT_FAILURE);
    }

    strcpy(strBuf, hello);
    strcpy(strBuf + helloLength, s);

    puts(strBuf);

    free(strBuf);

}

int main (void) {
    appendToHello("blah blah");
    return 0;
}

I am not sure whether it is correct/safe but right now I could not find a better way to do this in ANSI C.

Do not forget to initialize the output buffer. The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string:

char out[1024] = ""; // must be initialized
strcat( out, null_terminated_string ); 
// null_terminated_string has less than 1023 chars

You can write your own function that does the same thing as strcat() but that doesn't change anything:

#define MAX_STRING_LENGTH 1000
char *strcat_const(const char *str1,const char *str2){
    static char buffer[MAX_STRING_LENGTH];
    strncpy(buffer,str1,MAX_STRING_LENGTH);
    if(strlen(str1) < MAX_STRING_LENGTH){
        strncat(buffer,str2,MAX_STRING_LENGTH - strlen(buffer));
    }
    buffer[MAX_STRING_LENGTH - 1] = '\0';
    return buffer;
}

int main(int argc,char *argv[]){
    printf("%s",strcat_const("Hello ","world"));    //Prints "Hello world"
    return 0;
}

If both strings together are more than 1000 characters long, it will cut the string at 1000 characters. You can change the value of MAX_STRING_LENGTH to suit your needs.

The first argument of strcat() needs to be able to hold enough space for the concatenated string. So allocate a buffer with enough space to receive the result.

char bigEnough[64] = "";

strcat(bigEnough, "TEXT");
strcat(bigEnough, foo);

/* and so on */

strcat() will concatenate the second argument with the first argument, and store the result in the first argument, the returned char* is simply this first argument, and only for your convenience.

You do not get a newly allocated string with the first and second argument concatenated, which I'd guess you expected based on your code.

Avoid using strcat in C code. The cleanest and, most importantly, the safest way is to use snprintf:

char buf[256];
snprintf(buf, sizeof buf, "%s%s%s%s", str1, str2, str3, str4);

Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile, but most compilers already issue a warning if this is the case.

As people pointed out string handling improved much. So you may want to learn how to use the C++ string library instead of C-style strings. However here is a solution in pure C

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

void appendToHello(const char *s) {
    const char *const hello = "hello ";

    const size_t sLength     = strlen(s);
    const size_t helloLength = strlen(hello);
    const size_t totalLength = sLength + helloLength;

    char *const strBuf = malloc(totalLength + 1);
    if (strBuf == NULL) {
        fprintf(stderr, "malloc failed\n");
        exit(EXIT_FAILURE);
    }

    strcpy(strBuf, hello);
    strcpy(strBuf + helloLength, s);

    puts(strBuf);

    free(strBuf);

}

int main (void) {
    appendToHello("blah blah");
    return 0;
}

I am not sure whether it is correct/safe but right now I could not find a better way to do this in ANSI C.

It is undefined behaviour to attempt to modify string literals, which is what something like:

strcat ("Hello, ", name);

will attempt to do. It will try to tack on the name string to the end of the string literal "Hello, ", which is not well defined.

Try something this. It achieves what you appear to be trying to do:

char message[1000];
strcpy (message, "TEXT ");
strcat (message, var);

This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it. Just be careful with buffer overflows. If you control the input data (or check it before-hand), it's fine to use fixed length buffers like I have.

Otherwise, you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it. In other words, something like:

const static char TEXT[] = "TEXT ";

// Make *sure* you have enough space.

char *message = malloc (sizeof(TEXT) + strlen(var) + 1);
if (message == NULL)
     handleOutOfMemoryIntelligently();
strcpy (message, TEXT);
strcat (message, var);

// Need to free message at some point after you're done with it.

The first argument of strcat() needs to be able to hold enough space for the concatenated string. So allocate a buffer with enough space to receive the result.

char bigEnough[64] = "";

strcat(bigEnough, "TEXT");
strcat(bigEnough, foo);

/* and so on */

strcat() will concatenate the second argument with the first argument, and store the result in the first argument, the returned char* is simply this first argument, and only for your convenience.

You do not get a newly allocated string with the first and second argument concatenated, which I'd guess you expected based on your code.

Folks, use strncpy(), strncat(), or snprintf().
Exceeding your buffer space will trash whatever else follows in memory!
(And remember to allow space for the trailing null '\0' character!)

Do not forget to initialize the output buffer. The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string:

char out[1024] = ""; // must be initialized
strcat( out, null_terminated_string ); 
// null_terminated_string has less than 1023 chars

Strings can also be concatenated at compile time.

#define SCHEMA "test"
#define TABLE  "data"

const char *table = SCHEMA "." TABLE ; // note no + or . or anything
const char *qry =               // include comments in a string
    " SELECT * "                // get all fields
    " FROM " SCHEMA "." TABLE   /* the table */
    " WHERE x = 1 "             /* the filter */ 
                ;

Folks, use strncpy(), strncat(), or snprintf().
Exceeding your buffer space will trash whatever else follows in memory!
(And remember to allow space for the trailing null '\0' character!)

The first argument of strcat() needs to be able to hold enough space for the concatenated string. So allocate a buffer with enough space to receive the result.

char bigEnough[64] = "";

strcat(bigEnough, "TEXT");
strcat(bigEnough, foo);

/* and so on */

strcat() will concatenate the second argument with the first argument, and store the result in the first argument, the returned char* is simply this first argument, and only for your convenience.

You do not get a newly allocated string with the first and second argument concatenated, which I'd guess you expected based on your code.

This was my solution

#include <stdlib.h>
#include <stdarg.h>

char *strconcat(int num_args, ...) {
    int strsize = 0;
    va_list ap;
    va_start(ap, num_args);
    for (int i = 0; i < num_args; i++) 
        strsize += strlen(va_arg(ap, char*));

    char *res = malloc(strsize+1);
    strsize = 0;
    va_start(ap, num_args);
    for (int i = 0; i < num_args; i++) {
        char *s = va_arg(ap, char*);
        strcpy(res+strsize, s);
        strsize += strlen(s);
    }
    va_end(ap);
    res[strsize] = '\0';

    return res;
}

but you need to specify how many strings you're going to concatenate

char *str = strconcat(3, "testing ", "this ", "thing");

It is undefined behaviour to attempt to modify string literals, which is what something like:

strcat ("Hello, ", name);

will attempt to do. It will try to tack on the name string to the end of the string literal "Hello, ", which is not well defined.

Try something this. It achieves what you appear to be trying to do:

char message[1000];
strcpy (message, "TEXT ");
strcat (message, var);

This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it. Just be careful with buffer overflows. If you control the input data (or check it before-hand), it's fine to use fixed length buffers like I have.

Otherwise, you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it. In other words, something like:

const static char TEXT[] = "TEXT ";

// Make *sure* you have enough space.

char *message = malloc (sizeof(TEXT) + strlen(var) + 1);
if (message == NULL)
     handleOutOfMemoryIntelligently();
strcpy (message, TEXT);
strcat (message, var);

// Need to free message at some point after you're done with it.

Try something similar to this:

#include <stdio.h>
#include <string.h>

int main(int argc, const char * argv[])
{
  // Insert code here...
  char firstname[100], secondname[100];
  printf("Enter First Name: ");
  fgets(firstname, 100, stdin);
  printf("Enter Second Name: ");
  fgets(secondname,100,stdin);
  firstname[strlen(firstname)-1]= '\0';
  printf("fullname is %s %s", firstname, secondname);

  return 0;
}

Also malloc and realloc are useful if you don't know ahead of time how many strings are being concatenated.

#include <stdio.h>
#include <string.h>

void example(const char *header, const char **words, size_t num_words)
{
    size_t message_len = strlen(header) + 1; /* + 1 for terminating NULL */
    char *message = (char*) malloc(message_len);
    strncat(message, header, message_len);

    for(int i = 0; i < num_words; ++i)
    {
       message_len += 1 + strlen(words[i]); /* 1 + for separator ';' */
       message = (char*) realloc(message, message_len);
       strncat(strncat(message, ";", message_len), words[i], message_len);
    }

    puts(message);

    free(message);
}

It is undefined behaviour to attempt to modify string literals, which is what something like:

strcat ("Hello, ", name);

will attempt to do. It will try to tack on the name string to the end of the string literal "Hello, ", which is not well defined.

Try something this. It achieves what you appear to be trying to do:

char message[1000];
strcpy (message, "TEXT ");
strcat (message, var);

This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it. Just be careful with buffer overflows. If you control the input data (or check it before-hand), it's fine to use fixed length buffers like I have.

Otherwise, you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it. In other words, something like:

const static char TEXT[] = "TEXT ";

// Make *sure* you have enough space.

char *message = malloc (sizeof(TEXT) + strlen(var) + 1);
if (message == NULL)
     handleOutOfMemoryIntelligently();
strcpy (message, TEXT);
strcat (message, var);

// Need to free message at some point after you're done with it.

The first argument of strcat() needs to be able to hold enough space for the concatenated string. So allocate a buffer with enough space to receive the result.

char bigEnough[64] = "";

strcat(bigEnough, "TEXT");
strcat(bigEnough, foo);

/* and so on */

strcat() will concatenate the second argument with the first argument, and store the result in the first argument, the returned char* is simply this first argument, and only for your convenience.

You do not get a newly allocated string with the first and second argument concatenated, which I'd guess you expected based on your code.

You are trying to copy a string into an address that is statically allocated. You need to cat into a buffer.

Specifically:

...snip...

destination

Pointer to the destination array, which should contain a C string, and be large enough to contain the concatenated resulting string.

...snip...

http://www.cplusplus.com/reference/clibrary/cstring/strcat.html

There's an example here as well.