以下是使用C语言实现KMP算法和字符串替换函数的示例代码：

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

// KMP算法
int kmp_search(char* text, char* pattern) {
    int n = strlen(text);
    int m = strlen(pattern);
    int* lps = (int*)malloc(sizeof(int) * m);
    int i = 0, j = 0;
    int count = 0;

    compute_lps(pattern, m, lps);

    while (i < n) {
        if (pattern[j] == text[i]) {
            j++;
            i++;
        }

        if (j == m) {
            count++;
            j = lps[j - 1];
        }
        else if (i < n && pattern[j] != text[i]) {
            if (j != 0)
                j = lps[j - 1];
            else
                i++;
        }
    }

    free(lps);
    return count;
}

void compute_lps(char* pattern, int m, int* lps) {
    int len = 0;
    int i = 1;
    lps[0] = 0;

    while (i < m) {
        if (pattern[i] == pattern[len]) {
            len++;
            lps[i] = len;
            i++;
        }
        else {
            if (len != 0)
                len = lps[len - 1];
            else {
                lps[i] = 0;
                i++;
            }
        }
    }
}

// 字符串替换函数
void replace_string(char* text, char* pattern, char* replace) {
    int n = strlen(text);
    int m = strlen(pattern);
    int s = strlen(replace);
    int count = kmp_search(text, pattern);
    int* positions = (int*)malloc(sizeof(int) * count);
    int i, j, k;

    compute_positions(text, pattern, positions);

    for (i = count - 1; i >= 0; i--) {
        for (j = positions[i] + m - 1; j < n - m + s; j++) {
            text[j + s - m] = text[j + 1];
        }

        for (k = 0; k < s; k++) {
            text[positions[i] + k] = replace[k];
        }
    }

    free(positions);
}

void compute_positions(char* text, char* pattern, int* positions) {
    int n = strlen(text);
    int m = strlen(pattern);
    int* lps = (int*)malloc(sizeof(int) * m);
    int i = 0, j = 0;
    int count = 0;

    compute_lps(pattern, m, lps);

    while (i < n) {
        if (pattern[j] == text[i]) {
            j++;
            i++;
        }

        if (j == m) {
            positions[count] = i - j;
            count++;
            j = lps[j - 1];
        }
        else if (i < n && pattern[j] != text[i]) {
            if (j != 0)
                j = lps[j - 1];
            else
                i++;
        }
    }

    free(lps);
}

int main() {
    char text[100];
    char pattern[100];
    char replace[100];

    printf("请输入主串T：");
    scanf("%s", text);

    printf("请输入模式串P：");
    scanf("%s", pattern);

    printf("请输入替换串S：");
    scanf("%s", replace);

    int count = kmp_search(text, pattern);
    printf("匹配成功次数：%d
", count);

    int* positions = (int*)malloc(sizeof(int) * count);
    compute_positions(text, pattern, positions);
    printf("P在T中的定位：");
    for (int i = 0; i < count; i++) {
        printf("%d ", positions[i]);
    }
    printf("
");

    replace_string(text, pattern, replace);
    printf("替换后的主串：%s
", text);

    free(positions);

    return 0;
}

你可以将以上代码保存为一个C文件，然后编译运行。在运行时，你需要依次输入主串T、模式串P和替换串S。程序将输出匹配成功次数、P在T中的定位以及替换后的主串。